<< >> NASA ASRS Report 403819, May 1998

A climbing L1011-500 experiences an altitude undershoot when the FMC does not capture the climb to altitude. Aircraft slowly descended until ATC controller corrected situation.

Attributes
ACN	403819
Time
Date	199805
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : tlh
State Reference	FL
Altitude	msl bound lower : 36500 msl bound upper : 37000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx
Operator	common carrier : air carrier
Make Model Name	L-1011-500 Series
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 80 flight time total : 10000 flight time type : 2000
ASRS Report	403819
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : undershoot non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation other

Narrative:

I was out of the cockpit (in the lavatory) when I felt a sharp pull up. Upon returning, saw captain adding full power to climb back to cruise altitude. He said aircraft was in last 400 ft of climb, autoplt on in LNAV mode when vertical speed command dropped out (as normally does during last 300 ft of altitude capture). Center had cleared him direct to fix, which he entered into FMS. However, prior experience has taught me, this overloads this old type of FMS while it computes new course. The black magic was in the process of retarding the autothrottles prior to capture and the pitch mode was in neutral. The aircraft started a slow descent when center asked for our altitude. There is no warning other than the 'C' chime on the altitude alerter, however those parameters were short-cut because we hadn't been within 250 ft of leveloff. The next chime is at 750 ft off selected altitude.

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Original NASA ASRS Text

Title: A CLBING L1011-500 EXPERIENCES AN ALT UNDERSHOOT WHEN THE FMC DOES NOT CAPTURE THE CLB TO ALT. ACFT SLOWLY DSNDED UNTIL ATC CTLR CORRECTED SIT.

Narrative: I WAS OUT OF THE COCKPIT (IN THE LAVATORY) WHEN I FELT A SHARP PULL UP. UPON RETURNING, SAW CAPT ADDING FULL PWR TO CLB BACK TO CRUISE ALT. HE SAID ACFT WAS IN LAST 400 FT OF CLB, AUTOPLT ON IN LNAV MODE WHEN VERT SPD COMMAND DROPPED OUT (AS NORMALLY DOES DURING LAST 300 FT OF ALT CAPTURE). CTR HAD CLRED HIM DIRECT TO FIX, WHICH HE ENTERED INTO FMS. HOWEVER, PRIOR EXPERIENCE HAS TAUGHT ME, THIS OVERLOADS THIS OLD TYPE OF FMS WHILE IT COMPUTES NEW COURSE. THE BLACK MAGIC WAS IN THE PROCESS OF RETARDING THE AUTOTHROTTLES PRIOR TO CAPTURE AND THE PITCH MODE WAS IN NEUTRAL. THE ACFT STARTED A SLOW DSCNT WHEN CTR ASKED FOR OUR ALT. THERE IS NO WARNING OTHER THAN THE 'C' CHIME ON THE ALT ALERTER, HOWEVER THOSE PARAMETERS WERE SHORT-CUT BECAUSE WE HADN'T BEEN WITHIN 250 FT OF LEVELOFF. THE NEXT CHIME IS AT 750 FT OFF SELECTED ALT.

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.