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You fly in actual instrument weather conditions and make enough approaches to keep "current," take your biennial flight review from a good instructor, know the "Normal" and "Emergency" procedure sections of your Pilot's Operating Handbook, and feel you are qualified to cope with any emergency. Are you? Maybe not. 

Rare, But Sometimes Fatal, Accidents

 The most disturbing factor is the remaining half—an average of about one accident per year—occurred to instrument rated pilots who recognized the pneumatic system failure, flew on partial panel in instrument weather conditions for 30 to 45 minutes, and then lost control during high task loads, such as during an instrument approach. Another common denominator was that all aircraft involved were high performance, retractable gear, single engine aircraft. 

Lessons Learned

 An airplane with a single pneumatic system with no back-up system, or back-up instruments, should not be flown in any IFR conditions that do not provide for quick access to VfR conditions. IFR flight "on top" of cloud layers with good ceiling underneath should create minimal problems with pneumatic system failure, but flying in actual IFR with low ceilings and visibilities underneath sets the stage for serious difficulties.

 The second lesson is that any airplane used regularly in IFR weather should be equipped with either a back-up power source, such as dual pneumatic systems, or back-up electrically powered gyroscope instruments. Although it is legal to fly single engine aircraft without dual power sources for gyroscope instruments, and the exposure rate to accidents due to pneumatic system failure while in actual instrument weather is low (1 accident for each 40-50,000 general aviation instrument flight plans filed), prudence suggests that a back-up power source is good insurance against being forced to fly partial panel in adverse weather without sufficient practice. 

Gyroscope Instrument Power

 These gyrosopic instruments may be powered by pneumatic (vacuum or pressure) or by airplane electrical systems. Which power source is used for which instruments may vary in the same make and model of airplane, depending on use intended at time of manufacture or modifications made after manufacture.

 The most common arrangement for single engine airplanes without back-up instrumentation, or systems, is a single vacuum system which powers the directional and attitude gyroscope instruments. The other gyro instrument, the "turn and bank" or "turn coordinator" is usually electrically driven.

 The gage on the instrument panel may be marked as either a "suction gage," a "vacuum gage" or a "pressure gage" and indicates in inches of mercury. The correct operating range (around 4.5" to 5.5t HG.) is given in the Handbook for each airplane. Some airplanes also have warning lights when the vacuum or pressure is out of tolerance.

 Pneumatic systems, like other mechanical systems, can malfunction suddenly or slowly. A slow decrease in gage indication may indicate a dirty filter, dirty screens, sticking regulator, worn out air pump or leak in the system. Zero pressure could indicate a sheared pump drive, pump failure, a collapsed line, or a malfunctioning gage. Any operation out of the normal range requires immediate attention by a mechanic.

 A complete pneumatic loss is noticeable immediately on the gage or within minutes by incorrect gyro readings. A slow deterioration may lead to sluggish or incorrect readings which may trap a pilot who is not constantly cross-checking all instruments—including the vacuum or pressure gage.

 An additional factor involves an initial lack of recognition of the cause of the conflicting instrument indication which develops when one Instrument, usually the attitude indicator, malfunctions. Although possibly proficient in flying "partial panel," many pilots are not trained or skilled in deciding to revert to a "partial panel" scan unless an instructor or safety pilot has forced the scan by covering the attitude indicator. It is important for pilots to scan all instruments whenever conflicting information develops and not attempt to make control inputs on the basis of the attitude indicator alone. Once the all-important first step of recognition of the need for partial panel scan is accepted, it is also helpful to remove the malfunctioning instrument from the scan, usually by covering it with a disk or piece of paper.

 The possibility of pneumatic system or gyroscope instrument failure is the reason every instrument instructor drills students on partial panel flying without reference to gyroscope heading and attitude instruments. It is very rare that the failure itself results in a fatal accident, but it can set the stage for one if the pilot is not proficient in partial panel flying and the failure occurs during instrument flight conditions.

Know Your Airplane

Know Yourself

 Professional pilots who are required to take semiannual simulator training practice a lifetime of emergencies each training session although they rarely encounter emergencies in daily operations. Most general aviation pilots remain "current" by flying in the system and may rarely face or practice emergency situations. For most pilots, continued flight in IFR conditions with failed gyro heading and attitude instruments is a high work load situation that could lead to a fatality.

If You Are Not Instrument Rated

If You Are Instrument Rated

 Do not try to be a hero and continue on bravely as if loss of pneumatic power was no big deal. It can be a serious emergency unless you have maintained high proficiency in partial panel flying.

 Also, cover the dead or lying instruments. Most partial panel practice is done with covered instruments, but in real cases the artificial horizon will be sagging and giving erroneous information that your instincts want to accept as correct. Autopilots using these instruments as sensors must be turned off immediately.

 Finally, if your airplane has no back-up capability be cautious in the type of IFR you fly. Solid IFR from takeoff to touchdown can be very difficult on partial panel.

Back up The Better Way

(END OF DOCUMENT FAA-P-8740-52 AFS-800-08-87)

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