The air carrier flight crew of a B737 increases their airspeed and changes the assigned altitude to a lower one in compliance with a controller's clearance. The aircraft started a slow climb as it accelerated, even though the 'altitude hold' mode was, apparently, still engaged.

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Attributes
ACN	377247
Time
Date	199708
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : lax
State Reference	CA
Altitude	msl bound lower : 35000 msl bound upper : 36000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	arrival other arrival star : star
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 5500 flight time type : 1000
ASRS Report	377247
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 8000 flight time type : 2500
ASRS Report	377680
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : far non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation other

Narrative:

Level cruise flight, asked by ATC to accelerate to 300 KTS and given pilot's discretion to FL280. Aircraft was engaged in VNAV and LNAV mode with FL280 set in the altitude selector and 300 KTS in the speed selector. As the aircraft was accelerating from .74 mach to 300 KTS it started a 600 FPM climb, but was not noticed, since the altitude alert did not sound until FL359 had been reached. Aircraft was returned immediately to FL350 and soon thereafter descended to FL280. When any change is made in speed or altitude be especially alert that VNAV does not disengage. Although in this occurrence maintenance was notified because it did remain engaged. Supplemental information from acn 377680: in cruise at FL350 autoflt, both VNAV and LNAV, engaged, given pilot's discretion to FL240. 24000 ft set in mode selector panel altitude window, many mi prior to top of descent. Aircraft was in steady state cruise at FL350 with 'altitude hold' displayed.

Original NASA ASRS Text

Title: THE ACR FLC OF A B737 INCREASES THEIR AIRSPD AND CHANGES THE ASSIGNED ALT TO A LOWER ONE IN COMPLIANCE WITH A CTLR'S CLRNC. THE ACFT STARTED A SLOW CLB AS IT ACCELERATED, EVEN THOUGH THE 'ALT HOLD' MODE WAS, APPARENTLY, STILL ENGAGED.

Narrative: LEVEL CRUISE FLT, ASKED BY ATC TO ACCELERATE TO 300 KTS AND GIVEN PLT'S DISCRETION TO FL280. ACFT WAS ENGAGED IN VNAV AND LNAV MODE WITH FL280 SET IN THE ALT SELECTOR AND 300 KTS IN THE SPD SELECTOR. AS THE ACFT WAS ACCELERATING FROM .74 MACH TO 300 KTS IT STARTED A 600 FPM CLB, BUT WAS NOT NOTICED, SINCE THE ALT ALERT DID NOT SOUND UNTIL FL359 HAD BEEN REACHED. ACFT WAS RETURNED IMMEDIATELY TO FL350 AND SOON THEREAFTER DSNDED TO FL280. WHEN ANY CHANGE IS MADE IN SPD OR ALT BE ESPECIALLY ALERT THAT VNAV DOES NOT DISENGAGE. ALTHOUGH IN THIS OCCURRENCE MAINT WAS NOTIFIED BECAUSE IT DID REMAIN ENGAGED. SUPPLEMENTAL INFO FROM ACN 377680: IN CRUISE AT FL350 AUTOFLT, BOTH VNAV AND LNAV, ENGAGED, GIVEN PLT'S DISCRETION TO FL240. 24000 FT SET IN MODE SELECTOR PANEL ALT WINDOW, MANY MI PRIOR TO TOP OF DSCNT. ACFT WAS IN STEADY STATE CRUISE AT FL350 WITH 'ALT HOLD' DISPLAYED.

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.