High Altitude Operations

It is advantageous to fly at high altitudes because a plane is more efficient (consumes less fuel).  Also, weather can be avoided for the most part as you can fly above it.  There are certain oxygen requirements that must be met when flying at higher altitudes.

Regulations
If flying above 12,500 ft and up to 14,000 ft., the flight crew must use supplemental oxygen after 30 mins.
Above 14,000 ft., the flight crew must wear oxygen at all times.
Above 15,000 ft, the passengers must be provided with (not required to use) supplemental oxygen.

•In order to fly a pressurized aircraft that has a service ceiling or max. altitude above 25,000 ft. unless they have an endorsement proving they have received ground/flight training in that aircraft. (FAR 61.31)

•In an aircraft with a pressurized cabin, you may not fly at altitudes above FL 250 unless at least a 10-min. supply of oxygen is available for each occupant to use in the event that an emergency descent is needed due to loss of cabin pressure.

•Above FL 350, one pilot at the controls must be wearing supplemental oxygen that supplies oxygen at all times or automatically delivers oxygen above 14,000 ft cabin pressure altitude.  If one pilot is away from the controls, the one at the controls must wear an oxygen mask at all times.

•Aviation oxygen must be at least 99.5% oxygen.  Medical oxygen and industrial types are not allowed as they contain too much water, which could freeze and block the breathing lines.

Aeromedical
•Hypoxia is the primary risk for high-altitude operations.  Hypoxic Hypoxia is the altitude-specific variant - resulting from the overall lack of oxygen to the lungs, blood, and brain.  Other factors such as smoking, alcohol, or drug use can hamper the body's ability to function at altitude.

•Prolonged use of oxygen can also be harmful, as it is somewhat toxic.

•Nitrogen is inhaled when we breathe.  It is often naturally exhaled with carbon dioxide, but some gets absorbed by the body tissues.  If the surrounding atmospheric pressure lowers drastically (as in sudden decompression), the nitrogen could change from its liquid state and become a gas and form bubbles (the bends).

•Vision deteriorates with altitude

Operation of Pressurization Systems
Engine air is compressed by use of turbocharger or superchargers and may use a flow control venturi to control the amount entering the cabin.

An outflow valve is used to allow air inside the cabin to exit.  This allows a more constant stream of air into the cabin.

A common pressure altitude used in pressurized aircraft is 8,000 ft.  First, some more


TERMS


Aircraft altitude - actual height above Sea Level that aircraft is flying
Ambient Temperature - Temp. in area immediately surrounding aircraft
Ambient Pressure - pressure in area immediately surrounding aircraft
Cabin Altitude - Cabin pressure in terms of equivalent altitude above sea level.
Differential Pressure - Difference between cabin pressure and atmospheric pressure.

A Cabin Pressure Regulator controls cabin pressure to selected value and limits it to a preset in the differential range.  Differential control prevents excess pressure on the cabin from occurring.  When its max is reached, cabin altitude increases.

Cabin Air Pressure Safety Valve consists of pressure relief, vacuum relief, and dump valve. Pressure Relief prevents cabin pressure from exceeding differential pressures.  Vacuum relief prevents ambient pressure from exceeding cabin pressure by allowing external air to enter cabin.  Dump Valve allows cabin air to be dumped to atmosphere.

Instruments include a Cabin Differential Pressure gauge to indicate difference between inside and outside pressure.

Cabin Altimeter provides a check on performance of system.

Cabin Rate of Climb shows rate in change of cabin pressure.




Decompression


Decompression is the inability of the system to maintain the desired pressure differential.  There are two kinds.


Explosive Decompression - a change in cabin pressure faster than the lungs can decompress (potentially causing lung damage).  Decompression that occurs in less than 0.5 seconds is considered explosive.
Rapid Decompression - Change in cabin pressure in which lungs are able to decompress faster than cabin.  No likelihood of lung damage.

In explosive decompression, the cabin may fill with fog, dust, debris.  Air rushes out of the body.

In rapid decompression, the period of useful consciousness is reduced because pressure on the body is reduced.  A pilot's effective performance is therefore reduced.

In decompression, prompt emergency descent is necessary to minimize hypoxia and other factors.

Types of Oxygen Systems


Cannula is a piece of plastic tubing similar in appearance to what an emphysema sufferer would wear, but these may only be used up to 18,000 ft.

Diluter-Demand supplies oxygen only during inhalation.  Is safe up to 40,000 feet.  Can supply either 100% oxygen or mix with cabin air.

Pressure-Demand supplies oxygen under pressure at cabin altitudes above 34,000 ft.  Users lungs can be pressurized with oxygen.  Can be used above 40,000 feet.

Continuous Flow are commonly provided for passengers.  They consist of a reservoir bag, which collects oxygen during time when user is exhaling.

Electrical Pulse-Demand deliver oxygen by detecting individual user's inhalation effort.  This reduces oxygen needed and is thus more efficient.

Pulse Oximeters detect amount of oxygen in blood by detecting color changes in red blood cells.

Oxygen should be stored in high pressure containers from 1800 to 2200 PSI.