Narrative:

On descent to 10000 ft to cross ritzs intersection at 10000 ft ritzs 1 arrival, had a map failure (glass map on HSI). While pre-occupied reinstating map, copilot asked if I had changed altitude alerting device (aad), which had been set to 10000 ft, but had changed by itself to 8000 ft. I said no, and we confirmed 10000 ft with ATC. Although we are positive altitude alerting device changed itself, we are not sure of the circumstances that caused it to change. I suspect momentary electrical power interruption, perhaps caused by high load electric hydraulic pump being turned on in descent in preparation for approach checklist, caused map failure and altitude alerting device to change. This is an extremely unsafe situation as autoplt would have descended to and captured 8000 ft vice the required 10000 ft. This altitude alerting device has had a history of changing itself, but usually only by 1000 ft. An old fashion mechanical altitude alerting device would be more reliable and much safer, as they never changed themselves once set. FAA needs to rethink standards for these devices. Electrical power interruptions, relay transfer, etc, should not allow altitude alerting device to change. It's difficult enough to meet all requirements without such a poorly designed altitude alerting device that adds another gotcha. Callback conversation with reporter revealed the following: the reporter stated the md-88 has some ergonomics problems. He could write a book on deficiencies of the design of the mode control panel. No longer does the altitude alerter have to be pulled out to arm it each time, but only once at the beginning of each flight. That is an improvement in the md- 88. However, the selector knob has an out position which arms the subsequent selected altitudes and the mid position where it is spring loaded to. Twisting the knob changes the altitude in 1000's of ft. Also, there is a pushed in position which will cause the selector to change altitudes in 100's of ft. If a pilot allows the knob to relax out of the pushed in position as he is turning for hundreds of ft, the knob will spring load out and change the 1000's of ft and zero out the hundreds he was trying to achieve. Normal airplanes have little knobs for 100's of ft and bigger knobs for 1000's of ft and no need ever to arm them. The airspeed selector has a similar awkward troublesome ergonomic push in 3 position knob to achieve 1 KT change, 10 KT change, and a mach change. In this event he believes the altitude shift occurred at the same time as the cathode ray tube map failure -- both of which were provoked by the power surge when he turned on the electric hydraulic pumps as per the descent checklist. There is usually a flicker on the cathode ray tube with the power surge of starting the electric hydraulic pumps but the map does not go dead as it did in this case. The reporter and first officer are sure the 8000 ft altitude was a spurious change.

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Original NASA ASRS Text

Title: THE ALT ALERTER JUMPED 2000 FT BELOW THE 10000 FT IT WAS SET TO MYSTERIOUSLY.

Narrative: ON DSCNT TO 10000 FT TO CROSS RITZS INTXN AT 10000 FT RITZS 1 ARR, HAD A MAP FAILURE (GLASS MAP ON HSI). WHILE PRE-OCCUPIED REINSTATING MAP, COPLT ASKED IF I HAD CHANGED ALT ALERTING DEVICE (AAD), WHICH HAD BEEN SET TO 10000 FT, BUT HAD CHANGED BY ITSELF TO 8000 FT. I SAID NO, AND WE CONFIRMED 10000 FT WITH ATC. ALTHOUGH WE ARE POSITIVE ALT ALERTING DEVICE CHANGED ITSELF, WE ARE NOT SURE OF THE CIRCUMSTANCES THAT CAUSED IT TO CHANGE. I SUSPECT MOMENTARY ELECTRICAL PWR INTERRUPTION, PERHAPS CAUSED BY HIGH LOAD ELECTRIC HYD PUMP BEING TURNED ON IN DSCNT IN PREPARATION FOR APCH CHKLIST, CAUSED MAP FAILURE AND ALT ALERTING DEVICE TO CHANGE. THIS IS AN EXTREMELY UNSAFE SIT AS AUTOPLT WOULD HAVE DSNDED TO AND CAPTURED 8000 FT VICE THE REQUIRED 10000 FT. THIS ALT ALERTING DEVICE HAS HAD A HISTORY OF CHANGING ITSELF, BUT USUALLY ONLY BY 1000 FT. AN OLD FASHION MECHANICAL ALT ALERTING DEVICE WOULD BE MORE RELIABLE AND MUCH SAFER, AS THEY NEVER CHANGED THEMSELVES ONCE SET. FAA NEEDS TO RETHINK STANDARDS FOR THESE DEVICES. ELECTRICAL PWR INTERRUPTIONS, RELAY TRANSFER, ETC, SHOULD NOT ALLOW ALT ALERTING DEVICE TO CHANGE. IT'S DIFFICULT ENOUGH TO MEET ALL REQUIREMENTS WITHOUT SUCH A POORLY DESIGNED ALT ALERTING DEVICE THAT ADDS ANOTHER GOTCHA. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE RPTR STATED THE MD-88 HAS SOME ERGONOMICS PROBS. HE COULD WRITE A BOOK ON DEFICIENCIES OF THE DESIGN OF THE MODE CTL PANEL. NO LONGER DOES THE ALT ALERTER HAVE TO BE PULLED OUT TO ARM IT EACH TIME, BUT ONLY ONCE AT THE BEGINNING OF EACH FLT. THAT IS AN IMPROVEMENT IN THE MD- 88. HOWEVER, THE SELECTOR KNOB HAS AN OUT POS WHICH ARMS THE SUBSEQUENT SELECTED ALTS AND THE MID POS WHERE IT IS SPRING LOADED TO. TWISTING THE KNOB CHANGES THE ALT IN 1000'S OF FT. ALSO, THERE IS A PUSHED IN POS WHICH WILL CAUSE THE SELECTOR TO CHANGE ALTS IN 100'S OF FT. IF A PLT ALLOWS THE KNOB TO RELAX OUT OF THE PUSHED IN POS AS HE IS TURNING FOR HUNDREDS OF FT, THE KNOB WILL SPRING LOAD OUT AND CHANGE THE 1000'S OF FT AND ZERO OUT THE HUNDREDS HE WAS TRYING TO ACHIEVE. NORMAL AIRPLANES HAVE LITTLE KNOBS FOR 100'S OF FT AND BIGGER KNOBS FOR 1000'S OF FT AND NO NEED EVER TO ARM THEM. THE AIRSPD SELECTOR HAS A SIMILAR AWKWARD TROUBLESOME ERGONOMIC PUSH IN 3 POS KNOB TO ACHIEVE 1 KT CHANGE, 10 KT CHANGE, AND A MACH CHANGE. IN THIS EVENT HE BELIEVES THE ALT SHIFT OCCURRED AT THE SAME TIME AS THE CATHODE RAY TUBE MAP FAILURE -- BOTH OF WHICH WERE PROVOKED BY THE PWR SURGE WHEN HE TURNED ON THE ELECTRIC HYD PUMPS AS PER THE DSCNT CHKLIST. THERE IS USUALLY A FLICKER ON THE CATHODE RAY TUBE WITH THE PWR SURGE OF STARTING THE ELECTRIC HYD PUMPS BUT THE MAP DOES NOT GO DEAD AS IT DID IN THIS CASE. THE RPTR AND FO ARE SURE THE 8000 FT ALT WAS A SPURIOUS CHANGE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.