Air carrier medium large transport speed deviation at cruise altitude.

Attributes
ACN	153599
Time
Date	199007
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : lvs
State Reference	NM
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	enroute airway : zab
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 12000 flight time type : 350
ASRS Report	153599
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe inflight encounter : weather other anomaly
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

During cruise, the stabilizer trim on the medium large transport sounded 1 tone and pilots noted airspeed was decreasing form desired FMS speed of 280 KIAS toward 250 KIAS. Aircraft was being maneuvered around WX systems and our attention was primarily devoted to radar presentation and heading deviations. Aside from being in and out of flight cirrus clouds, ride was fairly smooth with no visible moisture or icing noted. FMS override selected and airspeed 280 KIAS, EPR's indicated maximum cruise 1.86. Airspeed continued to decrease and mct was selected at 240 KIAS. WX avoidance was still in progress but flight condition was mostly topping clouds to in the clear. Large thunderstorms to left and lesser cells to the right. As airspeed continued to decrease, captain clicked off autothrottles, selected takeoff thrust on the trp and applied nearly full forward thrust on the throttle levers. EPR indicated as high as 2.20 with command indices at 2.08; however, the N1 and N2 thrust settings indicated only 92% and 96% respectively. Airspeed began to increase. Lowest airspeed was 227 KIAS before increase. Even though we were in the clear with no windshield bolt icing, the ignition was placed on and engine anti-ice turned on. EPR indicators flashed and fluctuated wildly before indicating properly. FMS then indicated los temperature conditions for the first time. It is suspected that the PT2 probe had iced up and that the FMS commanded full power with no resultant increase in engine performance. Captain can remember seeing N2 readings as low as 78% prior to manually taking control of the throttles. Commanded EPR was takeoff thrust on trp. I believe that this condition, if repeated, could result in airspeed deteriorating to the point of stall. The really weird thing is that at no time were the classic icing clues present. Fortunately, the flight did not encounter any moderate or greater gusts during the low speed episode. Another anomaly is that at no time did the EPR appear to be erratic or stuck. The EPR followed the trp to each higher setting west/O a subsequent change in other engine INS or thrust levers. It was also apparent that the autothrottle clutches were engaged and that the requested power was not being applied.

Original NASA ASRS Text

Title: ACR MLG SPEED DEVIATION AT CRUISE ALT.

Narrative: DURING CRUISE, THE STABILIZER TRIM ON THE MLG SOUNDED 1 TONE AND PLTS NOTED AIRSPD WAS DECREASING FORM DESIRED FMS SPD OF 280 KIAS TOWARD 250 KIAS. ACFT WAS BEING MANEUVERED AROUND WX SYSTEMS AND OUR ATTN WAS PRIMARILY DEVOTED TO RADAR PRESENTATION AND HDG DEVIATIONS. ASIDE FROM BEING IN AND OUT OF FLT CIRRUS CLOUDS, RIDE WAS FAIRLY SMOOTH WITH NO VISIBLE MOISTURE OR ICING NOTED. FMS OVERRIDE SELECTED AND AIRSPD 280 KIAS, EPR'S INDICATED MAX CRUISE 1.86. AIRSPD CONTINUED TO DECREASE AND MCT WAS SELECTED AT 240 KIAS. WX AVOIDANCE WAS STILL IN PROGRESS BUT FLT CONDITION WAS MOSTLY TOPPING CLOUDS TO IN THE CLR. LARGE TSTMS TO LEFT AND LESSER CELLS TO THE RIGHT. AS AIRSPD CONTINUED TO DECREASE, CAPT CLICKED OFF AUTOTHROTTLES, SELECTED TKOF THRUST ON THE TRP AND APPLIED NEARLY FULL FORWARD THRUST ON THE THROTTLE LEVERS. EPR INDICATED AS HIGH AS 2.20 WITH COMMAND INDICES AT 2.08; HOWEVER, THE N1 AND N2 THRUST SETTINGS INDICATED ONLY 92% AND 96% RESPECTIVELY. AIRSPD BEGAN TO INCREASE. LOWEST AIRSPD WAS 227 KIAS BEFORE INCREASE. EVEN THOUGH WE WERE IN THE CLR WITH NO WINDSHIELD BOLT ICING, THE IGNITION WAS PLACED ON AND ENG ANTI-ICE TURNED ON. EPR INDICATORS FLASHED AND FLUCTUATED WILDLY BEFORE INDICATING PROPERLY. FMS THEN INDICATED LOS TEMP CONDITIONS FOR THE FIRST TIME. IT IS SUSPECTED THAT THE PT2 PROBE HAD ICED UP AND THAT THE FMS COMMANDED FULL PWR WITH NO RESULTANT INCREASE IN ENG PERFORMANCE. CAPT CAN REMEMBER SEEING N2 READINGS AS LOW AS 78% PRIOR TO MANUALLY TAKING CTL OF THE THROTTLES. COMMANDED EPR WAS TKOF THRUST ON TRP. I BELIEVE THAT THIS CONDITION, IF REPEATED, COULD RESULT IN AIRSPD DETERIORATING TO THE POINT OF STALL. THE REALLY WEIRD THING IS THAT AT NO TIME WERE THE CLASSIC ICING CLUES PRESENT. FORTUNATELY, THE FLT DID NOT ENCOUNTER ANY MODERATE OR GREATER GUSTS DURING THE LOW SPD EPISODE. ANOTHER ANOMALY IS THAT AT NO TIME DID THE EPR APPEAR TO BE ERRATIC OR STUCK. THE EPR FOLLOWED THE TRP TO EACH HIGHER SETTING W/O A SUBSEQUENT CHANGE IN OTHER ENG INS OR THRUST LEVERS. IT WAS ALSO APPARENT THAT THE AUTOTHROTTLE CLUTCHES WERE ENGAGED AND THAT THE REQUESTED PWR WAS NOT BEING APPLIED.

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.