Inlet Barrier Filters + Icing = Engine Flameout x 2?

Poor weather, icing, and helicopters often do not go together very well. Many different accident investigations have proven this over the years.

The Norwegian HEMS crew of an H145 had a close call with two engines flaming out within the same mission, with quite a few interesting learning points! 💡

Most helicopters out there are not certified to fly in known icing conditions, and there are many good reasons for that.

We’ll discover some of those here, welcome to The Why Spotlight #3!

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Accident Overview

On November 20, 2021, a Norsk Luftambulanse AS H145 (LN-OOS) took off on a mission in the snowy Kongsvikdalen valley. The crew had to deal with challenging conditions: it was dark, and snow showers reduced the visibility.

After flying an approach to a location close to the scene, the crew decided to wait for the rescue party while in the hover, using its lights and noise.

After a 45 minute hover, and 20 minutes rotors running on the ground, the party in distress reached the helicopter, and the crew departed southeast-bound towards Kongsvika. The crew knew of a power line crossing the valley they were flying through, but while they were searching for it the left engine suddenly shut down.

The commander made a swift 180-degree turn and carried out an emergency landing. After landing, the right engine also unexpectedly shut down while idling.

Timeline of Events

The crew departed Harstad at 16:34 local, tasked with locating three hikers and their dog who had become stranded in the mountains.

Using Night Vision Goggles (NVG) in intermittent snow showers, the crew navigated towards the distressed party’s GPS coordinates.

Upon arrival, the team assessed that it was unsafe to fly closer due to reduced visibility and mountainous terrain.

As the crew hovered nearby and waited for the group to descend, the helicopter’s IBF CLOG TREND alert indicated that snow was beginning to clog the engine air inlet filter. The pilot followed SOP, opening a bypass door to ensure the engine received enough air.

The report highlights:

“The crew have explained that while they waited, they assessed several times whether they should leave the area. The varying intensity of the snowfall, however, meant they did not find an opening in the weather suitable to leave.”

Snow intensity fluctuated throughout the 45-minute hover, creating concerns over safe departure timing. It was a further 20 minutes of rotors running on the ground before departure.

Once the party boarded, the crew opted for a south-eastern route down Kongsvikdalen, where they expected better visibility:

While flying through the valley and searching for a nearby power line (which the crew was aware of), the helicopter’s left engine suddenly failed. This happened with high terrain on both sides of the helicopter:

As they hadn’t spotted the wire yet, the commander was not satisfied with attempting to climb on one engine. The crew managed to carry out a safe emergency landing after performing a 180° turn.

Once they were on the ground, with the right engine on idle, it also unexpectedly shut down without warning. The helicopter was airlifted by another helicopter for technical inspections several days later.

Investigation Findings

So what did the investigation list as the main findings for this occurrence? Let’s have a look:

🔸 No technical faults or issues with the helicopter were found.
🔸 Icing conditions were not forecast except for inside the snow shower clouds.
🔸 It took longer than the crew anticipated for the party in distress to reach the helicopter.
🔸 At some points, visibility was below the minima required.
🔸 Two of the passengers were not able to sit in a seat with a safety belt.
🔸 The IBF system did not fulfil the certification requirements for icing protection.
🔸 Under unfavourable weather conditions, significant amounts of ice can build up in the IBF system, and this ice can enter the engine unhindered.
🔸 The Norwegian coast is exposed to winter conditions that are conducive to icing in the IBF system.
🔸 The investigation has not found any technical faults with the engines.
🔸 The engines do not have an auto ignition function that can autonomously start them if they were to stop

The most critical one here is that both engines experienced flameout due to ice ingestion, a serious issue linked to the IBF system installed on LN-OOS.

While no mechanical faults were detected in the engines themselves, the investigation revealed that ice had accumulated within the IBF system while the helicopter hovered in a snow shower.

When the helicopter transitioned to higher power for takeoff, the accumulated ice was likely drawn into the engine, leading to the sudden flameouts.

A post-incident analysis by Airbus Helicopters corroborated the risk of ice build-up within the IBF system.

Flight tests in January 2022 showed that, under specific conditions common to Norway’s winter climate (temperatures near freezing and high humidity) significant amounts of ice could form in the IBF and be ingested by the engine.

The report further highlights:

“The flight testing showed that ice can form in the IBF system and that this ice can enter the engine unhindered. Especially on the lower side of the bypass grid when the bypass door is open. If the temperature varies and goes above 0 °C, the ice can quickly melt on the contact surfaces leading to dislodging of ice or slush.”

Here is a picture of what that looks like (picture taken from the underside of the IBF tray):

And another one:

The investigation concludes:

“Both the engines on LN-OOS most likely stopped due a flameout caused by the ingestion of ice. The investigation has shown that this ice most likely came from the IBF system. Flight tests conducted by Airbus Helicopters after the incident have revealed that under certain weather conditions, significant amounts of ice can build up in the IBF system and this ice can then enter the engine unhindered.”

What can we Learn from this?

🚨 Be Aware of the Threats of Flying in Winter

We’ve covered the threats of flying in winter extensively in the article linked below ⬇️

Familiarising yourself with these is important, especially if you fly across different seasons and regions with long periods of not having to deal with winter.

How to Survive Flying in Winter as an Aviator

❄️ Understand and Respect Icing Conditions

The definition of icing condition varies amongst different regulators, OPS manuals, and even pilots. Usually it’s a combination of temperature and visibility, but visibility isn’t always part of it.

Either way, open up that OPS manual and make sure you’re on the same page as your operator! 📖

💡 Know the Limitations of your Inlet Barrier Filters

After the incident, Airbus published a notice that prohibited flights in snowy weather for helicopters with the IBF installed. It also set out that flights should be avoided when the temperature is below 5 °C and there is visible moisture.

EASA then also published additional measures in an Airworthiness Directive (linking into our previous point).

Flight in the following environmental conditions is prohibited:

• Flight in falling snow and sleet with visibility due to snow/sleet less than 1500 m
• Hover or flight in blowing snow for longer than 1 minute
• Flight in fog/clouds when visibility is less than 800 m and OAT ≤ 5 °
• Flight in icing or ice crystals conditions

No matter how many different definitions we look at, no situation is the same – which makes this harder to deal with.

The supplement of your RFM will lay out the limitations of the installed IBF’s, which might be worth a review.

↩️ Always have a Plan B

Complacency is something all of us have to deal with to some degree at some point. It could just be the fact you’re in a job for a very long time, and have become very proficient at the day-to-day that could result in complacency.

It’s the basics that matter, like where the wind is on a given day that could make the difference between a successful or unsuccessful unplanned landing. In the case of the incident here, the commander was able to swiftly put the aircraft down. Had it taken a long time, who knows what the outcome of a 2nd engine flameout could have been.

We’ve covered complacency here:

Defensive Flying 101: Are you Vigilant or Complacent?

🗣️ Brief all the Threats

Most operations in aviation involve multiple people that all need to share the same mental model. The wire going across the valley was a huge threat for the crew that massively impacted their decision-making.

If that wire wasn’t briefed or present in all crew member’s mental model, imagine the confusion and potential danger that would’ve occured when shit hits the fan.

Threat and Error management aims to deal with this, which we’ve covered here:

Keeping it Simple: Threat and Error Management as a Pilot

Conclusion

Fortunately no lives were lost in this serious occurrence, but that could’ve been very different if the second flame-out had happened earlier. It’s important for pilots, especially those of us who fly in different seasons, to not become complacent about the threats of cold weather.

Hopefully this Why Spotlight can help crews refamiliarise themselves with those. For other accidents or incidents you’d like us to cover, please get in touch via the About section, or simply leave a comment!

References

Full investigation Report

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️