A loss of airspeed creates a partial loss of aircraft control and subsequent loss of altitude during...

Attributes
ACN	589618
Time
Date	200308
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zlc.artcc
State Reference	UT
Altitude	msl bound lower : 29000 msl bound upper : 33000
Environment
Flight Conditions	Mixed
Weather Elements	Windshear Thunderstorm
Light	Night
Aircraft 1
Controlling Facilities	artcc : zlc.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	589618
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : company policies other anomaly other anomaly
Independent Detector	other flight crewa other flight crewb
Resolutory Action	aircraft : equipment problem dissipated flight crew : regained aircraft control flight crew : overrode automation flight crew : overcame equipment problem flight crew : declared emergency
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	Weather Environmental Factor Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance

Narrative:

The captain and I departed for dfw. About 1 hour into the flight, while in and out of clouds and deviating around WX, a loud rumbling sound started up, seemingly out of nowhere. We had just recently returned to VFR conditions and were waiting for the tail deice to cycle before turning off the deicing and anti-icing for the airframe and engines. The rumbling sound seemed to start at the back of the plane and move forward, enveloping the plane like a giant wave breaking over a small boat. As the sound reached the cockpit, the airplane began to buffet, lightly at first, then stronger. At first the captain and I thought we had a structural problem, but then we noticed the airplane was struggling to maintain altitude. I disengaged the autoplt and autothrottles and lowered the nose while the captain declared an emergency with ATC and asked for the closest suitable airport. It was not until we were in the descent that I realized we may had nearly approached a stall situation. During the descent from FL330 to FL290, the airplane went through a series of buffets from light to stronger with breaks in between, each break leading me to believe the problem was solved. At about FL300, the captain asked to take control of the plane. I relinquished control and he requested to level off at FL290. After leveloff, the captain returned control of the airplane to me and after checking the trim, I re-engaged the autothrottles and autoplt. The airplane flew normally. At the time of the event, the airplane weight indicated approximately 129000 pounds and performance mode engaged. I had initially trimmed the airplane upon reaching FL330 at .77 mach. All was normal. At the time prior to the event, we were involved in WX avoidance. However, I do not believe we were so fixated on the WX that we missed any large clues that set the stage for this event. Whatever caused the airplane to lose airspeed, such as mountain wave or a severe downdraft, came on in an insidious manner. Neither the captain nor I became aware of any trim or throttle movement that would have directed our attention to the airspeed indicator. As an ex-air carrier Y pilot, I had to perform all types of stalls at the beginning of every recurrent simulator training session. What I experienced in the airplane was nothing like the stall training experiences I had in the simulator. I believe that is the reason I did not immediately react as if this was a stall recovery situation.

Original NASA ASRS Text

Title: A LOSS OF AIRSPD CREATES A PARTIAL LOSS OF ACFT CTL AND SUBSEQUENT LOSS OF ALT DURING AN INFLT ENCOUNTER WITH WX ENRTE AT FL330 NEAR FMN, NM.

Narrative: THE CAPT AND I DEPARTED FOR DFW. ABOUT 1 HR INTO THE FLT, WHILE IN AND OUT OF CLOUDS AND DEVIATING AROUND WX, A LOUD RUMBLING SOUND STARTED UP, SEEMINGLY OUT OF NOWHERE. WE HAD JUST RECENTLY RETURNED TO VFR CONDITIONS AND WERE WAITING FOR THE TAIL DEICE TO CYCLE BEFORE TURNING OFF THE DEICING AND ANTI-ICING FOR THE AIRFRAME AND ENGS. THE RUMBLING SOUND SEEMED TO START AT THE BACK OF THE PLANE AND MOVE FORWARD, ENVELOPING THE PLANE LIKE A GIANT WAVE BREAKING OVER A SMALL BOAT. AS THE SOUND REACHED THE COCKPIT, THE AIRPLANE BEGAN TO BUFFET, LIGHTLY AT FIRST, THEN STRONGER. AT FIRST THE CAPT AND I THOUGHT WE HAD A STRUCTURAL PROB, BUT THEN WE NOTICED THE AIRPLANE WAS STRUGGLING TO MAINTAIN ALT. I DISENGAGED THE AUTOPLT AND AUTOTHROTTLES AND LOWERED THE NOSE WHILE THE CAPT DECLARED AN EMER WITH ATC AND ASKED FOR THE CLOSEST SUITABLE ARPT. IT WAS NOT UNTIL WE WERE IN THE DSCNT THAT I REALIZED WE MAY HAD NEARLY APCHED A STALL SIT. DURING THE DSCNT FROM FL330 TO FL290, THE AIRPLANE WENT THROUGH A SERIES OF BUFFETS FROM LIGHT TO STRONGER WITH BREAKS IN BTWN, EACH BREAK LEADING ME TO BELIEVE THE PROB WAS SOLVED. AT ABOUT FL300, THE CAPT ASKED TO TAKE CTL OF THE PLANE. I RELINQUISHED CTL AND HE REQUESTED TO LEVEL OFF AT FL290. AFTER LEVELOFF, THE CAPT RETURNED CTL OF THE AIRPLANE TO ME AND AFTER CHKING THE TRIM, I RE-ENGAGED THE AUTOTHROTTLES AND AUTOPLT. THE AIRPLANE FLEW NORMALLY. AT THE TIME OF THE EVENT, THE AIRPLANE WT INDICATED APPROX 129000 LBS AND PERFORMANCE MODE ENGAGED. I HAD INITIALLY TRIMMED THE AIRPLANE UPON REACHING FL330 AT .77 MACH. ALL WAS NORMAL. AT THE TIME PRIOR TO THE EVENT, WE WERE INVOLVED IN WX AVOIDANCE. HOWEVER, I DO NOT BELIEVE WE WERE SO FIXATED ON THE WX THAT WE MISSED ANY LARGE CLUES THAT SET THE STAGE FOR THIS EVENT. WHATEVER CAUSED THE AIRPLANE TO LOSE AIRSPD, SUCH AS MOUNTAIN WAVE OR A SEVERE DOWNDRAFT, CAME ON IN AN INSIDIOUS MANNER. NEITHER THE CAPT NOR I BECAME AWARE OF ANY TRIM OR THROTTLE MOVEMENT THAT WOULD HAVE DIRECTED OUR ATTN TO THE AIRSPD INDICATOR. AS AN EX-ACR Y PLT, I HAD TO PERFORM ALL TYPES OF STALLS AT THE BEGINNING OF EVERY RECURRENT SIMULATOR TRAINING SESSION. WHAT I EXPERIENCED IN THE AIRPLANE WAS NOTHING LIKE THE STALL TRAINING EXPERIENCES I HAD IN THE SIMULATOR. I BELIEVE THAT IS THE REASON I DID NOT IMMEDIATELY REACT AS IF THIS WAS A STALL RECOVERY SIT.

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.

A loss of airspeed creates a partial loss of aircraft control and subsequent loss of altitude during an in-flight encounter with WX en route at FL330 near fmn, NM.