MD88 crew had altitude excursion.

Attributes
ACN	421745
Time
Date	199811
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : vxv
State Reference	TN
Altitude	msl bound lower : 30650 msl bound upper : 31000
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	MD-88
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute : direct enroute airway : ztl
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 9000 flight time type : 6500
ASRS Report	421745
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Consequence	Other
Supplementary
Air Traffic Incident	Pilot Deviation

Narrative:

We fly, deviating around thunderstorm at night, with autoplt coupled, using both radar and visual means to identify cells. I was looking out of the windshield (face nearly pressed against the glass, so I could see better), and was controling my heading with the heading select knob on the flight guidance control panel. The first indication that we had left FL310 was from the altitude alert device, chiming at 250 ft below selected altitude. At that point, I disconnected the autoplt and climbed back to FL310. While we're not sure why the aircraft left altitude, I think that I may have inadvertently brushed against the pitch control wheel, which is adjacent to the heading select knob on the flight guidance control panel. Often, the altitude hold detent on this wheel is less secure than on other units. This incident could have been prevented had the pitch wheel detent been more secure. Also, one of us in the cockpit should have been watching the instruments and the flight guidance control panel. Instead, we were both looking outside at the cells. It was due to the altitude alert device that I only descended 350 ft below assigned altitude.

Original NASA ASRS Text

Title: MD88 CREW HAD ALT EXCURSION.

Narrative: WE FLY, DEVIATING AROUND TSTM AT NIGHT, WITH AUTOPLT COUPLED, USING BOTH RADAR AND VISUAL MEANS TO IDENT CELLS. I WAS LOOKING OUT OF THE WINDSHIELD (FACE NEARLY PRESSED AGAINST THE GLASS, SO I COULD SEE BETTER), AND WAS CTLING MY HDG WITH THE HDG SELECT KNOB ON THE FLT GUIDANCE CTL PANEL. THE FIRST INDICATION THAT WE HAD LEFT FL310 WAS FROM THE ALT ALERT DEVICE, CHIMING AT 250 FT BELOW SELECTED ALT. AT THAT POINT, I DISCONNECTED THE AUTOPLT AND CLBED BACK TO FL310. WHILE WE'RE NOT SURE WHY THE ACFT LEFT ALT, I THINK THAT I MAY HAVE INADVERTENTLY BRUSHED AGAINST THE PITCH CTL WHEEL, WHICH IS ADJACENT TO THE HDG SELECT KNOB ON THE FLT GUIDANCE CTL PANEL. OFTEN, THE ALT HOLD DETENT ON THIS WHEEL IS LESS SECURE THAN ON OTHER UNITS. THIS INCIDENT COULD HAVE BEEN PREVENTED HAD THE PITCH WHEEL DETENT BEEN MORE SECURE. ALSO, ONE OF US IN THE COCKPIT SHOULD HAVE BEEN WATCHING THE INSTS AND THE FLT GUIDANCE CTL PANEL. INSTEAD, WE WERE BOTH LOOKING OUTSIDE AT THE CELLS. IT WAS DUE TO THE ALT ALERT DEVICE THAT I ONLY DSNDED 350 FT BELOW ASSIGNED ALT.

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.