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Something To Squawk About

Air Safety Week, Oct 9, 2006

Was The Legacy Transponder Switched Off - Or Just Faulty With A Known Flaw?

In the aftermath of the Sept. 29, 2006 Brazilian collision between a 737- 800 of GOL and an Embraer Legacy 600 of ExcelAire, the two pilots of the Legacy have been widely accused in the press of not having their transponder on and squawking their allocated code at the time of the collision. They are also being accused of being northwest bound on the airway at an inappropriate altitude.

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On joining that airway, they should have descended from Flight Level 370 to FL 360, odd levels being inappropriate when on a northwesterly heading. The press reports have theorized that the two pilots had switched off their transponder so that they could deviate in altitude and track without those maneuvers being detected by Brazilian ATC radar. Why would they have needed to do that? Reporters claim that they might have been demonstrating features of the aircraft to its new owner, who was on board for the delivery flight. However, they may have some reasonable defenses in the form of other factors in play.

First, the crew was experiencing communications difficulties. Even though a crew may have planned to change levels at an airways intersection, normal practise is to first advise ATC when doing so. If comms fail, then the standard procedure is to maintain the last assigned level and trust that ATC will check and see that on your transponder's mode Charlie squawk and vector other traffic out of your way. Relaying through other airborne traffic is another means of overcoming spotty comms over vast tracts of the Brazilian jungle - or so we're told. Assertions that the Legacy crew diverted by thousands of feet from their assigned altitude may eventually be borne out by their flight data recorder. We have also picked up on a report that both of the GOL 737's recorders are quite badly damaged.

On the subject of whether the crew could have switched off their transponder, we suspect that this proposition is based on their "squawk" not being received by either/both of Manaus and Brasilia centers. It's believed that the crew has denied switching off their transponder. Might there be another answer?

Honeywell transponders have long had a fault that causes them to lapse into an idle or standby mode "if the crew takes longer than 5 seconds to change codes when using the rotary knob on the control unit". This is a quote from FAA Airworthiness Directive AD 2006-19-04 (replicated in a similar European Directive EASA AD 2005-0021). The AD expands upon a prior Jan. 27, 2006 Alert Service Bulletin from Honeywell. In comments to the Notice of Proposed Rulemaking, Embraer as makers of the Legacy, requested an extension of the compliance date. The effective date of the AD's Final Rule was 17 October 2006. The condition affected 1365 airplanes worldwide. The compliance action required by the AD was:

a. Within 14 days after the effective date of this AD, revise the Normal Procedures section of the applicable AFM to include the following statement:

"After completion of any 4096 ATC Code change (also referred to as Mode A Code), check the status of the transponder. If the transponder indicates that it is in standby mode, re-select the desired mode (i.e., the transponder should be in the active mode).

b. Within 18 months (of 17 Oct 06), replace the mode S transponder of the COM unit with a new or modified unit .....

Prima facie therefore, it would seem that replacement of the faulty unit could take up to 18 months and in the interim flight-crews were to double check the status light on their box after any code change (something that could easily be overlooked).

The potential impact of this fault was described in an earlier Aug. 2005 document that said: "This type of failure will increase ATC workload and will result in improper functioning of TCAS."

In other words, head two aircraft along the same airway in opposite directions at the same height and a collision is inevitable. The only thing that could avert that is an operating TCAS in one aircraft and an operating transponder (and preferably, but not necessarily, its associated TCAS set). Just one TCAS (the 737's) would have averted the collision by warning the GOL Flightcrew - as long as the Legacy's transponder had been operating.

A Swiss ATC document from SkyGuide dated as early as April 13, 2004 expressed great concern about the fault and its potential impact upon aircraft separation. In a later Skyguide Safety bulletin dated Sept. 11, 2005, a safety officer agonizes over why their SMS (Safety Management System) had failed to get action to rectify the anomaly.

At that stage, the final deadline for fixing the fault was May 2006, some nine months after the publication of the European AD. Recall that the FAA AD is dated Oct. 17, 2006 for effectivity, with compliance at some later date. You can read that SkyGuide Bulletin at: http://www.iasa.com.au/skyguide.htm. To see why this delay has possibly helped generate a collision, read also the two articles linked from http://www.iasa.com.au/offset.htm. The moral of the fatal Legacy incident, so far: it takes more than two operating transponders to sidestep collisions.

Government Industry

Something To Squawk About

Air Safety Week, Oct 9, 2006

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