Icing in Flight

Icing in the Climb and Cruise: The Invisible Threat

Why what you can’t see at FL330 still deserves your full attention

When most people think of aircraft icing, they picture the frosty buildup during taxi or take-off, but some of the most dangerous forms of icing happen far above the ground - in the cruise, well after the seatbelt signs are off.

Modern jets operate in high-moisture, high-altitude environments where ice crystal ingestion, probe icing and undetected airframe accumulation can all threaten flight safety. And the tricky part? It’s often invisible.

💧 Supercooled Liquid Droplets: the Classic Climb Threat

During climb - especially in layered frontal weather systems - aircraft can encounter supercooled water droplets. These remain liquid below 0°C and instantly freeze upon contact with cold aircraft surfaces.

Risk areas:

• Wings and stabilisers.

• Engine nacelles.

• Pitot-static probes and AOA vanes.

• Windscreens and wipers.

Even small accumulations can change stall margins, affect autopilot behaviour and/or degrade engine efficiency.

❄️ Ice Crystal Icing at Cruise: a Modern Jet’s Quiet Enemy

At higher altitudes, above weather systems and well below freezing, aircraft can still experience icing. But it’s different.

These are ice crystals, not supercooled droplets:

• They’re dry and don’t adhere at first.

• Once inside hot engine parts, however, they melt, refreeze and accumulate.

This can cause:

• Engine rollback.

• Flameout.

• Surge or stall.

• Sensor misreads (e.g. incorrect N1/N2 or SAT).

• Autopilot or automation misbehaviour due to probe icing.

🔎 Warning Signs of High-altitude Icing

• Erratic SAT readings or air data fluctuations.

• Unusual EPR/N1/N2 mismatches.

• Autothrottle hunting or throttles "stuck".

• Ice detection system alerts without visible ice.

• Reports of engine rollback or flameouts in nearby airspace

💬 It is important to remember that these symptoms can appear without visible moisture because ice crystals do not show up on surveillance systems like typical cloud moisture can.

🧊 System Protection - When It Fails

Modern aircraft are equipped with:

• Engine anti-icing.

• Wing anti-ice (bleed air).

• Pitot and probe heat.

• TAT probes with self-heating elements.

• Some aircraft even have automatic icing detection.

But:

• Crews sometimes delay activation of anti-ice until “visible moisture” is confirmed.

• Icing may form before systems are turned on manually.

• Late activation of engine anti-ice may be ineffective in ice crystal zones.

Case Study:

Some snow was forecast on our arrival with a cloud base not far from minimums. Moderate icing was forecast in the area of the approach. Extra fuel was carried in case of delays or a possible missed approach.

As we approached the destination, the weather looked about as expected but it was possible to commence an approach. On checking in with the approach controller we learnt that the cloud base was below that needed to make being visual realistic. We decided to delay the approach and monitor for improvement. 

Whilst holding we suddenly started to pick up some moderate ice. Anti-icing was immediately switched on and we asked ATC for a climb. Vectors were given and, during the climb, ice rapidly started to build up to the point where we felt we were in severe icing. We got into VMC at FL160 and reported the ice to ATC. The engines and airframe seemed normal, even with a couple of engine accelerations to be sure. At this point, the ice was visible on our winglets, and around windscreens.

There was no improvement in the weather so whilst out of icing conditions we decided to initiate a diversion. En-route we encountered more icing conditions and decided to climb a further 1000ft to try and get back to VMC. Shortly after initiating the climb, we were reminded of what ‘startle factor’ is. 

Nothing could fully describe the sound the crew described. There was an incredibly loud noise, and the airframe was vibrating to such an extent that it was hard to focus on anything else. An alert was triggered: ENG 1 high vibration with higher vibration indicated momentarily on engine 2 as well.

Our training kicked in and we went back to basics, fly the aircraft! We levelled at FL170 and actioned the QRH, suspecting ice. This was not successful at reducing the vibration levels so Engine 1 was run at idle (vibrations around 5 units). Engine 2 had normal vibration levels by this point without any intervention needed. Shortly after, fumes were perceived with a burning plastic smell, so we donned our oxygen masks, declared a MAYDAY, and completed the Smoke / Fumes / AVNCS Smoke checklist. 

Pack 1 was suspected to be the source, so this was turned off. The crew eventually completed a single engine approach and landed uneventfully, although the event was a strong reminder of the challenges that winter operations, and in flight icing in particular, can pose. 

High-altitude icing is silent, invisible, and can be quickly unforgiving. It respects no cruise altitude and no aircraft generation. But with the right anticipation, detection, and action, we can manage it before it becomes a threat. When being familiar with associated checklists such as the engine vibration and/or smoke/ fumes checklist we can further manage any escalation to a safe outcome. 

Low Temperature Corrections

As the winter months are approaching, this section aims to review the effect that low temperatures have on true altitude and highlight how this can be corrected for. 

Altimetry

ATC is responsible for providing altitudes corrected for temperature when under radar control, although flight crew retain the responsibility to ensure that all clearances issued by ATC are safe in respect of terrain and obstacle clearance (considering non-ISA conditions). Our chart supplier for example provides 2 MRCs for GVA to accommodate for this responsibility: one for temperatures between -8 degrees Celsius and 1 degree Celsius, and one for temperatures of 2 degrees Celsius and above. 

When to apply corrections and how

Temperature corrections should be applied when the surface temperature is -10°C or below. Unless otherwise specified, the elevation of the aerodrome in use is taken as the elevation of the altimeter source. Pending the use of approved automated cold temperature corrections by the FMS, when conducting an instrument approach and when required, these corrections must where applicable be applied to:

• To Decision Altitude (DA) and Minimum Decision Altitude (MDA).

• Minimum altitudes after passing the Final Approach Fix.

• To the Final Approach Fix. ATC must be informed when temperature corrections are applied at the FAF to ensure vertical separation with other traffic.

Ensure adequate knowledge on when to apply temperature corrections, pending the equipment on board your aircraft and the available approach type.

👥 Decision-Making Under Uncertainty

Winter flight planning often includes vague icing alerts that can lead to complacency or overreliance on surveillance system returns.

Best practice:

• Don’t wait for visual confirmation-go by conditions and likelihood.

• Use PIREPs and recent sector data.

• Monitor SAT, TAT, and EPR/N1 trends actively.

• Be ready to descend, deviate and/or isolate engines if signs develop.

High-Altitude Icing - Safety Pillars Summary

🧠 Mindset

Icing at cruise isn’t fiction. Trust the data and reports, not your eyes. Stay proactive, not reactive.

👥 People

Brief the crew on likely icing zones and handover strategies if automation becomes unreliable. Empower FO/PMs to call out anomalies without hesitation.

⚙️ Equipment

Keep anti-ice systems tested and active when required. Verify probe heat and air data stability regularly. Post-flight reports can help maintenance pinpoint subtle icing effects.

📋 Compliance

Follow SOPs on anti-ice activation based on temperature and moisture forecasts - not just visual indicators. Refer to QRH actions promptly if any signs of engine or probe icing arise.

⚠️ Risks

• Engine rollback or flameout.

• Erroneous probe readings leading to instrumentation/automation confusion.

• Stall margin degradation from airframe contamination.

• Unrecognised icing onset due to misinterpreted instruments.

📚 Learning

Several past incidents (real and fictional) show that misreading or delaying anti-ice activation led to emergency descents, engine shutdowns and/or automation disengagement. Most scenarios were recoverable; critically, all were preventable.

Final Word

Icing is silent, invisible, and unforgiving. It respects no cruise altitude and no aircraft generation, but with the right anticipation, detection and action, we can manage it-before it becomes a threat.

If you’re unsure, switch anti-ice on. No-one ever got in trouble for being too early.