Flight crew of MD80 unable to maintain cruise mach at FL320. Forced to descend to avoid stall.

Attributes
ACN	665350
Time
Date	200507
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : png.vor
State Reference	FO
Altitude	msl single value : 32000
Environment
Weather Elements	Ice
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zhu.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	665350
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	665349
Events
Anomaly	other anomaly other anomaly other anomaly other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : exited adverse environment flight crew : overrode automation flight crew : regained aircraft control
Supplementary
Problem Areas	Weather Aircraft Flight Crew Human Performance Chart Or Publication Company
Primary Problem	Aircraft

Narrative:

Approaching png VOR with ZHU; FL320; ram air temperature -1 degree; weight approximately 138000 pounds; on top of clouds; mach .76; mach atl. Noticed light airframe vibration like landing light or spoiler panel out. Commented this to first officer. We both did a quick check; gear doors; MCP; spoilers and ice detection rings. Shudder became more pronounced; airspeed declining. I called for engine and airfoil anti-icing. First officer contacted ZHU for FL260. We were cleared and I disconnected autoplt and autothrottles; and began a 1000 FPM descent with mcl selected manually. At FL315; got 1 stall aural and 1 claxon/red light. We never got a speed low or stick shaker. Airspeed now accelerating in a continued controled descent. Could have leveled at FL290 with plenty of airspeed; but continued to FL280 and mach .76 with engine and airfoil anti-ice on. I feel that perhaps we had picked up enough ice in the climb to give a flawed EPR rating; with less thrust than atl would have normally provided. At no time were there visual signs of icing either below the wiper or on the ice detection ring either side!

Original NASA ASRS Text

Title: FLT CREW OF MD80 UNABLE TO MAINTAIN CRUISE MACH AT FL320. FORCED TO DSND TO AVOID STALL.

Narrative: APCHING PNG VOR WITH ZHU; FL320; RAM AIR TEMP -1 DEG; WT APPROX 138000 LBS; ON TOP OF CLOUDS; MACH .76; MACH ATL. NOTICED LIGHT AIRFRAME VIBRATION LIKE LNDG LIGHT OR SPOILER PANEL OUT. COMMENTED THIS TO FO. WE BOTH DID A QUICK CHK; GEAR DOORS; MCP; SPOILERS AND ICE DETECTION RINGS. SHUDDER BECAME MORE PRONOUNCED; AIRSPD DECLINING. I CALLED FOR ENG AND AIRFOIL ANTI-ICING. FO CONTACTED ZHU FOR FL260. WE WERE CLRED AND I DISCONNECTED AUTOPLT AND AUTOTHROTTLES; AND BEGAN A 1000 FPM DSCNT WITH MCL SELECTED MANUALLY. AT FL315; GOT 1 STALL AURAL AND 1 CLAXON/RED LIGHT. WE NEVER GOT A SPD LOW OR STICK SHAKER. AIRSPD NOW ACCELERATING IN A CONTINUED CTLED DSCNT. COULD HAVE LEVELED AT FL290 WITH PLENTY OF AIRSPD; BUT CONTINUED TO FL280 AND MACH .76 WITH ENG AND AIRFOIL ANTI-ICE ON. I FEEL THAT PERHAPS WE HAD PICKED UP ENOUGH ICE IN THE CLB TO GIVE A FLAWED EPR RATING; WITH LESS THRUST THAN ATL WOULD HAVE NORMALLY PROVIDED. AT NO TIME WERE THERE VISUAL SIGNS OF ICING EITHER BELOW THE WIPER OR ON THE ICE DETECTION RING EITHER SIDE!

Data retrieved from NASA's ASRS site as of January 2009 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.