Narrative:

Request review of message number XXXX which announces deactivation of ground proximity inhibit switch on B767's. Failure of captain's air data computer causes bad inputs to GPWS even after bootstrapping air data computer. On B757's and on B767's with deactivated inhibit switch for GPWS, no method exists short of pulling GPWS circuit breakers to quiet invalid GPWS warnings on approach with failed captain's air data computer, even after bootstrapping, so GPWS input from left air data computer must be upstream of captain's air data selector switch selecting alternate. Why not insert a procedure in operating manual for failure of left air data computer which includes inhibition of GPWS, and instead of deactivating the inhibit switch on B767's, why not reactivate this switch on B767's and install it on B757's to enable pilots to quiet incorrect GPWS warnings after failure of left air data computers? Callback conversation with reporter revealed the following information: the reporter did not know why the company disconnected the function of the GPWS inhibit switch. The company has not yet replied to his debrief of the incident. On takeoff the crew had lost the captain's side of his panel due to the air data computer failure, with the airspeed indicator, rate of climb and altimeter information being lost. The crew used the 'bootstrap switch' to obtain information from the first officer's side for the PIC's use. The crew then discussed what was going to be affected on their approach when they returned for an approach to mia. They were unpleasantly surprised to receive the loud GPWS alert. The captain said it was impossible to pull the circuit breakers while on approach at 1000 ft. The B757 is believed to not have this switch at all and the captain thought that his incident was a graphic case of why those aircraft should have a GPWS inhibit switch installed instead of just having all of the B767 inhibit switches being disconnected. The aircraft design feature of the GPWS alert sensor being upstream of the alternate source switch does not then divert or avoid the alert signal when using the alternate source selector.

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Original NASA ASRS Text

Title: AN AIRLINE CAPT RPT REGARDING THE ACR'S DEACTIVATION OF THE GND PROX INHIBIT SWITCH ON THEIR B767'S. THIS SIT BECOMES AN EVENT WHENEVER THE CAPT'S AIR DATA COMPUTER MALFUNCTIONS. THEN, THE ONLY WAY TO STOP THE GPWS ALERT WHEN AN UNWANTED SIGNAL IS HEARD, IS TO PULL THE CIRCUIT BREAKERS. CAPT REQUESTS ACR TO INCLUDE THE INHIBITING OF THE GPWS FOR ANY AIR DATA COMPUTER FAILURES AND THE PARALLEL RESTORATION OF THAT SWITCH. HE CITES A DEFICIENCY IN THE B757 FLEET OF ANY INHIBITOR SWITCH.

Narrative: REQUEST REVIEW OF MESSAGE NUMBER XXXX WHICH ANNOUNCES DEACTIVATION OF GND PROX INHIBIT SWITCH ON B767'S. FAILURE OF CAPT'S AIR DATA COMPUTER CAUSES BAD INPUTS TO GPWS EVEN AFTER BOOTSTRAPPING AIR DATA COMPUTER. ON B757'S AND ON B767'S WITH DEACTIVATED INHIBIT SWITCH FOR GPWS, NO METHOD EXISTS SHORT OF PULLING GPWS CIRCUIT BREAKERS TO QUIET INVALID GPWS WARNINGS ON APCH WITH FAILED CAPT'S AIR DATA COMPUTER, EVEN AFTER BOOTSTRAPPING, SO GPWS INPUT FROM L AIR DATA COMPUTER MUST BE UPSTREAM OF CAPT'S AIR DATA SELECTOR SWITCH SELECTING ALTERNATE. WHY NOT INSERT A PROC IN OPERATING MANUAL FOR FAILURE OF L AIR DATA COMPUTER WHICH INCLUDES INHIBITION OF GPWS, AND INSTEAD OF DEACTIVATING THE INHIBIT SWITCH ON B767'S, WHY NOT REACTIVATE THIS SWITCH ON B767'S AND INSTALL IT ON B757'S TO ENABLE PLTS TO QUIET INCORRECT GPWS WARNINGS AFTER FAILURE OF L AIR DATA COMPUTERS? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR DID NOT KNOW WHY THE COMPANY DISCONNECTED THE FUNCTION OF THE GPWS INHIBIT SWITCH. THE COMPANY HAS NOT YET REPLIED TO HIS DEBRIEF OF THE INCIDENT. ON TKOF THE CREW HAD LOST THE CAPT'S SIDE OF HIS PANEL DUE TO THE AIR DATA COMPUTER FAILURE, WITH THE AIRSPD INDICATOR, RATE OF CLB AND ALTIMETER INFO BEING LOST. THE CREW USED THE 'BOOTSTRAP SWITCH' TO OBTAIN INFO FROM THE FO'S SIDE FOR THE PIC'S USE. THE CREW THEN DISCUSSED WHAT WAS GOING TO BE AFFECTED ON THEIR APCH WHEN THEY RETURNED FOR AN APCH TO MIA. THEY WERE UNPLEASANTLY SURPRISED TO RECEIVE THE LOUD GPWS ALERT. THE CAPT SAID IT WAS IMPOSSIBLE TO PULL THE CIRCUIT BREAKERS WHILE ON APCH AT 1000 FT. THE B757 IS BELIEVED TO NOT HAVE THIS SWITCH AT ALL AND THE CAPT THOUGHT THAT HIS INCIDENT WAS A GRAPHIC CASE OF WHY THOSE ACFT SHOULD HAVE A GPWS INHIBIT SWITCH INSTALLED INSTEAD OF JUST HAVING ALL OF THE B767 INHIBIT SWITCHES BEING DISCONNECTED. THE ACFT DESIGN FEATURE OF THE GPWS ALERT SENSOR BEING UPSTREAM OF THE ALTERNATE SOURCE SWITCH DOES NOT THEN DIVERT OR AVOID THE ALERT SIGNAL WHEN USING THE ALTERNATE SOURCE SELECTOR.

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.