Aviation Investigation Report A98H0003
2.11 ACARS and VHF Communications Gap Anomalies
Anomalies with the aircraft communications addressing and reporting system (ACARS) were evaluated for any possible connection to the technical failure event that preceded the in-flight fire. The first indication of an ACARS anomaly was the non-recording of the ACARS tracker message at about 0031:18, more than 39 minutes prior to the detection of the unusual odour in the cockpit (see Section 18.104.22.168).
The two ACARS tracker messages that were expected at about 0031:18 and 0041:18 were not recorded by the ACARS system providers; however, the count update information obtained from subsequent ACARS messages shows that these two messages had been sent from the aircraft. The most plausible explanation is that the ACARS system logged onto another network, as would happen if the system that was initially being used became saturated. No data was available to confirm this hypothesis.
The communications system in the MD-11 incorporates redundancy, in that it has three VHF communication paths available. It would take a complete failure of the entire system before communication capability would be lost. This could happen with a failure of the digitally controlled audio system (DCAS); however, this eventuality is unlikely in that the DCAS worked normally after communication had been re-established. The only other circumstance that would completely disable the communications system would be the simultaneous failure of all nine push-to-talk switches; this is considered unfeasible. Therefore, it is highly unlikely that the communications gap was related to a technical failure.
The most plausible explanation for the 13-minute communications gap is that the pilots selected an incorrect frequency during the attempted frequency change between 0033:12 and 0033:21. The communication sequence leading up to this frequency change differed from previous and subsequent frequency changes in that following the first instruction by ATS to change to a different en route frequency, only a short, clipped, unintelligible communication was heard from SR 111. However, when the new frequency assignment was repeated by the controller, SR 111 immediately acknowledged in a normal manner by repeating the assigned frequency. The subsequent keying events on both VHF 1 and VHF 2 that were recorded by the FDR during the communications gap are consistent with attempts by the pilots to re-establish radio contact. As frequency settings are not recorded on the FDR, it is not known which frequency the pilots were attempting to use.
When the pilots eventually re-established contact, it was on an unassigned frequency, and they made no mention of a technical problem. There is no record of any subsequent radio communication difficulties until after the effects of the fire had started to also affect other aircraft systems, about 40 minutes later.
The loss of ACARS on VHF 3 at 0047:06 can be explained by the pilots switching VHF 3 from data mode to voice mode in an attempt to use this radio for communications. The message protocol data from the ACARS provider indicates that the pilots must have switched VHF 3 to voice mode at 0047:06 when the ACARS changed to the satellite mode of transmission; the pilots then switched VHF 3 back to data mode at 0104:14.
The odour in the cockpit of SR 111 was initially detected about 37 minutes after the start of the VHF communications gap, and about 24 minutes after communication had been restored. No connection could be established between the ACARS anomalies, the 13-minute VHF communications gap, and the ignition of the fire in the aircraft.
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