Updated: Dec 8, 2020
High Altitude Operations Lesson by wifiCFI
High Altitude Operations (AC 61-107)
Pilots who are not familiar with operations in the high-altitude and high-speed environment are encouraged to obtain thorough and comprehensive training.
This should include a checkout in complex high-performance aircraft before engaging in extensive high-speed flight, particularly at high altitudes.
The training should enable the pilot to become thoroughly familiar with aircraft performance charts, aircraft systems, and procedures.
The pilot should also review the more critical elements of high-altitude flight planning and operations.
Training Required for a High Altitude Endorsement
No person may act as pilot in command of a pressurized aircraft (an aircraft that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL), unless that person has received and logged ground training from an authorized instructor and obtained an endorsement in the person's logbook or training record from an authorized instructor who certifies the person has satisfactorily accomplished the ground training.
Training Required for a High Altitude Endorsement
The ground training must include at least the following subjects:
High altitude aerodynamics and meteorology
Effects, symptoms, and causes of hypoxia
Durations of consciousness with supplemental oxygen
Effects of prolonged usage of supplemental oxygen
Causes and effects of gas expansion
Preventative measures for high altitude sickness
Physical impacts of decompression
Other physiological aspects of high altitude flight
Regulatory Oxygen Use Requirements
>12,500ft MSL = 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.
>14,000ft MSL = the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes
>15,000ft MSL = each occupant of the aircraft is provided with supplemental oxygen.
Pressurized Aircraft Requirements
>Flight Level 250 must have a 10 minute supply of oxygen available for each occupant
>Flight Level 350 one pilot at the controls of the airplane is wearing and using an oxygen mask that is secured and sealed and that either supplies oxygen at all times or automatically supplies oxygen whenever the cabin pressure altitude of the airplane exceeds 14,000 feet (MSL), except that the one pilot need not wear and use an oxygen mask while at or below flight level 410 if there are two pilots at the controls and each pilot has a quick-donning type of oxygen mask that can be placed on the face with one hand from the ready position within 5 seconds, supplying oxygen and properly secured and sealed.
Physiological Hazards of High Altitude Flight
To ensure safe flights at high altitudes, pilots of high-altitude aircraft must understand the physiological effects of high-altitude flight and the effect of hypoxia on an individual’s ability to perform complex tasks in a changing environment.
Hypoxia (PHAK C17)
Hypoxia means “reduced oxygen” or “not enough oxygen.”
Although any tissue will die if deprived of oxygen long enough, the greatest concern regarding hypoxia during flight is lack of oxygen to the brain, since it is particularly vulnerable to oxygen deprivation.
Any reduction in mental function while flying can result in life-threatening errors.
Hypoxia can be caused by several factors, including an insufficient supply of oxygen, inadequate transportation of oxygen, or the inability of the body tissues to use oxygen.
Hypoxic Hypoxia (PHAK C17)
Hypoxic hypoxia is a result of insufficient oxygen available to the body as a whole.
A blocked airway and drowning are obvious examples of how the lungs can be deprived of oxygen, but the reduction in partial pressure of oxygen at high altitude is an appropriate example for pilots.
Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases.
As an aircraft ascends during flight, the percentage of each gas in the atmosphere remains the same, but there are fewer molecules available at the pressure required for them to pass between the membranes in the respiratory system.
This decrease in number of oxygen molecules at sufficient pressure can lead to hypoxic hypoxia.
Not enough oxygen in the environment around you.
Hypemic Hypoxia (PHAK C17)
Hypemic hypoxia occurs when the blood is not able to take up and transport a sufficient amount of oxygen to the cells in the body. Hypemic means “not enough blood.”
This type of hypoxia is a result of oxygen deficiency in the blood, rather than a lack of inhaled oxygen, and can be caused by a variety of factors.
It may be due to reduced blood volume (from severe bleeding), or it may result from certain blood diseases, such as anemia.
More often, hypemic hypoxia occurs because hemoglobin, the actual blood molecule that transports oxygen, is chemically unable to bind oxygen molecules.
The most common form of hypemic hypoxia is CO poisoning.
CO Blocks O2 From Attaching to Blood Cells.
Stagnant Hypoxia (PHAK C17)
Stagnant means “not flowing,” and stagnant hypoxia or ischemia results when the oxygen-rich blood in the lungs is not moving, for one reason or another, to the tissues.
An arm or leg “going to sleep” because the blood flow has accidentally been shut off is one form of stagnant hypoxia.
This kind of hypoxia can also result from shock, the heart failing to pump blood effectively, or a constricted artery.
During flight, stagnant hypoxia can occur with excessive acceleration of gravity (Gs).
Blood not flowing.
Histotoxic Hypoxia (PHAK C17)
The inability of the cells to effectively use oxygen is defined as histotoxic hypoxia. “Histo” refers to tissues or cells, and “toxic” means poisonous.
In this case, enough oxygen is being transported to the cells that need it, but they are unable to make use of it.
This impairment of cellular respiration can be caused by alcohol and other drugs, such as narcotics and poisons.
Brain Tissue Rejecting O2.
Symptoms of Hypoxia
Cyanosis (blue fingernails and lips)
Decreased response times
Tingling in fingers and toes
Hypoxia Corrective Actions
Descend to a lower altitude
Stop pulling G’s
Put on an O2 mask
DCS and Scuba Diving (PHAK C17)
Decompression sickness (DCS) describes a condition characterized by a variety of symptoms resulting from exposure to low barometric pressures that cause inert gases (mainly nitrogen), normally dissolved in body fluids and tissues, to come out of physical solution and form bubbles.
Scuba diving subjects the body to increased pressure, which allows more nitrogen to dissolve in body tissues and fluids.
Scuba Diving Wait Times (AIM 8-1-4)
Wait 24 hours after Scuba Diving prior to flying to be safe.
Pressurization Characteristics (AC 61-107)
Several systems and equipment are unique to aircraft that fly at high altitudes, and pilots should be familiar with their operation before using them.
Before any flight, a pilot should be familiar with all the systems on the aircraft to be flown.
Most light piston engine airplanes that fly above 25,000 ft MSL are turbocharged.
Turbochargers compress air in the carburetor or cylinder intake by using exhaust gases from an engine-driven turbine wheel.
The increased air density provides greater power and improved performance.
Turbocharged engines are particularly temperature-sensitive.
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.
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.
Pressure Control and Outflow Valves:
Outflow valves regulate the flow of compressed air out of the cabin, which keeps the pressure constant by releasing excess pressure into the atmosphere.
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.
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.
Some aircraft have a negative pressure relief valve to equalize pressure in the event of a sudden decompression.
Supplemental Oxygen Systems
Continuous Flow System
The continuous flow system supplies oxygen at a rate that may be controlled automatically or by the user.
Diluter and Pressure Demand Systems
Diluter demand and pressure demand systems supply oxygen only when the user inhales through the mask.
An automix lever allows the regulators to automatically mix cabin air and oxygen or supply 100 percent oxygen,
The demand mask provides a tight seal over the face to prevent dilution with outside air and can be used safely up to 40,000 ft MSL.
Pressure demand regulators also create airtight and oxygen-tight seals, but they also provide a positive pressure application of oxygen to the mask facepiece, which allows the user’s lungs to be pressurized with oxygen.
This feature makes pressure demand regulators safe at altitudes above 40,000 ft MSL.
Quick Donning Masks
These masks must demonstrate the ability to be donned with one hand in 5 seconds or less, as seen in the video below.
Aviator’s Breathing Oxygen
Aviator’s breathing oxygen is 100% pure oxygen.
It is different from medical oxygen because it does not contain water.
Water cannot be present in aviator’s breathing oxygen so it doesn’t freeze in the lines.
Care and Storage of Oxygen Bottles
Inspect oxygen storage containers.
Make sure that they are securely fastened in the aircraft, as turbulence or abrupt changes in attitude can cause them to come loose.
Proper inspections are important, so your oxygen equipment should be inspected regularly at an authorized Federal Aviation Administration inspection station.
Oxygen is highly flammable.
Do not allow anyone to smoke around oxygen equipment that is being used. Likewise, no one should smoke around oxygen equipment that is being recharged.
Ensure that the aircraft is properly grounded before loading oxygen.
A change in cabin pressure where the lungs can decompress faster than the cabin.
The risk of lung damage is significantly lower in this decompression compared to an explosive decompression.
A change in cabin pressure faster than the lungs can decompress.
Most authorities consider any decompression that occurs in less than 0.5 seconds as explosive and potentially dangerous.
This type of decompression is more likely to occur in small volume pressurized aircraft than in large pressurized aircraft and often results in lung damage.
Times of Useful Consciousness
This 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.
FAA Sources Used for this Lesson
FAA Advisory Circular 61-107
Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 17
14 CFR Part 61
14 CFR Part 91