Procedural Drift at Saab 340 Operator Leads to Taxiway Excursion

Procedural Drift at Saab 340 Operator Leads to Taxiway Excursion

Saab 340A LV-BMD, operated by SOL Líneas Aéreas was damaged beyond economic repair in a taxiway excursion accident at Mendoza-El Plumerillo Airport, Argentina on 2 January 2013. There were no injuries among the 31 passengers and crew.

Argentina’s air accident investigation agency, the Junta de Investigación de Accidentes de Aviación Civil (JIAAC), issued their final report (in Spanish only) on 23 December 2015.

Saab 340 LV-BMD

Saab 340 LV-BMD

The crew initially attempted to start the right-hand engine.  This failed, but they were able to start the left-hand engine and then successfully start the right-hand engine after that.

Upon taxying, the Captain encountered difficulties in maintaining directional control of the aircraft and in particular when trying to turn right found the aircraft was in fact turning left.  The checklist was not consulted.  Asymmetric braking and asymmetric engine power were both attempted but proved ineffectual.  The aircraft left the paved surface of the taxiway and came to a halt 58 meters from the edge of the taxiway with both propellers contacting obstacles.

The Saab 340 uses hydraulic power for landing gear retraction, braking, nose wheel steering and flaps.  The investigation identified that the electric pump that powered the hydraulic system was not operating.  Consequently, the HYD warning on the central panel warning (CWP) was illuminated (although it does not appear to have been noticed)

The pump is controlled using a panel with three settings: OVRD / AUTO / OFF

Hydraulic Controls (Credit: via JIAAC)

Hydraulic Controls & Indications (Credit: via JIAAC)

The switch would normally be in the AUTO position (and the loss of directional control checklist, if followed, would have first checked this switch was in AUTO).

However, during post-accident interviews it emerged that an undocumented, but orally communicated and unnecessary practice had developed within the operator of setting the switch to OFF during engine starts. This was allegedly done with the intention of protecting the electronic system flight instrument system (EFIS) from electrical spikes.

While the JIAAC don’t comment, its possible that the initial right-hand engine failure to start may have distracted the crew from reselecting AUTO.

Procedural Drift / Practical Drift

The accident investigators comment of the phenomena of procedural drift / practical drift.

This is something we discussed in our article: ‘Procedural Drift’: Lynx CFIT in Afghanistan

One major study of an accident that featured drift was by Scott Snook, then of the US Army.  His book, Friendly Fire, examined the accident shoot down of two US Army Black Hawk helicopters on a peacekeeping mission in Iraq in 1994 by the US Air Force, and what he called ‘practical drift’.

Such drift occurs when group norms and practices start to deviate from formal procedures. In some cases this may be because procedures no longer match operational circumstances, however practicality can be a factor too.  These do not appear to have been factors here. Instead the practice seems simply to have been a misguided ‘improvement’.

In The Field Guide to Understanding Human Error Prof Sidney Dekker lists several potential reasons for procedural drift:

  • Rules or procedures are over-designed and do not match up with the way work is really done.
  • There are conflicting priorities which make it confusing about which procedure is most important.
  • Past success (in deviating from the norm) is taken as a guarantee for safety. It becomes self-reinforcing.
  • Departures from the routine become routine. Violations become compliant behaviour with local norms.

Safety Management System 

The JIACC note that the operator’s Safety Management System (SMS) did not detect this drift.

The JIACC question the effectiveness of the SMS but provide no further evidence on the functioning of the SMS so it’s not possible for us to discuss this aspect further.


Not for the first time the failure to follow the appropriate checklists has contributed to an accident:

The JIACC do not examine training or wider flight operations standards, so it’s not possible to discuss those aspects further ere.

Safety Recommendations

Thirteen recommendations are raised in the report,

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