An MD80 flight crew, operating under a thunderstorm overhang, was unable to maintain assigned altitu...

Attributes
ACN	441781
Time
Date	199906
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : vxv.vortac
State Reference	TN
Altitude	msl bound lower : 29300 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl.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
Navigation In Use	other vortac
Flight Phase	cruise : level
Route In Use	enroute airway : ns.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 200 flight time total : 11000 flight time type : 4000
ASRS Report	441781
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather inflight encounter other non adherence : clearance
Independent Detector	other controllera other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were approaching a large, diffuse overhang. I turned on the WX radar. Return was minimal -- a small patch of green on the nose. A few mins after exiting the overhang, in clear air, I noticed the airspeed decaying through 230 KTS (we had been at 280 KTS). The captain increased thrust, but the airspeed continued to drop. We turned on all anti-ice system, even though there had been no evidence of ice accumulation. As the airspeed began to increase, we commenced a slow climb back to FL310. We secured all anti-ice system and had no further recurrence of the problem. There was never any ice on the windshield, and a quick scan of the engine instruments when we noticed the airspeed decay revealed no discrepancies. Airframe icing seems to be the only explanation.

Original NASA ASRS Text

Title: AN MD80 FLC, OPERATING UNDER A TSTM OVERHANG, WAS UNABLE TO MAINTAIN ASSIGNED ALT NEAR VXV VOR.

Narrative: WE WERE APCHING A LARGE, DIFFUSE OVERHANG. I TURNED ON THE WX RADAR. RETURN WAS MINIMAL -- A SMALL PATCH OF GREEN ON THE NOSE. A FEW MINS AFTER EXITING THE OVERHANG, IN CLR AIR, I NOTICED THE AIRSPD DECAYING THROUGH 230 KTS (WE HAD BEEN AT 280 KTS). THE CAPT INCREASED THRUST, BUT THE AIRSPD CONTINUED TO DROP. WE TURNED ON ALL ANTI-ICE SYS, EVEN THOUGH THERE HAD BEEN NO EVIDENCE OF ICE ACCUMULATION. AS THE AIRSPD BEGAN TO INCREASE, WE COMMENCED A SLOW CLB BACK TO FL310. WE SECURED ALL ANTI-ICE SYS AND HAD NO FURTHER RECURRENCE OF THE PROB. THERE WAS NEVER ANY ICE ON THE WINDSHIELD, AND A QUICK SCAN OF THE ENG INSTS WHEN WE NOTICED THE AIRSPD DECAY REVEALED NO DISCREPANCIES. AIRFRAME ICING SEEMS TO BE THE ONLY EXPLANATION.

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.

An MD80 flight crew, operating under a thunderstorm overhang, was unable to maintain assigned altitude near vxv VOR.