<< >> NASA ASRS Report 398690, April 1998

B757 aircraft had inoperative left inertial reference unit which made GPWS inoperative. Flight crew thought the TCASII was still working, though the ground check failed, but with GPWS inoperative the TCASII is inoperative. MEL doesn't reflect that and reporter flight crew says the MEL should be updated.

Attributes
ACN	398690
Time
Date	199804
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : bos
State Reference	MA
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	ground : preflight
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
ASRS Report	398690
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	398689
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Chart Or Publication
Air Traffic Incident	other
Situations
Publication	Unspecified

Narrative:

Location: 30 DME west of bos. Aircraft arrived with inoperative left inertial reference unit. GPWS was placarded inoperative in accordance with MEL. During ground checks TCASII system check failed. Captain and I misread MEL and assumed that TCASII was ok, just failed check due to left inertial reference unit off and GPWS inoperative. Maintenance advised that the TCASII is disabled by the GPWS being turned off, but the MEL doesn't address this fact or require a placard. Suggest the MEL be updated to reflect TCASII loss with left inertial reference unit failure, and GPWS off, with placard warning added. We made an erroneous assumption and should have had the TCASII placarded before we departed.

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Original NASA ASRS Text

Title: B757 ACFT HAD INOP L INERTIAL REF UNIT WHICH MADE GPWS INOP. FLC THOUGHT THE TCASII WAS STILL WORKING, THOUGH THE GND CHK FAILED, BUT WITH GPWS INOP THE TCASII IS INOP. MEL DOESN'T REFLECT THAT AND RPTR FLC SAYS THE MEL SHOULD BE UPDATED.

Narrative: LOCATION: 30 DME W OF BOS. ACFT ARRIVED WITH INOP L INERTIAL REF UNIT. GPWS WAS PLACARDED INOP IN ACCORDANCE WITH MEL. DURING GND CHKS TCASII SYS CHK FAILED. CAPT AND I MISREAD MEL AND ASSUMED THAT TCASII WAS OK, JUST FAILED CHK DUE TO L INERTIAL REF UNIT OFF AND GPWS INOP. MAINT ADVISED THAT THE TCASII IS DISABLED BY THE GPWS BEING TURNED OFF, BUT THE MEL DOESN'T ADDRESS THIS FACT OR REQUIRE A PLACARD. SUGGEST THE MEL BE UPDATED TO REFLECT TCASII LOSS WITH L INERTIAL REF UNIT FAILURE, AND GPWS OFF, WITH PLACARD WARNING ADDED. WE MADE AN ERRONEOUS ASSUMPTION AND SHOULD HAVE HAD THE TCASII PLACARDED BEFORE WE DEPARTED.

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.