Flight MH370/2014-03-08/02-25-34 MYT: Difference between revisions
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Revision as of 11:15, 17 August 2026
Event Summary
The SATCOM link becomes available (for both voice and data - Class 3) once more and normal SATCOM operation resumes (except that there is no Data-2 ACARS traffic).
No Flight ID was sent to the GES during the Log-On. This implies that the SDU stopped receiving a valid Flight ID from the AIMS at sometime between 1642:04 and 1825:00.
The possible reasons for the link loss and the subsequent Log-On that took place at 1825:00 have been investigated and are detailed in Table 2.5A. There are many quite complicated scenarios that could have caused the 1825:00 Log-On. However, the most likely reason is a power interrupt to the SATCOM avionics, of a duration greater than 22 minutes (the time between events 7 and 9) and less than 78 minutes (the time between events 6 and 9).
Event Details
Safety Investigation Report MH370/01/2018 Satellite Ground Station Logs - Key Observations
SATCOM Log-On, successfully completed.
- The SATCOM link becomes available (for both voice and data - Class 3) once more and normal SATCOM operation resumes (except that there is no Data-2 ACARS traffic).
- No Flight ID was sent to the GES during the Log-On. This implies that the SDU stopped receiving a valid Flight ID from the AIMS at sometime between 1642:04 and 1825:00.
- The possible reasons for the link loss and the subsequent Log-On that took place at 1825:00 have been investigated and are detailed in Table 2.5A. There are many quite complicated scenarios that could have caused the 1825:00 Log-On. However, the most likely reason is a power interrupt to the SATCOM avionics, of a duration greater than 22 minutes (the time between events 7 and 9) and less than 78 minutes (the time between events 6 and 9).
- The GES recorded an abnormal BFO for the SATCOM Log-On Acknowledge transmissions (Sections 1.9.5 para. 5 and 2.5.3).
- 1825:00 Log-On Acknowledge - Most likely due to the power-on drift of the Oven Controlled Crystal Oscillator (OCXO), thus endorsing the belief that the 1825:00 Log On was preceded by a lengthy power interrupt.
An OCXO provides a stable reference frequency for the SDU Radio Frequency (RF) transmit and receive circuits and also for SDU modem timing. Within the OCXO, a regulated oven keeps the crystal at an almost constant temperature if the ambient temperature in the crown area is between the ranges -55oC up to above +70oC. The oven also contains extra electrical regulation and isolation to ensure frequency accuracy and stability. The OCXO includes an oven ready flag, which triggers the Log-On initiation when the OCXO reaches its operating temperature. Extensive laboratory testing has revealed that during warm up, the OCXO frequency may vary non-linearly with time, but then settles with almost negligible variation. At power-on, the OCXO can exhibit either a rising or falling frequency gradient, before decaying over time to its normal steady state value. The testing has indicated that reasonable stability (within 2Hz/minute) is typically reached by around five minutes after an initial peak or overshoot. The testing has also shown that there can still be a significant frequency offset at the time that the oven ready flag initiates the Log-On process, so the Log-On request, Log-On Acknowledge and subsequent data bursts can all exhibit significant frequency offsets.
Notes