CFI Brief: FAA Taxi Test

Monday on the blog we briefly discussed runway incursions and recommended practices for pilots to avoid such an occurrence. As air traffic grows and airports become busier, both general aviation and commercial, runway incursions become more of a growing concern to pilots and airport operators. In an effort to cut down on the potential of surface movement issues the FAA has implemented programs such as defining runway hotspots and identifying standardized taxi route.

Runway Hotspots
ICAO defines runway hotspots as a location on an aerodrome movement area with a history or potential risk of collision or runway incursion and where heightened attention by pilots and drivers is necessary. Hotspots alert pilots to complex or potentially confusing taxiway geometry that could make surface navigation challenging. Whatever the reason, pilots need to be aware that these hazardous intersections exist, and they should be increasingly vigilant when approaching and taxiing through these intersections. These hotspots are depicted on some airport charts as circled areas. [Figure 1-6] The FAA Office of Runway Safety has links to the FAA regions that maintain a complete list of airports with runway hotspots at

Hot Spots

Standardized Taxi Routes
Standard taxi routes improve ground management at high-density airports, namely those that have airline service. At these airports, typical taxiway traffic patterns used to move aircraft between gate and runway are laid out and coded. The ATC specialist (ATCS) can reduce radio communication time and eliminate taxi instruction misinterpretation by simply clearing the pilot to taxi via a specific, named route. An example of this would be Los Angeles International Airport (KLAX), where North Route is used to transition to Runway 24L. [Figure 1-7] These routes are issued by ground control, and if unable to comply, pilots must advise ground control on initial contact. If for any reason the pilot becomes uncertain as to the correct taxi route, a request should be made for progressive taxi instructions. These step-by-step routing directions are also issued if the controller deems it necessary due to traffic, closed taxiways, airport construction, etc. It is the pilot’s responsibility to know if a particular airport has preplanned taxi routes, to be familiar with them, and to have the taxi descriptions in their possession. Specific information about airports that use coded taxiway routes is included in the Notices to Airmen Publication (NTAP).

Standardized Taxi Routes
The best way you as a pilot can prevent a runway incursions is by being familiar with your surroundings and understanding the airport environment and standardized procedures that are in place. The FAA Safety Team has put together an excellent video and taxi test that will test your knowledge of procedures and operations on the airport movement area. I encourage you to spend 60 minutes and take the course.

The FAA Taxi Test

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Procedures and Airport Operations: Runway Incursions

Learn to reduce your risk of a runway incursion by following these simple FAA recommendations outlined in the Instrument Procedures Handbook (FAA-8083-16A).

On any given day, the NAS may handle almost 200,000 takeoffs and landings. Due to the complex nature of the airport environment and the intricacies of the network of people that make it operate efficiently, the FAA is constantly looking to maintain the high standard of safety that exists at airports today. Runway safety is one of its top priorities.

The FAA defines a runway incursion as:
“Any occurrence at an aerodrome involving the incorrect presence of an aircraft, vehicle, or person on the protected area of a surface designated for the landing and takeoff of aircraft.”

The four categories of runway incursions are listed below:
Category A—a serious incident in which a collision was narrowly avoided.
Category B—an incident in which separation decreases and there is a significant potential for collision that may result in a time critical corrective/evasive response to avoid a collision.
Category C—an incident characterized by ample time and/or distance to avoid a collision.
Category D—an incident that meets the definition of runway incursion, such as incorrect presence of a single vehicle/person/aircraft on the protected area of a surface designated for the landing and takeoff of aircraft but with no immediate safety consequences.

The below figure highlights several steps that reduce the chances of being involved in a runway incursion.

Fig 1-5


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Aerodynamics: Power

Today’s post is excerpted from the second edition of our textbook Aerodynamics for Aviators. This book features extensive illustrations and covers everything from the fundamentals of flight to high-speed flight, and includes an excellent compendium of formulae and equations used at all levels of aviation.

Aircraft aerodynamics involves the interaction of the four forces: lift, weight, thrust, and drag. The first basic issue to understand is the difference between propeller-driven aircraft power and jet engine thrust. Power is what a propeller-driven engine produces; thrust is what a jet engine produces. In a propeller-driven aircraft, the propeller—not the engine—is said to produce thrust. The thrust on a propeller-driven aircraft decreases with an increase in velocity; in a jet aircraft, thrust remains relatively constant with an increase in aircraft velocity.

(A) Thrust vs. velocity, jet engine; (B) Thrust vs. velocity, propeller-driven engine.

(A) Thrust vs. velocity, jet engine; (B) Thrust vs. velocity, propeller-driven engine.

Therefore, the power required curve versus the power available curve for a propeller driven aircraft and a jet aircraft will look different.

Power required versus power available (A) for a jet engine and (B) for a propeller-driven aircraft.

Power required versus power available (A) for a
jet engine and (B) for a propeller-driven aircraft.

Propeller Efficiency
Propeller efficiency is a measure of how much power is absorbed (transmitted) by the propeller and turned into thrust. In order to understand propeller efficiency, it’s helpful to start with a basic review of propeller principles. Propellers on aircraft consist of two or more blades and a hub. The blades are attached to the hub, and the hub is attached to the crankshaft on a piston-powered aircraft and to a gear reduction box on most turbo-prop aircraft. The propeller is simply a rotating wing that produces lift along the vertical axis. We call this lift force thrust.

Forces in flight.

Forces in flight.

Looking at a cross section of the propeller blade, we can see that it is similar to a cross section of an aircraft wing. The top portion of the blade is cambered like the top surface of a wing. The bottom portion is flat like the bottom surface of a wing.

Propeller cross section.

Propeller cross section.

The chord line is an imaginary line drawn from the leading edge of the propeller blade to the trailing edge of the propeller blade. Blade angle, measured in degrees, is the angle between the chord of the blade and the plane of rotation. The pitch of the propeller is usually designated in inches. A “78-52” propeller is 78 inches in length with an effective pitch of 52 inches. The effective pitch is the distance a propeller would move through the air in one revolution if there were no slippage. On a “78-52” propeller this distance would be 52 inches.

Propeller blade angle.

Propeller blade angle.

There are two types of propellers that can be installed on most general aviation aircraft: a fixed-pitch propeller or a controllable-pitch propeller. The fixed-pitch propeller is at the blade angle that will give it the best overall efficiency for the type of operation being conducted and for which the aircraft was designed. For most aircraft this would be a cruise setting. A controllable-pitch propeller allows the pilot to adjust the blade angle for the different phases of flight. On takeoff and climb out, a low pitch/high RPM setting is used. During cruise flight a high pitch/low RPM setting is generally used.

On the ground with the aircraft in a static condition, the propeller efficiency is very low because each blade is moving through the air at an angle of attack that produces a very low thrust to power ratio. This means that a lot of power is being used to sustain the engine and rotate the propeller while very little thrust is being produced. The propeller, unlike the wing, moves both rotationally and forward (dynamically). The angle at which the relative wind strikes the propeller blade is the AOA. This produces a higher dynamic pressure on the engine side, which in turn is called thrust. Thus thrust is the relationship of propeller AOA and blade angle.

Propeller blade angle with forward velocity.

Propeller blade angle with forward velocity.

Since an aircraft moves forward through the air, it is important that the pilot understands how forward velocity affects the AOA of the propeller. The figure above shows the propeller in a static condition on the ground. At this point the relative wind is opposing propeller rotation. As forward velocity increases, the relative wind moves closer to the chord line, decreasing the propeller AOA. This can easily be demonstrated in an aircraft with a fixed-pitch propeller by pitching up or down without changing power. When the aircraft is pitched down, RPM will increase as the relative wind moves closer to the chord line and the AOA is decreased. When the aircraft is pitched up, the RPM will decrease as the relative wind moves farther from the chord line and the AOA is increased.

Propeller efficiency is a ratio between thrust horsepower and brake horsepower. Brake horsepower (BHP) is the horsepower actually delivered to the output shaft. Brake horsepower is the actual usable horsepower. Thrust horsepower (THP) is the power that is imparted by the propeller to the air. Propeller efficiency is the relationship between brake horsepower and thrust horsepower. If the BHP of the engine is 200, the THP is less (20–40%). Some power is lost to turn the engine and propeller. Propeller efficiency usually varies between 50 and 80% on light general aviation aircraft.

The measure of efficiency is how much a propeller slips in the air. This is measured by the geometric pitch (theoretical) which shows a propeller with no slippage. Effective pitch is the distance that the propeller actually travels.

Propeller efficiency.

Propeller efficiency.

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CFI Brief: Calling all Student Pilots!

Share your flying story with the Learn to Fly Blog

We’re looking for student pilot stories dealing with overcoming challenges, lessons-learned, or insights-gained, humorous or serious. If selected, your story will be published on the Learn to Fly Blog! This is an ongoing call for submissions and there’s no deadline. Once selected your story will be professionally edited and published this summer to the Learn to Fly Blog for all your family, friends, and future employers to see!

Send you story (minimum 500 words) as a .doc or .rtf file to

Include your name, school, current number of hours, and a short bio (optional).



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Procedures and Airport Operations: Normal Landing

A good landing is most likely following a good approach, so aim to be well established in a stabilized approach with the airplane nicely trimmed by the time you reach short final, the last part of the approach. Short final for a training airplane may be thought of as the last 200 feet.

The landing starts with a flare commencing when the pilot’s eyes are about 15 feet above the runway. The pilot uses texture, height of peripheral objects, width of the runway and the perceived height of the horizon as cues to commence the flare and to judge the rate of rotation to achieve an almost level path over the runway. The landing is not complete until the end of the landing roll.

Once you reach the flaring height, forget the aim point because you will fly over and well past it before the wheels actually touch down. It has served its purpose and you should now look well ahead. Pick a point at the center of the far end of the runway. Transfer your visual attention to this point and slowly retard the throttle.

A normal landing is similar to the approach to the stall, with attitude being increased to keep the aircraft flying at the reducing airspeed. Touchdown will occur just prior to the moment of stall. Do not rush and try not to be tense. The aircraft will land when it is ready. This method of landing allows the lowest possible touchdown speed (significantly less than the approach speed), with the pilot still having full control.

The landing consists of four phases:

  • flare (or round-out);
  • hold-off;
  • touchdown; and
  • landing roll.

During the flare (round-out) the power is reduced and the nose is gradually raised to reduce the rate of descent. A small rate of sink is checked by a slight attitude change, a high rate of sink requiring a greater and quicker backward movement. A greater descent rate may require the pilot to add power momentarily to arrest the descent.

The hold-off should occur with the airplane close to the ground (with the wheels within a foot or so). The throttle is closed and the control column progressively brought back to keep the airplane flying a level path with the wheels just off the ground. If sinking, apply more back pressure; if moving away from the ground, relax the back pressure. The airspeed will be decreasing to a very low figure, but this is of no concern to you. You should be looking well ahead from the beginning of round-out until touchdown. Any sideways drift caused by a slight crosswind can be counteracted by lowering the upwind wing a few degrees and keeping straight with rudder.

On touchdown, the main wheels should make first contact with the ground (which will be the case following a correct hold-off ). The nose wheel will want to drop immediately but should be kept off the ground using the control column while the speed decreases. This may require a significant rearward pressure to allow it to touch gently.

Landing Roll
During the landing roll the airplane is kept straight down the centerline using rudder and the wings kept level with aileron. Look at the far end of the runway. The nose wheel is gently lowered to the ground before elevator control is lost. Brakes (if required) may be used once the nose wheel is on the ground. Remember that the landing is not complete until the end of the landing roll when the airplane is stationary or has exited the runway at taxiing speed.

For more on landing technique, and every maneuver required for certification, check out the brand new fifth edition of ThePilot’s Manual: Flight School (PM-1C).

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CFI Brief: Solo Flight

In Monday’s post we were introduced to the student pilot’s first solo flight. Today, we will take a look a little more in depth to understand exactly what the instructor needs to do to prepare his or her student for solo flight. As a student, this will give you a behind the scenes look at the regulations in which a CFI needs to follow to prepare you the student for solo flight.

A student pilot may not operate an aircraft in solo flight unless the student pilot’s logbook has been endorsed for the specific make and model aircraft to be flown, and unless within the preceding 90 days his/her pilot logbook has been endorsed by an authorized flight instructor who has provided instruction in the make and model of aircraft in which the solo flight is made, and who finds that the applicant is competent to make a safe solo flight in that aircraft.

Prior to being authorized to conduct a solo flight, a student pilot must have received and logged instruction in the applicable maneuvers and procedures for the make and model of aircraft to be flown in solo flight, and must have demonstrated proficiency to an acceptable performance level as judged by the instructor who endorses the student’s pilot certificate. As appropriate to the aircraft to be flown in solo flight, the student pilot must have received presolo flight training in:

  1. Flight preparation procedures, including preflight inspections, powerplant operation, and aircraft systems.
  2. Taxiing or surface operations, including runups.
  3. Takeoffs and landings, including normal and crosswind.
  4. Straight-and-level flight and turns in both directions.
  5. Climbs and climbing turns.
  6. Airport traffic patterns, including entry and departure procedure, and collision, wind shear, and wake turbulence avoidance.
  7. Descents with and without turns, using high and low drag configurations.
  8. Flight at various airspeeds from cruise to slow flight.
  9. Stall entries from various flight attitudes and power combinations with recovery initiated at the first indication of a stall, and recovery from a full stall.
  10. Emergency procedures and equipment malfunctions.
  11. Ground reference maneuvers.
  12. Approaches to a landing area with simulated engine malfunctions.
  13. Slips to a landing.
  14. Go-arounds.

A student pilot may not operate an aircraft in a solo cross-country flight, nor may he/she, except in an emergency, make a solo flight landing at any point other than the airport of takeoff, until he/she meets the requirements prescribed in Part 61. However, an authorized flight instructor may allow a student to practice solo takeoffs and landings at another airport within 25 NM from the airport at which the student receives instruction, if the instructor finds the student competent to make those landings and takeoffs, and the flight training specific to the destination airport (including the route to and from, takeoffs and landings, and traffic pattern entry and exit) has taken place. Also, the instructor must have flown with that student prior to authorizing those takeoffs and landings, and endorsed the student pilot’s logbook accordingly.

The term cross-country flight means a flight beyond a radius of 25 nautical miles from the point of takeoff. A flight instructor must endorse a student pilot’s logbook for solo cross-country flights. There are three types of these endorsements:

  1. An endorsement in the student pilot’s logbook that the instructor has reviewed the preflight planning and preparation for each solo cross-country flight, and the pilot is prepared to make the flight safely under the known circumstances and the conditions listed by the instructor in the logbook.
  2. The instructor may also endorse the logbook for repeated solo cross-country flights under stipulated conditions over a course of not more than 50 nautical miles from the point of departure if he/she has given the student flight instruction in both directions over the route, including takeoffs and landings at the airports to be used.
  3. The student pilot certificate must be endorsed for cross-country operations.

Hopefully the information above gives you a general idea of what needs to be accomplished prior to your instructor allowing you to conduct solo flight, whether it be your first solo in the traffic pattern or a more extensive solo cross country flight to the next town over.


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Regulations: First Solo Flight

You are able to fly solo when the instructor believes, with some confidence, that you can fly safely with a degree of consistency and you have mastered the presolo maneuvers defined in the regulations. Most important is evidence that you are taking control and responsibility for your own actions—that you are walking on your own two feet. Today’s post comes from the new fifth edition of The Pilot’s Manual: Flight School (PM-1C).

The instructor is looking to see you make corrections for inaccuracies without waiting to be told and without asking for instructions, responding to radio calls without question and saying what you intend to do rather than asking what is next. These are the signs of aviation maturity, of being in command. They are no different from other life skills, just applied at a higher altitude.

First solo is an unforgettable experience that you will remember and treasure all your life. When your instructor tells you to stop after turning off the runway, steps out of the airplane, secures the harness and then leaves you to your first solo flight, you are being paid a big compliment. Your instructor is confident that you can safely complete a solo traffic pattern. You have demonstrated sufficient awareness, skill and consistency to be trusted to take the aircraft up by yourself.

You may feel a little apprehensive (or very confident), but remember that the instructor is trained to judge the right moment to send you solo. Your instructor has a better appreciation of your flying ability than anybody (including you—especially you).

Your instructor will have observed your progress and have assessed your consistency, safety and predictability. It is not the occasional brilliant landing that is looked for, but a series of consistently safe ones. Your instructor will choose the conditions and the traffic so that they are not more demanding than you are used to.

You know instinctively when you are ready to fly solo. In some cases, you may feel you are ready before time. Your instructor knows when the time is right. Trust in that.

Your instructor will also advise the control tower that this is a first solo and the controller will keep a watchful eye open for this new fledgling. The controller will anticipate wind changes and try not to change the active runway while you are flying your first solo traffic pattern.

Presolo Written Exam
Before going solo, you must have passed a written examination administered and graded by the flight instructor who endorses your logbook for solo flight. The written examination will include questions on the applicable Federal Aviation Regulations, and the flight characteristics and operational limits of your airplane. By answering the review questions of each exercise during your training, you will be well prepared for the questions on the flight characteristics and operational limits of your airplane.

These next review questions prepare you for the regulations questions. They direct you into your current copy of the regulations to indicate the level of knowledge you require prior to going solo. Since regulation numbering changes from time to time, the part has been identified—for example, Part 91 and Part 61—but not the individual section, which you can easily find using the table of contents page in your book of regulations.

Fly your first solo traffic pattern in the same manner as you flew the pattern before the instructor stepped out. The usual standards apply to the takeoff, pattern and landing. Follow exactly the same pattern and procedures. Maintain a good lookout, fly a neat pattern, establish a stabilized approach and carry out a normal landing. Be prepared for better performance of the airplane without the weight of your instructor on board. If at any stage you feel uncomfortable, go around. Many students comment on how much better the airplane flies without an instructor and how much quieter it is!

Be in control. Do not be blown with the wind. The tower will try to avoid any interruptions or runway changes while you are airborne but, if there is a need for you to hold overhead the field or to change runway, then take your time, think through the best plan of action, ask for instructions if you are in doubt and then complete a normal pattern and landing.

If an emergency occurs, such as engine failure (and this is an extremely unlikely event), carry out the appropriate emergency procedure that you have been taught. If your radio fails simply complete the pattern and land normally. Be aware of other traffic. You have been taught to go around and it may happen even on your first solo. Simply complete another pattern.

Your flight instructor, when sending you solo, not only considers you competent to fly a pattern with a normal takeoff and landing, but also considers you competent to handle an abnormal situation. One takeoff, one pattern and one landing are the rites of passage to the international community of pilots.

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CFI Brief: Flight Controls of a typical Commercial Airliner

This week on the Learn to Fly Blog the theme has been aerodynamics, and rather than stick to Private Pilot level aeronautical information we’ve hit you with some “graduate level” knowledge. Today, I thought it would be interesting to take a look at the primary flight controls of a typical commercial airliner. Looking at the image below you’ll notice right off the bat that while there’s a few more controls it’s not all that different than the training aircraft you might be flying in today.

One of the biggest differences to point out revolves around the way the flight controls are moved. Because of the high air loads, it is very difficult to move the flight control surfaces of jet aircraft with just mechanical and aerodynamic forces. So flight controls are usually moved by hydraulic actuators. Flight controls are divided into primary flight controls and secondary (or auxiliary) flight controls. The primary flight controls are those that maneuver the aircraft in roll, pitch, and yaw. These include the ailerons, elevator, and rudder. Secondary (or auxiliary) flight controls include tabs, trailing-edge flaps, leading-edge flaps, spoilers, and slats.


Roll control of most jet aircraft is accomplished by ailerons and flight spoilers. The exact mix of controls is determined by the aircraft’s flight regime. In low speed flight, all control surfaces operate to provide the desired roll control. As the aircraft moves into higher speed operations, control surface movement is reduced to provide approximately the same roll response to a given input through a wide range of speeds.

Many aircraft have two sets of ailerons—inboard and outboard. The inboard ailerons operate in all flight regimes. The outboard ailerons work only when the wing flaps are extended and are automatically locked out when flaps are retracted. This allows good roll response in low speed flight with the flaps extended and prevents excessive roll and wing bending at high speeds when the flaps are retracted.

Spoilers increase drag and reduce lift on the wing. If raised on only one wing, they aid roll control by causing that wing to drop. If the spoilers rise symmetrically in flight, the aircraft can either be slowed in level flight or can descend rapidly without an increase in airspeed. When the spoilers rise on the ground at high speeds, they destroy the wing’s lift which puts more of the aircraft’s weight on its wheels which in turn makes the brakes more effective.

Often aircraft have both flight and ground spoilers. The flight spoilers are available both in flight and on the ground. However, the ground spoilers can only be raised when the weight of the aircraft is on the landing gear. When the spoilers deploy on the ground, they decrease lift and make the brakes more effective. In flight, a ground-sensing switch on the landing gear prevents deployment of the ground spoilers.

Vortex generators are small (an inch or so high) aerodynamic surfaces located in different places on different airplanes. They prevent undesirable airflow separation from the surface by mixing the boundary airflow with the high energy airflow just above the surface. When located on the upper surface of a wing, the vortex generators prevent shock-induced separation from the wing as the aircraft approaches its critical Mach number. This increases aileron effectiveness at high speeds.

As you progress through the ranks of aviation and begin flying larger aircraft you will start noticing some of these secondary flight controls installed on your aircraft. But many of the training aircraft like the Cessna 172’s, Piper Archers, or piston powered aircraft you might be flying today won’t have secondary controls such as spoilers installed. The majority of the time these aircraft are just not large enough, heavy enough or fast enough that spoilers would be an effective or beneficial flight control. It is however beneficial to gain experience in the knowledge of these flight control systems as it will help you later on in training when you merge your private and professional aerodynamics lessons into practice.

For more advanced information on aerodynamics check out our collegiate level textbook, Aerodynamics for Aviators, 2nd Edition.

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Aerodynamics: High Speed Flight

Today’s post comes from the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B).

In subsonic aerodynamics, the theory of lift is based upon the forces generated on a body and a moving gas (air) in which it is immersed. At speeds of approximately 260 knots or less, air can be considered incompressible in that, at a fixed altitude, its density remains nearly constant while its pressure varies. Under this assumption, air acts the same as water and is classified as a fluid. Subsonic aerodynamic theory also assumes the effects of viscosity (the property of a fluid that tends to prevent motion of one part of the fluid with respect to another) are negligible and classifies air as an ideal fluid conforming to the principles of ideal-fluid aerodynamics such as continuity, Bernoulli’s principle, and circulation.

In reality, air is compressible and viscous. While the effects of these properties are negligible at low speeds, compressibility effects in particular become increasingly important as speed increases. Compressibility (and to a lesser extent viscosity) is of paramount importance at speeds approaching the speed of sound. In these speed ranges, compressibility causes a change in the density of the air around an aircraft.

During flight, a wing produces lift by accelerating the airflow over the upper surface. This accelerated air can, and does, reach sonic speeds even though the aircraft itself may be flying subsonic. At some extreme angles of attack (AOA), in some aircraft, the speed of the air over the top surface of the wing may be double the aircraft’s speed. It is therefore entirely possible to have both supersonic and subsonic airflow on an aircraft at the same time. When flow velocities reach sonic speeds at some location on an aircraft (such as the area of maximum camber on the wing), further acceleration results in the onset of compressibility effects, such as shock wave formation, drag increase, buffeting, stability, and control difficulties. Subsonic flow principles are invalid at all speeds above this point.

The speed of sound varies with temperature. Under standard temperature conditions of 15 °C, the speed of sound at sea level is 661 knots. At 40,000 feet, where the temperature is –55 °C, the speed of sound decreases to 574 knots. In high speed flight and/or high-altitude flight, the measurement of speed is expressed in terms of a “Mach number”—the ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions. An aircraft traveling at the speed of sound is traveling at Mach 1.0. Aircraft speed regimes are defined approximately as follows:

  • Subsonic—Mach numbers below 0.75
  • Transonic—Mach numbers from 0.75 to 1.20
  • Supersonic—Mach numbers from 1.20 to 5.00
  • Hypersonic—Mach numbers above 5.00

While flights in the transonic and supersonic ranges are common occurrences for military aircraft, civilian jet aircraft normally operate in a cruise speed range of Mach 0.7 to Mach 0.90.

The speed of an aircraft in which airflow over any part of the aircraft or structure under consideration first reaches (but does not exceed) Mach 1.0 is termed “critical Mach number” or “Mach Crit.” Thus, critical Mach number is the boundary between subsonic and transonic flight and is largely dependent on the wing and airfoil design. Critical Mach number is an important point in transonic flight. When shock waves form on the aircraft, airflow separation followed by buffet and aircraft control difficulties can occur. Shock waves, buffet, and airflow separation take place above critical Mach number. A jet aircraft typically is most efficient when cruising at or near its critical Mach number. At speeds 5–10 percent above the critical Mach number, compressibility effects begin. Drag begins to rise sharply. Associated with the “drag rise” are buffet, trim, and stability changes and a decrease in control surface effectiveness. This is the point of “drag divergence.”


VMO/MMO is defined as the maximum operating limit speed. VMO is expressed in knots calibrated airspeed (KCAS), while MMO is expressed in Mach number. The VMO limit is usually associated with operations at lower altitudes and deals with structural loads and flutter. The MMO limit is associated with operations at higher altitudes and is usually more concerned with compressibility effects and flutter. At lower altitudes, structural loads and flutter are of concern; at higher altitudes, compressibility effects and flutter are of concern.

Adherence to these speeds prevents structural problems due to dynamic pressure or flutter, degradation in aircraft control response due to compressibility effects (e.g., Mach Tuck, aileron reversal, or buzz), and separated airflow due to shock waves resulting in loss of lift or vibration and buffet. Any of these phenomena could prevent the pilot from being able to adequately control the aircraft.

For example, an early civilian jet aircraft had a VMO limit of 306 KCAS up to approximately FL 310 (on a standard day). At this altitude (FL 310), an MMO of 0.82 was approximately equal to 306 KCAS. Above this altitude, an MMO of 0.82 always equaled a KCAS less than 306 KCAS and, thus, became the operating limit as you could not reach the VMO limit without first reaching the MMO limit. For example, at FL 380, an MMO of 0.82 is equal to 261 KCAS.

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CFI Brief: ATC Tower Light Gun Signals

A while back I was on a local area pleasure flight with a couple of friends showing off the sights in the club’s Piper Cherokee. I was so wrapped up in making sure my passengers were having a good time that I failed to immediately notice the illuminated low voltage light. By the time I did notice, my alternator had already completely failed and I was working with about 20 minutes of remaining battery. Lucky for me at the time I was operating on a VFR flight plan in uncontrolled airspace on a beautiful sunny day. The failure in itself did not present any sort of emergency situation but I knew I would soon lose all electrical power, including my radios and would be unable to communicate with air traffic control (ATC).

My home airport was about a 25 minute flight away and located in Class D airspace, meaning in a normal situation I would need to establish two-way radio communication prior to entering into the airspace and further clearance to land from the control tower. However, I knew with every click of the radio I would be draining the battery of precious power and more than likely have no battery left by the time I got near the airport. After running through the checklists and reducing the electrical load by switching off all non-essential equipment, I tuned in the control tower frequency for the class D airport. My goal at this point was to make a quick radio call to the tower advising them of my impending communications failure, intentions, and current position. Unfortunately for me I was in a bit of a mountainous area and still a little too far out that I was not able to hear any response back from the tower, so I was unsure if they had received my transmission or not. This still was not that big of a concern for me since I knew there were communication procedures in place for situations just like this one.

Light Gun

In the event of a radio communications failure, ATC towers have set procedures to communicate with aircraft via light gun signals. Every operating control tower is outfitted with hand held light guns like the one pictured that emit, Red, Green, and White Light.

After my failure to establish radio communication with the tower, I dialed 7600 into the transponder, which is the squawk code for communications failure. Keeping in mind that when my battery finally did die, my transponder would as well, the squawk code would disappear from ATC radar, and I would just appear as a blip on the screen. About 10 miles out from the airport I went to call tower again and sure enough lost battery power mid transmission. I was close enough to the airport now that I figured someone in the tower probably saw my 7600 squawk and knew I had a communications failure, but I still needed to be extremely cautious and aware of other traffic in the airspace and traffic pattern. I bee-lined it directly for the airport at an altitude of 2,500 ft MSL which was about 1,000 ft above traffic pattern altitude (TPA). My goal here was to overfly the airport looking for other aircraft in the traffic pattern so I could safely descend to TPA and enter into the pattern. Upon overflying the airport I noticed a bright green light emitting from the control tower window. Now remember those aforementioned light gun signals a paragraph earlier? The steady green light is visual communication for cleared to land. Tower must have either noticed my squawk code or put two and two together that some random aircraft was in their airspace without prior clearance and is one, either an idiot or two, more than likely has a communication failure. Without further incident I was able to safely land, receiving another steady green light while on final approach. Once taxing clear of the runway I looked back behind me at the tower and received a flashing green light which is the visual light cue for cleared to taxi.

There is a whole set of Airport Traffic Control Tower Light Gun Signals that you should become familiar with and know by memory. You can find all of these signals and procedures outlined in the AIM Section 4-3-13. I have also included a visual image for each of the light gun signals below.

Light Gun Signals

After parking and securing the aircraft I gave tower a quick phone call to make sure everything was good. They gave me the A-OK and said they had received the first transmission I made when still 30 miles out, so they had been expecting my arrival and tracked my 7600 squawk code up until I lost power. All in all everything worked out fine that day, other than our scenic flight being cut a bit short—but oh well, saved me a few bucks on the rental fee.

Light gun signals are something that you should know by memory; radio communication failures are not as rare as you might think they are. I have had two in my 15 years of flying. The second of which was a similar circumstance to the first, however that time ATC tower was not exactly on their A-game. After overflying the airport for about 5 minutes and entering the traffic pattern I never received any sort of visual light signal from the tower. I ended up landing, taxing, and parking without ever getting any clearance. I was starting to think their light gun was broken. After I parked and secured the aircraft, I called tower to see what was up. Turns out it was a slow day at the airport and no one in the tower ever even noticed me in the pattern or landing for that matter! I taxied to parking none-the-wiser to the controller’s. It’s not really advisable to land without clearance but sometimes everything doesn’t work out the way it should and you must adapt to the circumstances you are dealt with.

You have the light gun signals memorized yet? Time to find out!

1. A steady red light from the tower, for an aircraft on the ground indicates
A—Give way to other aircraft and continue circling.
C—Taxi clear of the runway in use.

2. A flashing white light signal from the control tower to a taxiing aircraft is an indication to
A—taxi at a faster speed.
B—taxi only on taxiways and not cross runways.
C—return to the starting point on the airport

3. An alternating red and green light signal directed from the control tower to an aircraft in flight is a signal to
A—hold position.
B—exercise extreme caution.
C—not land; the airport is unsafe.

Answers posted in the comments section.

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