NTSB · Safety Recommendation
Safety Recommendation A-10-060
Recommendation
TO THE FEDERAL AVIATION ADMINISTRATION: Require Honeywell to revise the software logic within the MK XXII Enhanced Ground Proximity Warning System so that a fault in the radio altimeter system would not prevent the look-ahead feature from functioning without notification to the pilot.
Safety Issue and Context
During recent investigative testing, the National Transportation Safety Board (NTSB) learned that a failure within a helicopter’s radio altimeter system could prevent the look-ahead feature of a Honeywell MK XXII Enhanced Ground Proximity Warning System (EGPWS) from functioning during flight and that the pilot would possibly be unaware that this feature within the system is not operational. The look-ahead feature provides visual and aural terrain avoidance alerts by comparing the aircraft’s projected flightpath to a database containing terrain and obstacle information.
The absence of these alerts, when the pilot does not know the alerts are not functioning, could mislead the pilot thereby significantly reducing the safety of flight.
Response History
March 6, 2012 · NTSB Correspondence · FAA
On October 26, 2010, Honeywell released a modification that corrects the software logic problem with the MK XXII EGPWS and is available at no charge to owners of these systems. This release completes the action recommended; consequently, Safety Recommendation A-10-60 is classified CLOSED—ACCEPTABLE ACTION.
November 22, 2011 · Recipient Response · FAA
-From J. Randolph Babbitt, Administrator: Honeywell has released publication number D201006000021, dated October 26, 20 10, to provide a software modification that corrects the software logic at no charge to MK XXII EGPWS owners. According to Honeywell, the new logic considers only the radar altitude valid signal and Honeywell has added the Ground Proximity Warning System (GPWS) inoperable (INOP) indication whenever radio altitude is invalid during flight below 1200 feet above ground level (agl). Thus, this new modification also corrects the issue for helicopters equipped with a weight-on-wheels switch.
January 11, 2011 · NTSB Correspondence · FAA
The NTSB disagrees with the FAA’s statement that “Helicopters equipped with a weight-on-wheels switch are not affected by this problem because the current logic configuration for the MK XXII EGPWS utilizes weight-on-wheels information instead of radio altimeter to transition from ground to flight mode. This transition enables the aural and visual warnings.” The NTSB believes that helicopters equipped with weight-on-wheels switches are also subject to the problem that this recommendation addresses.
Nevertheless, the NTSB is pleased to learn of Honeywell’s issuance of a service information letter (SIL) stating that it plans to provide a free software modification in the fourth quarter of 2010 that corrects the software logic problem that is the subject of this recommendation. Pending completion and release of this modification, Safety Recommendation A-10-60 is classified OPEN -- ACCEPTABLE RESPONSE.
June 29, 2010 · Recipient Response · FAA
MC# 2100239 - From J. Randolph Babbitt, Administrator: We agree a logic change in the MK XXII EGPWS software is necessary to correct the potential for unannunciated failures of the EGPWS look-ahead alerting function. Honeywell issued a service information letter (SIL), Honeywell Publication Number D20100400029, on April 16, 2010, stating they plan to provide a free software modification in the fourth quarter of 2010 that corrects the software logic problem. The Honeywell SIL can be found online at: http://www51.honeywell.com/aero/commonldocuments/egpwsdocumentslMaintenance-documents/SIL-documentsIMKXXII-documentsl D20 1004000029.pdf.
In this SIL, Honeywell states the EGPWS will experience an unannunciated loss of the Terrain Awareness Warning System (TAWS) aural alerting function if the radio altimeter is in either the no computed data (NCD) state or the radio altimeter data invalid state. The EGPWS terrain display will still be active. If the radio altimeter transmits valid data after takeoff, the TAWS aural alerts will become fully functional for the flight even if the radio altimeter encounters an NCD condition.
Helicopters equipped with a weight-on-wheels switch are not affected by this problem because the current logic configuration for the MK XXII EGPWS utilizes weight-on-wheels information instead of radio altimeter to transition from ground to flight mode. This transition enables the aural and visual warnings.