High Altitude Operations
This page covers Task N. High Altitude Operations - Supplemental Oxygen and Task O. High Altitude Operations - Pressurization from the FAA-S-ACS-25 Flight Instructor for Airplane Category Airman Certification Standards.
Regulatory Requirements
- 14 CFR §61.31(g) says that to 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) the pilot needs an endorsement.
- Unpressurized planes that fly at high altitudes don't need endorsement
- Pressurized plane with a max operating altitude or service ceiling 25k MSL or below don't need endorsement
- 14 CFR §91.121(a)(2) says when operating at or above 18,000 feet MSL the altimeter must be set to 29.92" Hg.
Oxygen Requirements
- 14 CFR §91.211 - Supplemental oxygen
- Oxygen requirements at various cabin pressure altitudes
- 12,500 - 14,000 ft MSL: flight crew uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration
- Above 14,000 ft MSL: flight crew must use oxygen
- Above 15,000 ft MSL: each occupant is provided oxygen
- Pressurized cabin requirements at various flight altitudes
- Above FL250 need at least 10 minutes oxygen supply for each occupant
- Above FL350 one pilot needs to wear oxygen mask that provides oxygen continuously or automatically whenever the cabin pressure altitude of the airplane exceeds 14,000 feet (MSL)
- Exception: below FL410 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 Factors
Impairment
- A primary concern when operating at high altitudes is impairment associated with hypoxia.
Hypoxia
Hypoxia
Not enough oxygen.
- The forms of hypoxia are based on their causes:
- Hypoxic hypoxia
- Insufficient oxygen available to the body as a whole
- E.g.: blocked airway, drowning
- The "normal" hypoxia we talk about in aviation
- Usually caused by the decreased pressure of oxygen at altitude
- Anemic or 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
- May be due to low blood supply, low hemoglobin, or CO poisoning
- Same symptoms as hypoxic hypoxia
- Stagnant hypoxia
- Blood not flowing to tissues that need it
- Also known as ischemia
- Can occur with excessive acceleration of gravity (Gs).
- Also from cold temperatures reducing circulation
- Histotoxic hypoxia
- The inability of the cells to effectively use oxygen
- This impairment of cellular respiration can be caused by alcohol and drugs
- Hypoxic hypoxia
- Symptoms of hypoxia
- Belligerence
- Euphoria
- Headache
- Decreased response to stimuli and increased reaction time
- Impaired judgment
- Visual impairment
- Drowsiness
- Lightheaded or dizzy sensation
- Tingling in fingers and toes
- Numbness
- False sense of security
- Blue colored lips and fingernails
- Tunnel vision
- Symptoms can take effect at 5,000 ft. at night
- Immediately reduce altitude, use oxygen, avoid alcohol
Time of Useful Consciousness
Effects of Rapid Decompression
- Decompression is the inability of the pressurization system to maintain its designated differential pressure.
- May be caused by a malfunction in the pressurization system or structural damage to the plane.
- Decompression types
- Explosive decompression
- Change in cabin pressure faster than the lungs can decompress
- Less than 0.5 seconds
- Rapid decompression
- Change in cabin pressure where lungs can decompress faster than the cabin
- No likelihood of lung damage
- Explosive decompression
- During explosive decompression, there may be noise, and one may feel dazed for a second
- During most decompressions, the cabin will fill with
- Fog (the result of the rapid change in temperature and change of relative humidity)
- Dust
- Flying debris
- Air will rush from the mouth and nose due to the escape from the lungs
- Differential air pressure on either side of the eardrum should clear automatically
- Exposure to wind blast and extremely cold temperatures may occur
- Primary danger-hypoxia.
- If proper use of oxygen equipment is not accomplished quickly, could quickly result in unconsciousness.
- Effective performance time-reduced to one third or one fourth of its normal time.
- Recovery
- Don oxygen masks
- Emergency descent
- Top priority: reaching safe altitude. Be aware that rapid descent from high altitude could result in cold shock in piston engines, and cylinder cracking.
- For explosive decompression, the time to make a recovery before loss of useful consciousness is even less.
Operational factors
Oxygen System
- In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature.
- Components
- Mask/cannula
- Supply (bottle/air bleed)
- Regulator
- For optimum protection, pilots are encouraged to use supplemental oxygen above 10,000 feet cabin altitude during the day and above 5,000 feet at night.
- Most regulators provide 100% cabin air at around 8,000 ft, and 100% oxygen at 34,000 ft, with the ratio changing in between.
- Be aware of the danger of fire when using oxygen
- Masks
- Face worn mask or cannula
- Most masks are the oronasal type that covers only the mouth and nose
- Cannula only goes in nose. More comfortable, but is not as reliable at providing adequate oxygen.
- Current regulations require aircraft with oxygen systems installed and certified for operations above 18,000 feet to be equipped with oxygen masks instead of cannulas.
- Oxygen delivery systems
- Diluter-Demand
- Supply oxygen only when the user inhales through the mask
- Used up to 40,000 ft
- Pressure-Demand
- Supply oxygen to mask at positive pressure above 34,000 ft
- Used above 40,000 ft
- Continuous-Flow
- Most common kind in general aviation aircraft
- Usually provided for passengers
- Electrical Pulse-Demand
- provide oxygen flow during the initial portion of inhalation
- do not waste oxygen during the breathing cycle
- reduce oxygen 50-85 percent compared to continuous-flow
- Diluter-Demand
Importance of Aviator's Breathing Oxygen
- Specified at 99.5% pure oxygen.
- Not more than 0.005 mg of water per liter.
- Medical oxygen-has too much water, which can collect in various parts of the system and freeze, reducing or stopping the flow of oxygen.
- Industrial oxygen-not intended for breathing, may have impurities in it.
Precautions
- TBD
Using Oxygen
- Take oxygen gradually to build up in small doses - the sudden supply of pure oxygen following decompression can aggravate hypoxia.
- Prolonged oxygen use can be harmful to health.
- 100% aviation oxygen can create toxic symptoms if used for too long.
- Symptoms: bronchial cough, fever, vomiting, nervousness, irregular heartbeat, lowered energy.
Nitrogen
- Decompression sickness-evolving and expanding gases in the body.
- Trapped gas-expanding/contracting gas in cavities during altitude changes can result in abdominal pain, toothache, or pain in ears and sinuses if the pressure change isn't equalized.
- Evolved gas-with a sufficient pressure drop, nitrogen forms bubbles which can have adverse effects on some body issues.
- Scuba diving compounds this problem.
- See more on scuba in aeromedical factors section
Vision
- Vision becomes impaired with lack of oxygen, especially at night
Care and Storage of Oxygen Bottles
Portable oxygen equipment must be accessible in flight if the airplane does not have a fixed installation. Oxygen usually stored at 1,800-2,200 psi. When the ambient temperature surrounding the cylinder decreases, the pressure within the cylinder will decrease-no reason to suspect supply depletion if you notice a drop in indicated pressure.
Fire danger-materials that are nearly fire proof in ordinary air may be susceptible to burning in pure oxygen. Oils and greases may catch fire if exposed to pure oxygen and cannot be in oxygen systems. Smoking is prohibited during any kind of oxygen equipment use.
Thoroughly inspect and test all oxygen equipment before each flight. Available supply, operational check, assure it is readily available. Do periodic inspections and servicing.
PRICE checklist to inspect oxygen equipment
- P - Pressure - is there enough oxygen pressure and quantity to complete the flight
- R - Regulator
- I - Indicator - check to assure steady flow of oxygen
- C - Connections - all secured
- E - Emergency - have equipment readily available, brief passengers
Pressurization
- Aircraft more efficient at altitude
- Can help avoid bad weather
- Cabin pressure typically maintained at 8,000 feet
- Pressurized air from turbine compressor or turbocharger used to pressurize aircraft
- Cabin and baggage compartments pressurized
- Differential pressure puts stress on airframe
- Decompression
- Rapid / explosive
- Dangers
- Hypoxia
- Gas decompression sickness (nitrogen bubbles out of blood)
- Relief valve
- Gauges to monitor pressure
- Cabin, flight, and baggage compartments are incorporated into a sealed unit capable of containing air under a differential pressure.
- Maximum differential pressure varies by airplane - be familiar with limitations
- Turbine-powered aircraft-bleed air from engine compressor section used to pressurized
- Light aircraft-turbocharger's compressor/engine-driven pneumatic pump used to pressurize. Compression heats the air, so it's routed through a heat exchange unit before entering the cabin.
- Provides pressure regulation, pressure relief, and vacuum relief, as well as the means for selecting the desired cabin altitude.
- Uses a cabin pressure regulator, an outflow valve, and a safety valve.
- Cabin pressure regulator (CPR)-controls cabin pressure.
- If we reach the maximum difference, an increase in outside altitude will result in an increase inside.
- Outflow valve-keeps pressure constant by regulating flow of compressed air.
- Safety valve-combination of a pressure relief, vacuum relief, and a dump valve.
- Pressure relief-prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure. Vacuum relief-prevents ambient pressure from exceeding cabin pressure by allowing external air to enter when ambient pressure exceeds cabin pressure.
- Dump valve-dumps cabin air to atmosphere.
- Cockpit switch.
- Cabin differential pressure gauge-indicates the difference between inside and outside pressure.
- Cabin altimeter-shows altitude inside the airplane. Differential pressure gauge and cabin altimeter could be combined into one instrument.
- Cabin rate of climb/descent.
