Acknowledgements:   European Union Aviation Safety Agency  

 (Ed.Note: Thanks to EASA for an informative explanation of the perils associated with oxygen deficiency, as demonstrated in the loss of a Helios Airlines flight over Greece a few years ago. If it can happen to a commercial aircraft fitted with all the equipment necessary to survive such a problem, it is certainly not an impossibility in General Aviation ...)  

 General Information 

Pilots may encounter hypoxia conditions in various situations: crossing high terrain, sight‐seeing tour in mountains, flying IFR at higher flight levels than expected due to degrading weather conditions, icing, turbulence, high performance gliding and ballooning, etc. 

 The purpose of this leaflet is to help the pilot assess the need to carry supplemental oxygen before the fight and to take the appropriate precautions in case oxygen equipment is not installed or portable bottles are not carried. This is particularly relevant for private pilots of light aircraft

 Human beings need oxygen to live. The higher you fly, the lower is the quantity of available oxygen, and lack of oxygen impairs a pilot’s awareness and a passenger’s comfort and health. As flight altitude increases, without oxygen supply dimness of vision, sleepiness and poor judgement are discreetly increasing threats, until the last fatal stage, fainting. Hypoxia also impairs night vision because the rod cells in the eye, which give us night vision, require a lot of oxygen. Unfortunately, our body doesn’t give us reliable signals at the onset of hypoxia. The brain is the first part of the body to reflect a diminished oxygen supply, and the evidence of that is usually a loss of judgement. 

 Early symptomsof hypoxia are subtle; they may include rapid breathing, headache, drowsiness, nausea, behavioural changes (e.g. euphoria, irritability), slurred speech, and diminished thinking capacity. A pilot experiencing hypoxia has a limited amount of time to recognise signs and symptoms. The time of useful consciousness ranges from a few minutes at lower altitudes to seconds at higher altitudes and it is within this time frame that a pilot must take the correct decisions and actions. 

  •  Assessment of individual conditions before the flight: Individuals react differently to lack of oxygen; some people are sensitive to hypoxia as low as 7000 feet, while others with outstanding aptitudes and training achieve ascent up to the Everest summit without need for supplemental oxygen. Personal factors are numerous: Being accustomed to high altitude conditions due to location of residence or work, smoking habits, stress, illness, medication, etc. The effects of hypoxia can be safely experienced under professional supervision at Aeromedical centres in altitude chambers. Pilots may consider undergoing such specific training to increase their ability to recognise hypoxia·      

  • Briefing passengers: In order to assess the individual situation of all persons on board before the flight, it is essential to have an open and trustful communication with your passengers. In order to generate mutual trust, use of this leaflet during briefing could encourage them to advise you about any relevant health conditions. So, they shouldn’t be shocked should you question them on personal matters, as this is purely aiming to prevent unsafe conditions in flight. Before the flight always ask your passengers to notify you immediately if they experience or even only think they are experiencing any of the symptoms during the flight

  • Always plan carefully for flights at high altitudes. Do not improvise!

  • Pilots should always exercise their own judgement even below the 10000 ft altitude threshold

  • During Flight, watch out for any early symptoms of hypoxia in yourself, using self‐assessment methods such as the following: 

    ·       If without any specialist equipment,run periodic ‘Mastermind’‐type drills, or carry out a simple exercise such as touching your nose with your fore-finger. You may also involve your passengers in monitoring your state. Be aware of possible false evaluation, or of no evaluation at all, due to euphoria triggered by hypoxia. 

·       With specialist equipment, e.g. a finger-mounted pulse oximeter or similar device. This measures the oxygen saturation in your blood with a relatively inexpensive sensor device which clips over your finger tip. Reading is immediate; a 100% level is normal, and 95% is considered a minimum. An oxygen saturation level below 90% is a warning sign. That’s when people usually begin to experience hypoxia

  • Suspicion of Hypoxia? No Question! Action!! Whatever the methods used, as Pilot‐in‐Command you must react immediately once you or one of your passengers exhibits early hypoxia symptoms. Only one mitigating measure exists - increase oxygen resource. To achieve that, two options are possible: 

1.    Descent to lower altitude, which is largely weather/terrain dependent

2.    Use of supplemental oxygen, which is equipment dependent

When planning a flight over high terrain areas, both options must have been assessed in detail during pre‐flight preparation.

  • When using supplemental oxygen equipment consider the following: 

·       Pure oxygen, or nearly pure oxygen gaseous oxygen, is stored and transported in high‐pressure cylinders that are typically painted green 

·       Different types of mask are on the market, appropriate for private pilot and private operations. Comprehensive reading of manufacturer’s instruction is mandatory 

·       Caution: Cylinders marked as “Aviator’s breathing oxygen” have been tested for the presence of water. At altitudes of hypoxia conditions, sub-zero temperatures are often encountered and water in cylinders could freeze and block valves and regulators. Don’t use oxygen cylinders aimed for medical use or diving

 Case examples: 

a)   A mountain sight-seeing tour at 7000-13000 ft. with no additional oxygen: 


1.    Identify “safe zones” where rapid descent is possible to 7000 ft and lower 

2.    Brief passengers about early symptoms of hypoxia and the possibility of descent to lower altitude to escape hypoxia conditions and to recover 

During flight 

1.   Monitor your own condition periodically and be prepared to descend as soon as any early symptoms show up 

2.   Ask your passengers about their condition and re-assure them about possible descent if necessary

b) An IFR Flight crossing high terrain with challenging weather conditions: 


1.    Identify altitudes of icing conditions, of turbulence, of towering cumulus (TCU), etc., and the possibility to fly at higher altitude 

2.    Check every passenger, even if holding a pilot licence, about preceding experience of altitude as high as the one planned, and about any health conditions or smoking habits 

3.    Clearly recognise that a combination of risk of insidious euphoria and risk of lack of opportunity to descend is detrimental

4.    Identify “safe zones” where rapid descent would be possible to 7000 ft and lower. 

5.   If not, consider carrying on‐board supplemental oxygen equipment. Pay serious attention to realistic risk of being over-burdened if having simultaneously to manage diversion options (due to degrading flight conditions), to negotiate with ATC, and to activate supplemental oxygen system for yourself and your passengers

6.   Once the decision is made to carry supplemental oxygen: 

·      Consider that its procurement might need advance planning, especially in small airports 

·      Read the usage instructions carefully, and make sure you know well all the steps in the process 

·      Brief and train your passengers to use the mask and regulator. This could be a very critical step as you could be very busy at the time of activation of supplemental oxygen

c) High Performance Gliding: 

Rapid descents are rarely a suitable option for glider pilots, especially when crossing high terrain. Usage of supplemental oxygen is more often the only option. The confined space of a glider cockpit is an issue for handling the cylinders and mask. Every practical issue must be addressed on the ground. Glider and balloon pilots can envisage much longer flights at higher altitude than private pilots of light aircraft. Preparation for such high-performance flights is a complex matter; supplemental oxygen is only one piece of that preparation and it is beyond the scope of this leaflet.

 d) Caution: Easy VFR Flight:

Flying in perfect VFR weather conditions at high altitude in order to enjoy a nice temperature, with autopilot engaged, could lead to false confidence and lower vigilance, thus increasing the risk of entering hypoxia conditions. 


Tony Birth