By Ralph Graves, President, ASCO Aeronautical, Inc.

After spending 50-plus years in the aviation industry, most of them in instrument repair stations, recent articles in Air Safety Week and in sister publication Avionics Magazine verify my observations of aircraft attitude director indicators (ADI) and the virtue of "twinning" the standby ADI with the primary instrument.

From my viewpoint, the ADI is the most used, the most misused and the least understood instrument in the cockpit. I might also add it has the highest failure rate of all the instruments in the aircraft.

While complete ADI failure is rare, erroneous indications are common and often are undetected by the flight crew.

Whether it is an air-driven gyro in a panel-mounted ADI or an electric remote gyro with a glass instrument panel, they all have faults that can produce erroneous displays. The ADI will never operate more accurately than it did when the manufacturer or service shop last tested it. Many times the ground crew or the pilot on preflight cannot duplicate erroneous indications detected on an airborne ADI.

As the saying goes, I wish I had a nickel for every time a pilot tried to duplicate for me an ADI problem on the ground that he or she had encountered in flight.

Several years ago while giving a lecture to a group of pilots, I remarked that the ADI "has the privilege of being the aircraft instrument with the highest failure rate." Moreover, I declared, "Any instrument-rated pilot who is not capable of flying the aircraft without using the ADI should not have an instrument rating."

This comment elicited many replies - not too many in my favor. Several pilots said they had been flying for years and never had an ADI fail, while conceding they would be lost without it. To be sure, it is difficult to sell pilots on the shortcomings of the ADI without destroying overall confidence in their flight instruments.

Without a doubt, moving the alternate or emergency ADI to a more visible position on the instrument panel would be a great improvement. Such pairing would give the pilot constant visual contact with the alternate ADI and would be a great aid in determining improper operation of the primary ADI should it occur.

In addition to placement, more partial panel training for pilots should have a very high priority, in my opinion.

Last but not least, and I say this fully recognizing it's from the perspective of someone in the instrument testing and repair business, there should be a requirement for periodic testing of all aircraft instruments. In recent years, many aircraft engine manufacturers have recognized the importance of having properly operating and accurate engine instruments in the aircraft. They have suggested the instruments should be tested annually or at engine installation. The Federal Aviation Administration (FAA) does not require such testing and volunteer testing just doesn't happen.

The sad part about this situation is that under most circumstances, instruments will not receive any testing from the time they leave the manufacturer's final inspection and testing until failure occurs. Should that occur in flight, a paired standby ADI can help pilots diagnose their situation and retain attitude awareness needed to bring the aircraft to a safe landing.

Byline: Graves is owner and manager of Columbus, Ohio-based ASCO Aeronautical, a distributor of many aviation-related products and a manufacturer of aviation-related test equipment. See http://members.tripod.com/ralph_graves . (For articles on twinning, see ASW, March 17, 2003; Aug. 4, 2003; Avionics Magazine, Oct. 2003) >> Graves, e-mail rgraves555@msn.com <<

Paired for Safety

Twinning - The Concept Defined

The philosophy of closely juxtaposing critical flight instruments that essentially back each other up (and so require cross-checking, since the failure of one to replicate the other's movement should become the cause for instant concern). Examples include the altimeter and standby altimeter, and also the main attitude director indicator (ADI) and its backup/standby attitude instrument.

Twinning - The Concept Illustrated

A notional example of pairing the primary and standby attitude indicators as one means of mitigating spatial disorientation should the primary instrument fail.

Twinning - The Concept Called For

The Transportation Safety Board (TSB) of Canada on standby instruments:

"When pilots are forced to use standby instruments, they must be able to quickly adjust their instrument cross-check scan. The transition to standby instruments, especially in adverse circumstances, could be significantly hampered by having the instruments positioned away from the normal line of vision, and by not having them in a standard grouping layout. The result could be disorientation of the flight crew, and loss of control of the aircraft."

Source: TSB Aviation Safety Advisory A010042-1, Sept. 28, 2002