Sol SF34 at Mendoza on Jan 2nd 2013, veered off taxiway
Last Update: December 29, 2015 / 19:43:59 GMT/Zulu time
The airline confirmed that the aircraft was taxiing for departure when the aircraft veered off the taxiway due to a mechanical malfunction. There were no injuries, the airport continued to operate normally.
Passengers reported the aircraft started to violently move around with a deafening noise and went off the taxiway. The nose gear appeared seriously damaged.
On Mar 6th 2013 Argentina's Accident Investigation Board (JIAAC) indicated the occurrence was rated an accident, an investigation (number 8313) has been opened, no further details were provided.
On Mar 21st 2013 The Aviation Herald learned that the airframe has been written off due to repair cost exceeding the commercial value of the aircraft. The aircraft sustained substantial damage to the landing gear, both engines, both propellers as well as parts of the left hand propeller penetrated the fuselage. The aircraft is currently being parted out for spares to keep other aircraft operating.
On March 22nd 2013 the JIAAC released a brief preliminary report in Spanish dated Mar 7th 2013 reporting, that during taxi for departure the captain felt there was no control over the nose wheel steering, the aircraft exited the taxiway to the left, the left propeller struck a pillar, the right main gear went over a sewer causing the right propeller to strike ground. The aircraft came to a stop 20 meters off the edge of the taxiway and was evacuated.
On Dec 29th 2015 the JIAAC released their final report in Spanish concluding the probable causes of the accident were:
While taxiing to the holding point of the active runway for an air transport flight the 8R-5420 flight crew lost control of the aircraft trajectory causing the aircraft to veer off the taxiway, which resulted in impact of the aircraft with objects in the field. This was due to the combination of following factors:
- the electric pump controlling the hydraulic pressure was not in operation causing insufficient hydraulic pressure.
- the low hydraulic pressure warning was not recognized by the crew
- the crew could not control the trajectory of the aircraft due to lack of nose wheel steering
- informal exchange of (editorial note: false) information about the operation of the aircraft systems between flight crew
The JIAAC reported that due to informal exchange between flights crew of the operator a practise had been established to shut down the electric pump controlling the hydraulic pressure prior to engine shut down and to turn the pump back on after engine start. This however was in contradiction to the design of the hydraulic system and the manuals.
The first officer (32, CPL, 1,818 hours total, 110 hours on type) was assigned to be pilot flying, the captain (54, ATPL, 7,000 hour total, 600 hours on type) was assigned to be pilot monitoring. As nose wheel steering was available only at the captain's side, the captain would taxi the aircraft to the runway then hand control to the first officer.
During the startup for the accident flight the crew attempted to start the right hand engine, the start however failed. The crew thus decided to start the left hand engine first, which went without problem. Then the crew attempted a second start of the right hand engine, which now succeeded. The crew requested and received taxi clearance. Beginning taxi the captain noticed difficulties in maintaining directional control of the aircraft and decided, that he would also fly the takeoff, there was no other reaction of either pilot to the control difficulty.
When the captain attempted to initiate a right hand turn he found that he had no directional control whatsoever, quite the opposite, the aircraft showed a tendency to the left. The captain attempted to use asymmetric braking but found the brakes were ineffective. The captain now attempted to maintain directional control by applying asymmetric engine power increasing power on the left hand engine, however to no avail. The aircraft veered left completely out of control and departed the paved surface of the taxiway despite application of beta range (reverse power). The left propeller impacted a concrete block and the right hand propeller the concrete wall of a sewer drain, the aircraft came to a stop 58 meters from the edge of the taxiway.
The occupants evacuated safely, there were no injuries. The aircraft received substantial damage.
The JIAAC reported the flight data recorder revealed the electric pump controlling the hydraulic pressure was not in operation at any time since the engine start up until impact with the obstacles. The low hydraulic pressure indication was active during that entire time. Taxi commenced at 17 knots, using maximum braking the aircraft could have been stopped within 25 meters from that speed.
The JIAAC reported that the low hydralic pressure warning consists of a visual indication as well as an aural warning. The cockpit voice recorder leaves the activation of the aural signal "oblivious". The cockpit voice recorder confirmed that the crew completed the "before engine start" checklist.
The JIAAC reported that the electric pump is controlled by a switch with 3 positions: OVRD, AUTO and OFF, the switch would normally lock the switch into the AUTO position. Before the investigators arrived on scene, that switch had been manipulated. The engineer reported he placed the switch into OVRD and the pump was working normally. Normal position would be AUTO, and the pump during a subsequent test worked normally with the switch in the AUTO position.
The JIAAC reported that through information from colleges the captain learned to turn that switch into the OFF position prior to engine shut down and keep it in that position until both engines had been successfully started. This procedure was in contradiction to the AOM which explicitely said the switch should never be in the OFF position except in emergency.
The JIAAC reported that in case of encountered steering control difficulties the related checklist as first item queries the position of that switch to be AUTO. However, that checklist was never referred to and was never worked.
The JIAAC analysed that the basic fact identified by the investigation was, that the crew began taxi of the aircraft despite low hydraulic pressure causing the loss of nose wheel steering and loss of braking. The investigation thus focussed on how this scenario did happen.
The investigation thus discovered that it was common practise amongst the operator's flight crews to shut the electric pump down prior to engine shut down and keep it shut down until both engines had been started and were running stable. This procedure, orally transmitted between flight crew, supposedly prevented damage of the EFIS screens during engine startup and resulting power spikes. The flights crew described the procedure as "standard operating procedure by oral tradition".
The AUTO position of that switch at all times, except in emergency, is regarded as highly important, the switch therefore is normally locked into the AUTO position. The switch is supposed to be put into the OVRD or OFF position only in case of an emergency.
In addition, the JIAAC analysed, the crew did not consult the related checklist after noticing nose wheel steering control issues, the first item of which would have been to verify that switch to be in the AUTO position.
The visual indication of the low hydraulic pressure was active in the center warning panel. The cause why the aural alert concerning the low hydraulic pressure could not be heard on the cockpit voice recorder could not be determined, the crew reported that the button to inhibit non-essential alerts for takeoff (Takeoff Inhibit) had not been activated. The investigation was unable to determine why the crew was not attracted by the warning light.
The operators safety management system (SMS) did not detect/pick up that drift procedure, that developed out of daily routine operation. One of the main tasks of the SMS is to monitor routine and detect drifting off the standard operating procedures. The non-detection of the drift procedure thus puts the effectiveness and efficiency of the operator's SMS into doubt.
In this case the drift procedure was in contradiction to the system design and requirements as well as was not covered by checklists.
ICAO requirements demand a safety area of 47.5 meters around taxiways, however, at Mendoza the obstacles (concrete blocks for the ILS system) were only 34.75 meters from the edge of the taxiway.
The JIAAC analysed that it is of considerable gravity and extremely disturbing that evidence following the accident was not preserved, in particular the switches were moved before the arrival of the investigators contrary to regulations.
This article is published under license from Avherald.com. © of text by Avherald.com.
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