Safair B733 near Cape Town on Sep 10th 2013, failure of primary flight instruments

Last Update: May 27, 2015 / 17:28:01 GMT/Zulu time

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Incident Facts

Date of incident
Sep 10, 2013

Classification
Incident

Airline
Safair

Departure
Cape Town, South Africa

Destination
Port Elizabeth, South Africa

Aircraft Registration
ZS-SMJ

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

A Safair Boeing 737-300 on behalf of South African Airways, registration ZS-SMJ performing freight flight SA-6836 from Cape Town to Port Elizabeth (South Africa) with no passengers and 3 crew, was climbing through about FL170 when all primary flight instruments failed. The crew stopped the climb at FL210 and returned to Cape Town for a safe landing on runway 01 about 25 minutes later.

On Nov 13th 2013 South Africa's Civil Aviation Authority reported the aircraft on a scheduled cargo flight from Cape Town to Port Elizabeth returned because of the failure of the primary flight instruments consistent with the failure of a central air data computer. An investigation has been opened.

Media in South Africa had reported in September the cargo flight returned because of smoke in the cockpit, an information that The Aviation Herald could not substantiate at the time.

On May 27th 2015 South Africa's Civil Aviation Authority (SACAA) released their final report concluding the probable cause of the incident was:

Electrical system failure resulting in the aircraft returning to Cape Town, however the cause of the failure of the incident could not be determined

The SACAA reported that the aircraft was climbing through FL170 out of Cape Town when several primary flight instruments at the Captain's panel failed. The crew continued the climb. When the aircraft climbed through FL210 the captain smelt some burning and noticed smoke emanating from behind and left of his seat, the captain identified the origin of the smoke at the circuit breaker panel, circuit breakers C498 and C425 had popped. The crew declared emergency and initiated a return to Cape Town, the "smoke, fire or fume" checklist was carried out, three or four minutes later the smoke began to dissipate. The captain's primary instruments remained unserviceable for the remainder of the flight. The crew flew a "monitored approach", the first officer flew approach and ILS Cat 1 approach until minimums, 200 feet AGL, the captain then took over and completed the landing.

The SACAA described the tests in an attempt to identify the source of the problem and source of the smoke:
A contracted AMO (No. 001) carried out troubleshooting once the aircraft had landed at FACT. Technicians initially thought that the central air data computer auto-transformer T98 was the source of the burning smell and smoke. The circuit breakers C498 and C425 opened as designed, cutting off power to the affected circuit. It was suspected that the auto transformer became hot due to low resistance or a short circuit on the output 28 Vac. The T98 transformer was replaced with a new component but this did not clear the fault on the failed instruments. There was no evidence of smoke during the testing phase. The circuit breakers did not pop during the test of the original transformer or of the replacement transformer. Examination of the replaced transformer also showed no signs of overheating or burning, and there was no related burning smell. Despite extensive efforts, technicians were unable to identify or duplicate the source of smoke with certainty.

Further troubleshooting traced a potential problem to plug D4449P, which was part of the original Boeing digital flight data recorder (DFDR) wiring for altitude/ airspeed excitation. The 28 Vac output of transformer T98 provides the reference for the course and fine altitude synchro and the airspeed synchro outputs from the digital air data computer #1 previously used for the DFDR. The wires had been disconnected and tied back at the DFDR end to satisfy the requirements for a supplemental type certificate (ST00599SE). This was part of a DFDR upgrade involving the installation of a digital flight data acquisition unit. During the repair work, the wires were further isolated with end-capping and stowing as per Boeing standard practices to prevent a possible reoccurrence of the problem. Ground tests and isolation tests were carried out. The defect was no longer apparent and all systems operated normally. Technicians were unable to identify the source of smoke with any certainty, or duplicate it.

The P18 circuit breaker panel, the P6 circuit breaker panel, and the overhead P5 panel were opened to inspect for any signs of smoke damage, be it visibility or in terms of odour. None was found. Connectors in the P18 panel were removed and cleaned in case of any moisture or contamination. No defects were found. (It should be noted that these connectors had previously been removed during the initial investigation after the event.) The two circuit breakers (C498 and C425) that tripped during the flight were removed and sent to Megchem for extensive analysis.

The T98 transformer was returned to the AMO for inspection and investigation by the engineering department. No significant findings were noted. After the inspection, the transformer was load-checked for approximately six hours. No problems were observed, and there was no heat gain in the unit under normal load conditions.

The SACAA analysed: "Despite extensive efforts, technicians were unable to identify or duplicate the source of smoke with certainty. Thus the contributory factor for the return of the aircraft to FACT could not be determined. The aircraft was placed on continuous monitored service and the event has not recurred."
Incident Facts

Date of incident
Sep 10, 2013

Classification
Incident

Airline
Safair

Departure
Cape Town, South Africa

Destination
Port Elizabeth, South Africa

Aircraft Registration
ZS-SMJ

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

This article is published under license from Avherald.com. © of text by Avherald.com.
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