Medium large transport unable to maintain FL350 leaves assigned altitude without clearance.

Attributes
ACN	171348
Time
Date	199102
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zdc
State Reference	DC
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc tracon : yyz
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 234 flight time total : 12500 flight time type : 3000
ASRS Report	171348
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 6000 flight time type : 3500
ASRS Report	171355
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter other non adherence : clearance other anomaly
Independent Detector	other flight crewa
Resolutory Action	flight crew : exited adverse environment
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation

Narrative:

Takeoff and climb to cruise altitude (FL350) was normal (takeoff weight was structural limit 110000#). After approximately 15 mins at cruise mach, a slow but steady decrease was noticed in indicated airspeed. I increased power to climb power and regained lost (normal) speed. With maximum allowable cruise thrust set I was able to maintain planned cruise mach (same condition as initial level-off). After 10 mins or so, IAS again started a slow but steady decrease. At 245 KIAS I requested FL310. ATC was unable to approve FL310 and said to stand by. IAS continued to decrease. At 215 KIAS I felt a slight buffet start. Satisfied this was a prestall condition and justified prompt action, I immediately started a descent and advised ATC I was leaving FL350. I followed ATC instructions for a left turn and descent to FL310. The remainder of the flight was uneventful. I contacted ATC by phone and discussed the vent. I was advised that there was no sep conflict and no action was to be taken. Considering all factors, including the fact that I had requested a lower altitude, ATC was satisfied my actions were prudent. Note: in my conversation with the ATC supervisor, he said 2 other aircraft in that area had reported unexplained losses of 40+ KIAS.

Original NASA ASRS Text

Title: MLG UNABLE TO MAINTAIN FL350 LEAVES ASSIGNED ALT WITHOUT CLRNC.

Narrative: TKOF AND CLB TO CRUISE ALT (FL350) WAS NORMAL (TKOF WEIGHT WAS STRUCTURAL LIMIT 110000#). AFTER APPROX 15 MINS AT CRUISE MACH, A SLOW BUT STEADY DECREASE WAS NOTICED IN INDICATED AIRSPD. I INCREASED PWR TO CLB PWR AND REGAINED LOST (NORMAL) SPD. WITH MAX ALLOWABLE CRUISE THRUST SET I WAS ABLE TO MAINTAIN PLANNED CRUISE MACH (SAME CONDITION AS INITIAL LEVEL-OFF). AFTER 10 MINS OR SO, IAS AGAIN STARTED A SLOW BUT STEADY DECREASE. AT 245 KIAS I REQUESTED FL310. ATC WAS UNABLE TO APPROVE FL310 AND SAID TO STAND BY. IAS CONTINUED TO DECREASE. AT 215 KIAS I FELT A SLIGHT BUFFET START. SATISFIED THIS WAS A PRESTALL CONDITION AND JUSTIFIED PROMPT ACTION, I IMMEDIATELY STARTED A DSNT AND ADVISED ATC I WAS LEAVING FL350. I FOLLOWED ATC INSTRUCTIONS FOR A LEFT TURN AND DSNT TO FL310. THE REMAINDER OF THE FLT WAS UNEVENTFUL. I CONTACTED ATC BY PHONE AND DISCUSSED THE VENT. I WAS ADVISED THAT THERE WAS NO SEP CONFLICT AND NO ACTION WAS TO BE TAKEN. CONSIDERING ALL FACTORS, INCLUDING THE FACT THAT I HAD REQUESTED A LOWER ALT, ATC WAS SATISFIED MY ACTIONS WERE PRUDENT. NOTE: IN MY CONVERSATION WITH THE ATC SUPVR, HE SAID 2 OTHER ACFT IN THAT AREA HAD RPTED UNEXPLAINED LOSSES OF 40+ KIAS.

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.