H2F Weekly Mayday #15

WEEK 15

H2F brings you the ‘helicopter mayday of the week’ with a short accident report summary. I tell you what happened. You think more about why it happened. We all learn from it. Because that’s what accident reporting is for.

TITLE

VFR helicopter flight into deteriorating weather results in controlled flight into terrain (CFIT).

WHAT?

Fatal accident involving an AS355 helicopter conducting a private VFR flight from near Cranfield, UK, to Dublin, Ireland when it entered cloud in deteriorating weather conditions over Snowdonia mountain range, colliding with the east side of Rhinog Fawr mountain. All five occupants were fatally injured.

WHERE?

Snowdonia, Wales, at an elevation of 2,060 ft amsl, around 300 ft below the mountain summit. The aircraft was flying from a private site near Cranfield Airport to a private destination near Dublin Airport.    

WHEN?

Accident occurred on 29 March 2017. The helicopter departed at about 1045 UTC and impacted terrain shortly after disappearing into cloud near Snowdonia.    

HOW?

The pilot had planned a VFR route to Dublin. Initially, weather conditions were suitable for VFR flight, but deteriorated significantly as forecast over North Wales. The helicopter climbed to 3,000 ft before descending toward Snowdonia. A witness observed the helicopter enter cloud at approximately 2,500 ft. The aircraft continued descending under autopilot control with heading-hold and vertical-speed modes engaged and struck rising terrain while still under power and at cruising speed. Investigation found no evidence of mechanical failure.   

CONDITIONS?

The weather forecast indicated progressively worsening conditions along the route. Conditions in Snowdonia included low cloud, hill fog, rain, and reduced visibility associated with frontal weather systems. Cloud bases in the western parts of Snowdonia were forecast between 200–400 ft amsl with widespread hill fog and moderate turbulence. METARs from nearby RAF Valley reported broken cloud around 600 ft and periods of rain and drizzle. These conditions were unsuitable for safe VFR navigation through mountainous terrain.    

OUTCOME?

The investigation determined that:

• The helicopter impacted the mountain at approximately cruise speed, constituting a classic CFIT accident.    
• Flight instruments and autopilot were functioning normally.
• The autopilot was engaged in HDG, T/C, and V/S modes during a controlled descent.
• The helicopter entered cloud while approaching rising terrain.
• No evidence indicated an attempt to perform a 180° turn, climb, or terrain avoidance manoeuvre after entering IMC.
• The pilot likely remained unaware of the immediate terrain threat until impact.

WHY?

The pilot was qualified for VFR only and did not hold an instrument rating.  

Weather deteriorated along the route exactly as forecast, with extensive low cloud and hill fog over Snowdonia.  

The pilot continued toward worsening weather rather than diverting, landing, or turning back.  

After entering cloud, there appears to have been no attempt to execute the standard inadvertent IMC recovery procedure (180° turn).  

Although the helicopter carried GPS-based navigation aids, neither the Garmin GNS430 nor the installed applications provided active terrain warnings in the configuration being used.    

The investigation referenced previous research showing that pilots often continue a chosen course of action when weather deteriorates gradually, a classic example of plan continuation bias.  

HUMAN FACTORS / CRM LESSONS

This accident is particularly significant for its human factors because it demonstrates examples of:

• Plan continuation bias during gradually worsening weather.
• Inadvertent IMC leading to loss of situational awareness and terrain clearance.
• Over-reliance on automation, with the autopilot continuing a descent into rising terrain.
• The importance of establishing personal weather minima and diversion triggers before departure.
• Failures in decision-making processes related to the transition from VFR to IMC.
• The need to decide and act early when approaching the VMC/IMC boundary rather than waiting until cloud penetration occurs.  

REFERENCE?

UK Air Accidents Investigation Branch (AAIB). (2018). Airbus Helicopters AS355F1 Ecureuil II, G-OHCP, Rhinog Fawr, Snowdonia, 29 March 2017. AAIB Bulletin 3/2018, EW/C2017/03/02.

Note:

Accident reports selected from the following open source databases: US NTSB; UK AAIB; Flight Safety Foundation’s Aviation Safety Network; Australia’s ATSB. Ireland’s AAIU; Taiwan’s TTSB; France’s BEA; Spain’s CIAIAC. Germany’s BFU.