As a result of a March 19, 2001, in-flight upset involving a Comair Airlines EMB-120 twin turboprop, the National Transportation Safety Board has recommended that the airplane's air data indicators (ADIs) be capable of remaining functional through the instruments' specified 128� per second operating range. Moreover, the board declared that the ADIs should be tested for performance at this roll rate.

In an upset believed to be the result of in-flight icing, the crew lost 7,000 feet of altitude in 24 seconds and rolled 360� with 60� nose-down pitch before they were able to regain control of the airplane. After recovering the airplane, the crew diverted and landed at West Palm Beach, Fla., without further incident. Under the aerodynamic forces of the upset and recovery, the horizontal stabilizer and elevators were damaged substantially (see photos in ASW, April 23, 2001).

During the upset, the electronic ADIs blanked out. When the units were tested, they failed at rates less than the operating specification. The units passed the manufacturer's production test and final alignment requirements (PTR). According to the NTSB recommendation for tougher testing, the manufacturer said the high-rate testing was not required "because the unit's performance could be inferred from the results obtained during low-rate testing." The captain's instrument was operating in a degraded condition. When asked for how long, the manufacturer replied that it was impossible to determine how long this "benign latent condition" had existed.

The icing aspects of the case are still under investigation. Nonetheless, the upset would seem to reinforce the argument for twinning the main and standby attitude indicators. The evidence at hand suggests that the crew did not go instantly to the standby horizon.

John Dow, a retired official in the small airplane directorate of the Federal Aviation Administration, (FAA) was asked to comment on this case. Herewith, extracts:

"The words 'benign latent condition' are interesting. Latent benign failure means that it is broken, but you don't know it, and that it won't kill you when you find out.

"At some point, when the airplane exceeded the maximum roll rate for the system - which it did - the crew would have lost both displays anyway. So in this regard, I am concerned with a crew that does not, or is not able to use all the tools available to it. The standby system is there for a purpose.

"On the ADIs, would the better question be, 'Are the primary flight instruments capable of providing reliable information under all possible flight conditions?' If the airplane could do 180� per second, then shouldn't they be able to operate under that condition? If the roll rate limit is part of the intended function of the instrument, then the instruments may not have met FAR 25.1301. Failing to meet a requirement does not automatically justify an airworthiness directive (AD). There must be an unsafe condition. Some would argue that because there was a standby ADI there was not an unsafe condition. Since there was not an unsafe condition, then an AD is not justified - although the boundary can be a bit soft at times.

"The ADI is of course one of those peripherals that sometimes complicates the [larger] issue. The fact of the matter is that the EMB-120 airplane in certain icing conditions can experience a roll upset in IMC (instrument meteorological conditions) without adequate stall warning that often fills the windshield with dirt. For the largest safety improvement, that is where I would spend money. It would not be with the ADIs, although they are critical.

"I do not recall loss of ADI in any of the prior EMB-120 upsets. In those events the airplane rolled around the sky. In the fatal Jan. 9, 1997, crash of a Comair EMB-120 at Monroe, Mich., the pilot did a great job of working the problem. A former FAA test pilot said if he'd had another 1,000 feet, he may well have recovered (see ASW, Aug. 31, 1998). I think he had his ADIs working. Everyone died in that one. No one died in this more recent upset.

"By weight of numbers, I would still go with fixing the airplane so the pilot doesn't have to find out at what roll rate he/she loses the ADIs." (For the NTSB recommendation, see http://www.ntsb.gov/Recs/letters/2003/A03_02_03.pdf) >> Dow, e-mail jdowsr@earthlink.net <<