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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.
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.