Clarity Emerges On Flight Level Assignments During The Legacy 600 And GOL 737 Collision Over Brazil
In scrutinizing the events that led up to the fatal midair collision over the Amazonian Rain Forest of a Legacy 600 with a GOL 737, it appears that between Brasilia and Manaus Air Traffic Control, nobody had the presence of mind to think through communications and radar difficulties, connect the dots and extrapolate the potential for the southeast bound GOL 737 to slam into the incommunicado Legacy 600.
GOL airlines Flight 1907, a new Boeing 737-800, collided with a Brazilian-made Legacy jet over the Amazon jungle on Sept. 29. The damaged Legacy landed safely at a nearby air force base, but the larger airliner crashed in dense jungle, killing all 149 passengers and six crew members. (See Oct. 23 ASW.) Of course, the wild card here is the aviation sector’s fierce dependence on technology, both in the aircraft and at the scope.
When both comms and secondary radar (i.e., the Legacy transponder’s replies) failed simultaneously yet temporarily, the probability of confliction greatly escalated. That is what happened, although the mechanics of timing and the precise actions being taken by ATC during the countdown to collision are still obscure.
It is nevertheless beginning to appear that the Brazilian military controllers had no timely and practical Plan B to cope with such a disabling event. Neither were the Rules of the Air adequate.
Were They On The Level?
What is clear is that the Legacy crew was correctly and logically flight-planned from turn-point Brasilia towards Manaus at the appropriate lower level of FL360. Countermanding this was the ATC clearance received and read back by the Legacy crew that they were cleared past Brasilia (to Manaus) at FL370 and that this was read back by the Legacy crew. That is believed to have been extracted from the ATC tapes.
If this hadn’t been the case, then the applicable rules, as set out in the Oct. 23 ASW, would have prevailed – and there would have been an obligation by the bizjet’s crew to revert to flying their flight plan’s Levels once radio contact had been overdue for seven minutes. That need was abnegated by their FL370 clearance limit being given as Manaus.
It isn’t unusual for crews to be cleared at (or request clearance at) ODLs (Opposite Direction Levels). It’s sometimes requested by pilots to optimize fuel burn. Aircraft may also be requested by an adjacent sector to cruise at an ODL, to resolve a traffic conflict.
The Brasilia newspaper Correio Brasiliense said Nov. 8 that a Brazilian Air Force source close to the investigation and who has seen and heard the recorder data, confirmed that the Legacy’s FDR indicates the transponder “jammed” (literal translation from the Portuguese word “travou” which can cover several other meanings including “went to standby”) shortly after overflying Brasilia and that this “impeded all radio transmission and reception”.
It’s not inconceivable that a defect in the transponder could affect long-range VHF radio communication through generation of an interfering harmonic, somewhat like the mike to earpiece feedback of a super-heterodyne squeal, but admittedly it would be a “first”, or at least a little known and even less appreciated phenomena. However, the Honeywell Primus range of transponders have gotten themselves a reputation for slipping off into standby. Supposedly, the Legacy’s transponder wasn’t from one of those bone-lazy batches – as established by its serial number.
The FDR also confirms their flight was stable at FL370 and not, as Cindacta primary radar recordings showed, varying between FL 320 and FL 420.
Communications Breakdown
The Legacy’s CVR recorded five attempts to contact Cindacta 1 (Brasilia) by radio and confirms that they were tuned to the correct Brasilia frequency. At the same time, Cindacta 1 was attempting to contact the Legacy, first requesting them to switch their transponder on and, when primary radar began showing the altitude variations, to follow its flight plan, equally without success.
Shortly before the collision, Cindacta asked on an open frequency for any aircraft in the vicinity to contact the Legacy. There was no reply until after the collision, when the Legacy switched to emergency frequency and radio and transponder resumed functioning. This resumption of normal operation might support contentions that the VHF frequency and transponder were affecting each other as a function of the prior VHF frequency and a “normal” ATC squawk.
Note that an emergency transponder squawk is also different in that it continuously squawks IDENT, and the return is thus enhanced on the controller’s screen. Consequently, the nature of its interrogation reply is electronically different. When ATC determines that a transponder is inoperative, squawking the wrong code or generating a duff flight level, the operator will normally ask the pilot to “recycle his transponder”.
That usually fixes the glitch. However, when comms are also out, everybody’s cast into a silent world of hurt. Exactly when the Legacy crew should have selected the NOCOMM squawk of 7600 is a very subjective argument, yet it’s one that they could easily lose. If selecting 7700 later, after the collision, restored their “squawk”, it’s likely that an earlier routine “lost comms” squawk of 7600 may have done the same thing.
The GOL collision may prove to be a signal event and an object lesson to pilots worldwide that they should not delay their switch to code 7600 whenever comms are in doubt. On the subject of comms failure, three aspects should be noted:
a. The Legacy crew heard no inputs from other airborne aircraft that were asked to relay. It is unusual for both pilots in an airplane to lose reception and transmission capabilities unless all sidetone disappears (indicating a blown fuse in a transceiver). This would indicate that they were being jammed, or were jamming themselves. It’s possible, and not entirely improbable, that the Legacy has a radio mute button that was being used to cut down on ATC chatter, or had an open mike caused by a faulty transmit button staying in.
b. The comms loss began above Brasilia, which may well be an electromagnetically “dirty” place from a frequency spectrum viewpoint. It’s not uncommon for aircraft radios to “play up” when in proximity to population centers. Whether this could cause a relay to stick, the radio to oversquelch or the transponder to go to standby? Squelch is an automated suppression of the hissing atmospherics in the background carrier wave. Oversquelch is an unwanted weakening of incoming signals at the receiver (aka, a loss of “gain”). A strong or highly directional radiated signal emitted by an unknown source from Brasilia could have caused a VHF Receiver to back off to an oversquelch condition.
Squelch typically affects a receiver only. However, if all sidetone is being squelched and pilots can’t hear themselves “going out”, they’ll assume that they aren’t “getting out” – and give up attempts at transmitting. Changing frequency will normally reset the squelch and restore reception, so oversquelch is a strong possibility and candidate for the basic cause of their nocomm.
c.As a new aircraft, the Legacy may have had an EMI vulnerability (electromagnetic interference). It’s a naturally occurring phenomenon that occurs when the electromagnetic field of one device disrupts the electromagnetic field of another device by coming into proximity with it, or through radiating a carrier that spectrally interferes.
The transponder is quite a quintessential piece of kit. If it’s not operating, then collision avoidance (TCAS) isn’t on line. However, shouldn’t there be an onboard attention-getting warning of that fact? Unfortunately, the failure or intermittency of a transponder is always poorly conveyed to the pilot.
Airline pilots worldwide are hoping that the long-advocated lateral track offsets on upper-level airways will be an outcome of the GOL Flt 1907 downing. The consensus seems to be that opposite direction traffic should each dial up a side step (to the right) of a mere 0.1nm into their Flight Management Computers. All could then rest easy that, even if ATC (or a comms loss or level “bust”) was to set up a similar scenario, the two opposite direction aircraft would be laterally separated in passing by 1,200 feet horizontally. For details of that advocacy and suggested procedure, go to: http://www.iasa.com.au/offset.htm.
The Brazilian Air Force is essentially in charge of the investigation of its own personnel and ATC system. The public pronouncements made thus far by public and military officials are understandably stemming from the angst generated by the loss and the underlying mystery. However, the statements haven’t helped. The two captive Legacy pilots remain bailed up in a Rio de Janeiro Hotel, afraid to go out in public and obviously concerned about the prospects for criminalization of the investigation. Airline pilots organizations worldwide are watching these developments with some understandable trepidation.
Ultimately it will come down to a two-pronged bottom line. A man in Honeywell overalls brandishing a multi-meter in the bowels of the Legacy will arise and say “eureka”, and an overworked air traffic controller will be reprimanded for erroneously issuing an inappropriate clearance limit. It won’t be the last time that a puny technical glitch and a minor human error link up to kill en masse.