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CFI Practical Test Standards: Technical Subject Areas


Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater Than .75

This advisory circular (AC) alerts pilots transitioning from aircraft with less performance capability to complex, high-performance aircraft that are capable of operating at high altitudes and high airspeeds. In particular, this AC stresses special physiological, equipment, and aerodynamic considerations involved in these kinds of operations. It also provides information to aid pilots in becoming familiar with the basic phenomena associated with high-altitude and high-speed flight.

Title 14 of the Code of Federal Regulations (14 CFR) part 61 prescribes the knowledge and skill requirements for the various airman certificates and ratings, including category, class, and type ratings authorized to be placed thereon. The civil aircraft fleet consists of numerous aircraft capable of high-altitude flight. Certain knowledge elements of high-altitude flight are essential for the pilots of these aircraft.

As required by 14 CFR 61.31, pilots who fly at altitudes at or above FL250 in a pressurized aircraft must receive training in the critical factors relating to safe flight operations under those circumstances. These critical elements include knowledge of the special physiological and/or aerodynamic considerations that should be given to high-performance aircraft operating in the high-altitude environment.

High-altitude flight has different effects on the human body than those experienced during lower altitude flight. An aircraft's aerodynamic characteristics displayed in high altitude flight may differ significantly from those experienced when penetrating at a lower altitude. Knowledge of and skill in operating high-performance aircraft will enhance the pilot's ability to easily transition into aircraft capable of high speed, high altitude flight.

Chapter 1: General Information

Chapter 2: Recommendations for High-Altitude Training

Chapter 3: Mach Flight at High Altitudes

List of Tables

List of Figures

Get the most recent version from FAA.gov

Advisory Circular 61-107: Operations at Altitudes Above 25,000

Advisory Circular 61-107, Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater Than .75, opens with a glossary of terms that are specific to operating at high altitudes/and or high speeds. Many of these terms will be new, even to commercial pilots with no high-altitude experience, such as "aileron buzz," "drag divergence," "mach tuck," and "Q-Corner or Coffin Corner." It's fun reading, so check it out.

Part 61 includes requirements for high-altitude training of pilots who transition to any pressurized airplane that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 ft MSL. This was a result of NTSB recommendations in 1982, after a review of fatal accidents involving flight crews operating at high altitudes with a lack of knowledge and proficiency.

The high-altitude training (defined in § 61.31(g)) includes knowledge of the special physiological and/or aerodynamic considerations, which should be given to high-performance aircraft operating in the high-altitude environment. Upon completion of the ground and flight training, the flight instructor who conducted the training should certify that he or she gave training in high-altitude operations by providing an endorsement in the pilot's logbook or training record.

§ 61.31(g) applies only to pilots who fly pressurized airplanes with a service ceiling or maximum operating altitude above 25,000 ft mean sea level (MSL), whichever is lower. It is highly recommend for all pilots who fly at altitudes above 10,000 ft MSL.

All pressurized aircraft have a specified maximum operating altitude above which they must not operate. An airplane's maximum operating altitude is limited to 25,000 ft MSL or lower, unless certain airworthiness standards are met.

Subjects covered in § 61.31(g) ground training include:

  • The High-Altitude Flight Environment
  • Weather
  • Flight Planning and Navigation
  • Physiological Training
  • High-Altitude Systems and Components
  • Aerodynamics and Performance Factors
  • Emergencies

Subjects covered in § 61.31(g) flight training include:

  • Preflight Briefing
  • Preflight Planning
  • Preflight Inspection
  • Run-Up, Takeoff, and Initial Climb
  • Climb to High Altitude and Normal Cruise Operations While Operating Above 25,000 ft MSL
  • Simulated Emergencies
  • Planned Descents
  • Shutdown Procedures
  • Postflight Discussion


Regulatory requirements for use of oxygen

This is private-pilot material, easily referenced from 104 CFR § 91.211. However, CFI candidates should memorize 12,500, 14,000, and 15,000 as top-of-mind values for this topic.

No person may operate a civil aircraft of U.S. registry—

  • At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;
  • At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and
  • At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.

Note that there's a trick-question in here that shows up on tests: Can a minimum flight crew fly at 12,500 feet for more than 30 minutes without oxygen? The answer is yes. Read 91.211 closely. (Oxygen requirements begin at 12,501, 14,001, and 15,001 feet.)


Physiological hazards associated with high altitude operations

When we are exposed to a high-altitude environment, our ability to obtain and utilize oxygen is dramatically affected.

The human body functions normally in the atmospheric area extending from sea level to 12,000 ft MSL. AC 61-107 recommends oxygen use when flying unpressurized above 10,000 ft MSL during the day and above 5,000 ft MSL at night, when the eyes become more sensitive to oxygen deprivation.

Body areas that may be affected by trapped gas during high-altitude flight. (AC 61.31 includes two reference tables, "Issues" and "In-Flight Treatment.")

The signs and symptoms of hyperventilation are easily confused with those of hypoxic hypoxia. Hyperventilation occurs as an early adaptive mechanism to hypoxia at altitude. The treatment of hyperventilation requires a voluntary reduction in the rate and depth of ventilation. Pilots also should administer 100 percent oxygen under pressure and descend to a lower altitude where hypoxia is unlikely to occur.

Hypoxia is a state of oxygen deficiency sufficient to cause an impairment of body functions. If the oxygen supply to the body is reduced, the brain will be one of the first organs to be affected, with the higher reasoning portions of the brain showing degraded function first.

The four types of hypoxia — hypoxic (altitude), hypemic, stagnant, and histoxic — are set out in a table in AC 61-107. This is private pilot stuff, but CFIs should be conversant in all four types.

  • The redundant-sounding hypoxic hypoxia is the one that aviators are most concerned with, since it's caused by low air pressure at higher altitudes.
  • Hypemic hypoxia is medical in nature, "bad blood," (think anemia), but it can be caused by C02 poisoning and smoking.
  • Stagnant hypoxia is when your hand or foot falls asleep, but it can be caused by excessive G-loading.
  • Histoxic hypoxia is when you've had one too many. And pilots aren't allow to have any eight hours before a flight.

A common technique for aviators to monitor themselves for hypoxia is the use of finger pulse oximeters that report the degree of hemoglobin oxygen saturation as a percent. However, the FAA cautions against relying on pulse oximeters as the sole indicator of hypoxia the results can be both delayed and inacurrate.

The FAA recommends that pilots fly at an altitude where oxygen is not required, fly in a pressurized cabin, or fly in accordance with current regulations in reference to the use of supplemental oxygen.

If hypoxia is suspected, immediately don an oxygen mask and breathe 100 percent oxygen slowly. Descend to a safe altitude. Once hypoxia is detected and 100 percent oxygen is administered, recovery usually occurs in a matter of seconds.

If supplemental oxygen is not available, initiate an emergency descent to an altitude below 10,000 ft MSL.


Characteristics of a pressurized airplane and various types of supplemental oxygen systems

Cabin pressurization is the compression of air in the aircraft cabin in order to maintain a cabin altitude lower than the actual flight altitude. Because of the ever-present possibility of decompression, the aircraft still requires supplemental oxygen.

Most high-altitude airplanes come equipped with some type of fixed oxygen installation. If the airplane does not have a fixed installation, portable oxygen equipment must be readily accessible during flight.

Turbine aircraft use a steady supply of engine bleed air for cabin pressurization. In the case of most pressurized light aircraft, the air supply is sent to the cabin from the turbocharger's compressor or from an engine-driven pneumatic pump.

The cabin altitude selection can be manual, or if available, electronic. A gauge that indicates the pressure difference between the cabin and ambient altitudes helps the pilot to monitor the cabin altitude.

Aircraft oxygen is usually stored in high-pressure system containers.

Regulators and masks work on continuous flow, diluter demand, or pressure demand systems.

  • Continuous flow: The continuous flow system supplies oxygen at a rate that may be controlled automatically or by the user.
  • Diluter demand: Diluter demand and pressure demand systems supply oxygen only when the user inhales through the mask.
  • Pressure demand: This system provides a positive pressure application of oxygen to the mask facepiece, which allows the user's lungs to be pressurized with oxygen


Importance of aviator's breathing oxygen

Only oxygen that meets or exceeds the Society of Automotive Engineers (SAE) International Aerospace Standard AS8010 (as revised), Aviator's Breathing Oxygen Purity Standard, should be used.

To assure safety, periodic inspection and servicing should take place by a certified maintenance provide found at some fixed base operations and terminal complexes.

Note that there are three types of oxygen: Aviator's, medical, and industrial. Aviator's oxygen has very little water content in order to prevent freezing. Medical oxygen has more water content and is not to be used on aircraft. Industrial oxygen is not meant for human consumption.


Care and storage of high-pressure oxygen bottles

When the ambient temperature surrounding an oxygen cylinder decreases, pressure within that cylinder will decrease because pressure varies directly with temperature if the volume of a gas remains constant. A drop in indicated pressure on a supplemental oxygen cylinder can be an indication that the oxygen supply has merely compacted (not necessarily a leak).

High-pressure oxygen containers should be marked with the psi tolerance (i.e., 1,800 psi) before filling the container to that pressure.

Only oxygen that meets or exceeds the Society of Automotive Engineers (SAE) International Aerospace Standard AS8010 (as revised), Aviator's Breathing Oxygen Purity Standard, should be used.

To assure safety, periodic inspection and servicing should take place by a certified maintenance provide found at some fixed base operations and terminal complexes.


Problems associated with rapid decompression and corresponding solutions

Rapid loss of aircraft pressurization dramatically reduces Time of Useful Consciousness (TUC). Above 43,000 ft, the TUC is reduced to the time it takes for the blood to circulate from the lung to the brain, plus any reserve oxygen stored in the brain. This is approximately 9–12 seconds from the start of a rapid decompression to the loss of functional capability. TUC following decompression to altitudes between 25,000 ft and 43,000 ft will be reduced by 50 percent.

The potential for impairment begins almost immediately. For this reason, the donning of the oxygen mask should be practiced from time to time.

Corresponding Solutions include:

  • Don mask: In five seconds or less. Check for flow. Breathe 100 percent oxygen.
  • Descend: Preferably below 10,000 ft.
  • Land ASAP: At nearest suitable installation where appropriate medical help can be found.

During a decompression event, decompression phenomena are evident during rapid and explosive decompressions. However, the phenomena below may not be evident during a slow decompression. Slow decompressions are the most dangerous, and the aviator must always be on guard against this insidious threat.

Signs of decompression include:

  • Noise
  • Fog
  • Flying Debris, Dust, and Dirt
  • Wind Blast
  • Cooler Temperatures
  • Gas Expansion

Time of Useful Consciousness (TUC) or Effective Performance Time (EPT) is the period of time from interruption of the oxygen supply, or exposure to an oxygen-poor environment, to the time when an individual is no longer capable of taking proper corrective and protective action.

Slow decompression is as dangerous as or more dangerous than a rapid decompression. Rapid decompression commands attention. A slow decompression may go unnoticed and the resultant hypoxia may be unrecognized by the pilot.

Warning: The TUC does not mean the onset of unconsciousness. Impaired performance may be immediate. Prompt use of 100 percent oxygen is critical.

AC 61-107 includes a TUC table. TUCs are based on data that represent average values and reflect wide variation among individuals in time to incapacitation.

Altitude-Induced Decompression Sickness (DCS): Explained in AC 61-107 per Henry's Law (carbonated soda).

Nitrogen Absorption: At sea level, the nitrogen pressure inside the body and outside of the body is in equilibrium. When atmospheric. pressure is reduced as a result of ascent, the equilibrium is upset. This can result in trapped gas and evolved gas, both of which have adverse effects. AC 61-107 includes a Decompression Sickness table.


Fundamental concept of cabin pressurization

A lot of people probably think that airplanes are pumped full of oxygen when at high altitudes, but this is not the case. There's oxygen at higher altitudes, but the air isn't very dense, so it's of little use to humans. The pressurization system establishes habitable air pressure in the cabin. On an airliner flying at 35,000 feet, the cabin pressure is approximately 6,000 feet.

Turbine aircraft use a steady supply of engine bleed air for cabin pressurization. In the case of most pressurized light aircraft, the air supply is sent to the cabin from the turbocharger's compressor or from an engine-driven pneumatic pump.

An outflow valve regulates the flow of compressed air out of the cabin, which keeps the pressure constant by releasing excess pressure into the atmosphere.

The oxygen on board is there in case the pressurization fails. If pressurization is functioning, the oxygen is a standby emergency system.


Operation of a cabin pressurization system

The cabin altitude selection can be manual, or if available, electronic. A gauge that indicates the pressure difference between the cabin and ambient altitudes helps the pilot to monitor the cabin altitude. A manually set backup control automatically controls the rate of change between these two pressures.

Each pressurized aircraft has a determined maximum pressure differential, which is the maximum differential between cabin and ambient altitudes that the pressurized section of the aircraft can support. The pilot must be familiar with these limitations, as well as the manifold pressure settings recommended for various pressure differentials.

Some aircraft have a negative pressure relief valve to equalize pressure in the event of a sudden decompression or rapid descent to prevent the cabin pressure from becoming lower than the ambient pressure.

Many airplanes are equipped with automatic visual and aural warning systems that indicate an unintentional loss of pressure.

Pressurized aircraft meeting specific requirements of 14 CFR part 23 or 25 have cabin altitude warning systems, which activate at a cabin altitude of 10,000 ft.

Pressurized aircraft meeting the more stringent requirements of part 25 have automatic passenger oxygen mask-dispensing devices that activate before exceeding 15,000 ft cabin altitude. It should be noted that some aircraft require that the flightcrew disable the automatic passenger oxygen mask-dispensing devices prior to landing at airports over 10,000 ft MSL to prevent inadvertent deployment when the cabin depressurizes at landing. The system is then re-armed after departure.


Practical Test Standards: Flight Instructor

II. Technical Subject Areas
Task I: High Altitude Operations

References

  1. 14 CFR part 91
  2. Aircraft Operations at Altitudes Above 25,000 Feet (AC 61-107)
  3. Aeronautical Information Manual (AIM)
  4. POH/AFM
  5. Airplane Flying Handbook (FAA-H-8083-3)
  6. Commercial Pilot: Airplane Practical Test Standards (FAA-S-8081-12) (cancelled)
  7. Commercial Pilot: Airplane Airman Certification Standards (FAA-S-ACS-7A)

Objective: To determine that the applicant exhibits instructional knowledge of the elements of high altitude operations by describing:

  1. Regulatory requirements for use of oxygen.
  2. Physiological hazards associated with high altitude operations.
  3. Characteristics of a pressurized airplane and various types of supplemental oxygen systems.
  4. Importance of "aviator's breathing oxygen."
  5. Care and storage of high-pressure oxygen bottles.
  6. Problems associated with rapid decompression and corresponding solutions.
  7. Fundamental concept of cabin pressurization.
  8. Operation of a cabin pressurization system.

Flight Instructor Test Questions

Hypoxia is the result of decreasing amounts of oxygen as your altitude increases.

During a climb to 18,000 feet, the percentage of oxygen in the atmosphere remains the same.

Which statement regarding hypoxia is true? Belligerence or a false sense of security may be symptoms of hypoxia.

Which statement is true regarding alcohol in the human system? Alcohol renders a pilot more susceptible to hypoxia.

What kind of oxygen is most commonly found in general aviation aircraft? Continuous flow.
— Demand and pressure-demand systems are used at altitudes up to 40,000 feet and beyond 40,000 feet.

What precautions should be taken with respect to aircraft oxygen systems? Ensure that industrial oxygen has not been used to replenish the system.
— "Aviators Breathing Oxygen" is the correct oxygen type. Smoking is prohibited when using oxygen, but not necessarily on the aircraft.

What type of oxygen should be used to replenish an aircraft oxygen system? Aviation.

What is the purpose of the rebreather bag on an oxygen mask in a continuous-flow system? Helps to conserve oxygen.

Which cabin pressure altitude allows a pilot to operate an aircraft up to 30 minutes without supplemental oxygen? 12,600 feet MSL. (91.211)
— A trick question, since 12,500 and 14,100 are offered as distractors. A pilot is permitted to operate up to 30 minutes (and more) at 12,500, so technically this answer appears to be correct. However, the 30-minute rule kicks in at 12,501 feet.

What physical change would most likely occur to occupants of an unpressurized aircraft flying above 15,000 feet without supplemental oxygen? A blue coloration of the lips and fingernails develop along with tunnel vision.
— Blue discoloration of the lips and fingernails is cynanosis.

Anemic hypoxia has the same result as hypoxic hypoxia, but it is most of the result of a leaking exhaust manifold.
— Anemic hypoxia is also known as hypemic hypoxia, and is caused by contaminated blood. CO poisoning would cause anemic/hypemic hypoxia.

Rapid or deep breathing while using oxygen can cause hyperventilation
— Hyperventilation is caused by a deficiency of carbon dioxide, which can be brought about by an excess of oxygen.

The advantage of experiencing hypoxia in an altitude chamber is it helps pilots learn to recognize their own symptoms in a controlled environment.

What physical change would most likely occur to occupants of an unpressurized aircraft flying above 15,000 feet without supplemental oxygen? A blue coloration of the lips and fingernails develop along with tunnel vision.

Although not required, supplemental oxygen is recommended for use when flying at night above 5,000 feet.

Oral Exam Questions

  1. What are the legal requirements for pilots operating an aircraft above FL 250 (or a cabin pressure above FL 250)?
  2. When are pilots required to use oxygen?
  3. Are passengers ever required to use oxygen? What are the flight-crew's responsibilities for passengers?
  4. When does the FAA recommend pilots use oxygen? Is there a different recommendation for night operations? Why?
  5. What type of hypoxia is of greatest risk for pilots?
  6. What type of hypoxia would a C02 leak cause?
  7. What type of hypoxia does a chain-smoking pilot risk experiencing?
  8. What type of hypoxia does an aerobatic pilot risk experiencing?
  9. What type of hypoxia does an intoxicated pilot risk experiencing?
  10. If hypoxia is suspected, what steps should a pilot or flight-crew take?
  11. Are pressurized airplanes pumped full of oxygen? Why is oxygen on board a pressurized airplane?
  12. What is aviator's breathing oxygen? Why is it important?
  13. What is more dangerous: rapid or slow decompression?
  14. What are some signs of a decompression event?
  15. What is "Time of Useful Consciousness?

Robert Wederquist   CP-ASEL - AGI - IGI
Commercial Pilot • Instrument Pilot
Advanced Ground Instructor • Instrument Ground Instructor


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