The Silent Emergency

(Pneumatic System Malfunction)

AVIATION SAFETY PROGRAM

Foreword

The purpose of this series of publications is to provide the flying public with safety information that is handy and easy to review. Many of the publications in this series summarize material contained in FAA Aviation Safety program audio-visual presentations.

Comments regarding these publications should be directed to the Department of Transportation, Federal Aviation Administration, Aviation Safety Program Branch, AFS-803, 800 Independence Avenue, S.W. Washington, D.C. 20591.

The Silent Emergency
(Pneumatic System Malfunction)

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 NTSB has reported pneumatic system failures as a factor in an average of two fatal accidents per year over the past ten years. A significant number of the reported cases involved other overriding factors such as loss of control with a back-up electrical gyro available, non-instrument rated pilots flying in instrument weather conditions, and departing with pneumatic systems known to be inoperative.

All too often a pneumatic system failure leads to a situation where a pilot is forced into "partial panel" instrument flying that he or she may not be fully prepared to handle. A number of these accidents occurred to instrument rated pilots who recognized the pneumatic system failure, flew on partial panel in Instrument Meteorological Conditions (IMC) for some period of time, and then lost control during high task-loads, such as during an instrument approach. Many of the aircraft involved were high performance, single engine aircraft.

Lessons Learned

The lessons are clear. The first is that loss of a pneumatic system in actual IMC, without a back-up system, is an emergency that may become life-threatening unless the airplane can be flown by partial panel into Visual Meteorological Conditions (VMC). This may not be possible either due to weather conditions or lack of pilot practice with partial panel flying.

An airplane with a single pneumatic (vacuum or pressure) system, and no back-up system or electric stand-by instruments, should not be flown under IFR. IFR flight "on top" of cloud layers with good ceilings underneath should create minimal problems with a pneumatic system failure, but flying in hard IMC with low ceilings and visibilities underneath sets the stage for serious difficulties.

The second lesson is that any airplane flown regularly under IFR should be equipped with either a back-up pneumatic source, such as dual pneumatic systems, or stand-by electrically powered gyroscopic instruments.

Although it is legal to fly single engine aircraft without dual power sources for gyroscopic instruments and even though the exposure rate to accidents due to pneumatic system failure while in actual instrument weather is relatively low, prudence suggests that a back-up pneumatic source or stand-by instruments are good insurance against being forced to fly partial panel in adverse weather (IMC) without sufficient practice.

Gyroscopic Instrument Power

Normal instrument flight relies in part on three gyroscopic instruments: an attitude indicator (artificial horizon), a heading indicator (directional gyro, or "D.G.") and a turn and slip indicator ("needle and ball," or "turn and bank," or "turn coordinator").

These gyroscopic instruments may be powered by pneumatic (vacuum or pressure) or by 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 pneumatic system which powers the directional and attitude gyroscopic 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 5.5 in. 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, malfunction suddenly or slowly. A decrease in gage indication may be the result of a dirty filter, dirty screen, sticking regulator, worn out air pump or a leak in the system. A zero reading could indicate an air pump failure, collapsed line or malfunctioning gage. Any indication out of the normal range requires immediate attention by a qualified technician. A schedule of periodic pneumatic system inspections is also recommended in order to check the overall health of the system, and at a minimum should address the items mentioned above. The goal of these inspections should be to ensure that the system is operating in complete accordance with the specifications outlined by the airframe manufacturer. In addition, maintenance instructions provided by manufacturers of components within pneumatic systems should also be strictly followed while performing these inspections.

A complete pneumatic system 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. During training an instructor or safety pilot forces a partial panel scan by covering the attitude indicator. Although possibly proficient in flying "partial panel," many pilots are not trained or skilled in recognizing when to revert to a "partial panel" scan. 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 gyroscopic instrument failure is the reason every instrument instructor drills students on partial panel flying without reference to gyroscopic heading and attitude instruments. These failures can set the stage for a fatal accident if the pilot is not proficient in partial panel flying and the failure occurs during IMC.

Know Your Airplane

Every pilot should know the instrument power sources (pneumatic or electrical) for each airplane flown. The pilot must understand the consequences of the loss of any power source or instrument and be prepared to act accordingly.

Know Yourself

Airplanes can be flown safely with loss of one or more gyroscopic instruments. Every instrument rated pilot demonstrated the ability to do so prior to receiving the rating. The problem is that many never practice the skill and only a few have ever practiced in turbulence as it seems an unlikely need in routine operations.

Professional pilots who are required to take semi-annual 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 many pilots, continued flight in IMC 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 not instrument rated and inadvertently encounter IMC, the 180 degree turn is usually the best course of action. If your pneumatic driven gyro instruments fail, it is still possible to make a 180 degree turn by using the turn and bank (or turn coordinator), magnetic compass and clock. Likewise a descent through clouds to VMC can be made using the turn indicating instrument. These procedures may be tailored to each airplane type and model and should be demonstrated by and practiced with an instructor. It may be too late to learn them when faced with actual need. Avoid conditions that risk encountering IMC.

If You Are Instrument Rated

If you are instrument rated and gyro instruments fail or mislead, do not be afraid to ask for help. ATC personnel know where to find better weather and are able to give "no gyro" heading directions. The whole system - radar, weather reports, communications, and personnel - is instantly available to assist you.

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

Also, cover the dead or unreliable 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 weather you fly. Solid IMC from take-off to touchdown can be very difficult on partial panel.

BACK-UP! The Better Way

IF YOUR AIRPLANE DOES NOT HAVE A BACK-UP, OR STANDBY PNEUMATIC SYSTEM, AND IF YOU USE YOUR AIRPLANE FOR IFR FLIGHT, YOU SHOULD INSTALL EITHER A BACK-UP PNEUMATIC POWER SOURCE OR ELECTRICALLY POWERED STAND-BY GYROSCOPIC INSTRUMENTS. Several manufacturers offer a variety of alternate electric gyroscopic instruments or pneumatic systems that will supply vacuum or pressure if the engine driven pump fails. Since pneumatic system - or pneumatic driven instrument - failure in actual IMC can and does occur, the cost of a stand-by system is far less than the too often fatal results of not having a back-up.

AVIATION SAFETY PROGRAM

Federal Aviation Administration
Aviation Safety Program (AFS-820)
800 Independence Avenue S.W.
Washington, D.C. 20591

Contact your local FAA Flight Standards District
Office's Safety Program Manager for more safety information.