One of the bogeys of winter and spring flying is the threat of helicopter icing conditions. Sean Reynolds recalls the value of a spot of hangar flying as a training tool and how it paid off one dark New Year’s Eve in Sarajevo.
This article, headlined “The Lollipop Effect “ was originally published in the February/March 2024 edition of RotorHub International. Apply for your complimentary subscription to RotorHub International to read more articles like this.
RAF Chinook pilots sharing critical icing experiences
When I arrived on a front-line RAF Chinook squadron in the mid-1990s, the crew room was for us, apart from the ops desk, the focal point of the organisation.
This is where we would congregate in the morning before the daily met brief, where we would relax and chat between sorties, and often where we would socialise at the end of the week.
Although the topics of conversation could be wide-ranging, the ones that I remember the most were the sharing of flying in icing conditions and anecdotes not covered in training guides or flight manuals. As such, it was a melting pot of collective learning and aviation safety education.
It was a conversation such as this, about an aircraft operating technique, which took place in the crew room of No 78 Squadron at RAF Mount Pleasant in the Falkland Islands, that was to have so much value more than 10 years later in Bosnia.
Dealing with snow: helicopter icing in cold weather operations
I was on my first operational detachment, and although we did not operate as constituted crews, you could only operate as the aircraft commander on task if you held a Combat Ready status.
Members of the 3rd Battalion, 22nd Parachute Regiment disembark a Chinook during Exercise BLIZZARD NORDIQUE. Image; Valcartier Imaging Services
Because I was still only Limited Combat Ready, I spent most of my time flying with the more experienced pilots down there. One of these pilots was Captain Dan, who was our Canadian Exchange Officer.
At that time, we had a number of exchanges with other nations who operated the Chinook (Canada and Australia), as well as the Royal Navy and Army Air Corps.
Dan had vast experience flying Chinook helicopters in cold weather operations, having flown all over Canada and spent a lot of time up in the Arctic, where helicopter icing conditions were a regular occurrence. He was also what I think we might term a “wise owl”. He didn’t always say a lot but listened intently and when he did say something it was invariably worth listening to.
As we peered out of the crew room window, all we could see was a blizzard of snow. It was even more galling that we were listening to the Wimbledon Tennis Championships on the radio with the UK experiencing a midsummer heat wave. Oh, the joys of a “summer” in the South Atlantic.
Cold weather helicopter techniques from Arctic flying
As we reviewed the weather, the conversation turned to helicopters flying in icing conditions, and specifically the Chinook’s icing clearance. Unlike most helicopters, the Chinook has an icing clearance and is cleared to fly in moderate icing conditions.
This seemed mainly due its significant power margins and six huge rotor blades with a chord that was larger than most light aircraft and a rotor diameter of more than 60 feet.
Apart from the obvious constraints about being able to maintain level flight and not exceeding torque and power limits, the overriding limitation was to do with the centrifugal droop stops on the rear rotor head.
Helicopter icing clearance and preventing droop stop failure
So what are droop stops? In simple terms, as the rotor blades slow down during the aircraft shutdown process, they lose lift and centrifugal force, which makes them descend or droop.
If the rotor blades droop too far then they will impact on the helicopter fuselage and in the case of the Chinook cut through the cabin. Clearly something to be avoided.
18 Squadron Chinooks deployed as part of Aviation Task Force 3 in Estonia.
There are two types of droop stops – fixed and centrifugal. Fixed droop stops are fixed lumps of metal or sometimes a ring of metal that stay in the same place and prop up the blades at rest. This is what the Chinook has on the front rotor head.
However, on the rear rotor head there is a centrifugal system which holds the droop stops out by a spring mechanism. Then, as the blades slow down, the springs gently allow the droop stops back into place.
But in icing conditions these springs can become iced up, resulting in the droop stops remaining out.
Because of this, we had to have 1,000 feet of non-icing conditions above the surface with a positive temperature to allow any ice on the centrifugal droop stops to melt before shutdown.
As we talked about this, much to the envy of our Sea King Search and Rescue colleagues who had no meaningful icing clearance, Captain Dan put down the magazine he was reading and cleared his throat.
“Droop stop icing was a daily occurrence in the Arctic and for eight months of the year it was almost impossible to get a positive temperature to de-ice the droop stops,” he said.
So what did they do? Clearly, they didn’t stop operating. Dan then ran through the technique they had developed and used, which then became a standard operating procedure for the Canadians in Artic and cold-weather conditions.
High demands fit for the Chinook helicopter
Fast-forward almost 10 years to New Year’s Eve 1995.
I was now the Chinook detachment commander based at Divulje Barracks in Split. As the NATO IFOR peacekeeping operation rapidly ramped up, we had been operating at a pretty high tempo for almost three weeks and the aircraft, due to its capability, was in huge demand by all of our coalition customers.
We were also experiencing the worst winter in the Balkans for more than 10 years, which again because of that icing clearance meant the Chinook was one of the few platforms that could operate most days.
We had also declared a 24-hour standby task line with a medic on board in case of a medical emergency/evacuation type scenario.
Given the high flying rate of the previous few weeks, we had negotiated a lighter tasking day on New Year’s Day, and I had decided to hold the standby task with my crew for the next 24 hours to give the others some downtime.
I had just got back to the accommodation when my mobile rang. We had a high priority task to try and evacuate a compassionate case from Sarajevo to Split for an onward move back to the UK by jet.
Heavy snowfalls during the day had closed the runway at Sarajevo and the only viable option was to evacuate by helicopter – and the only aircraft capable of attempting the task was the Chinook.
Compassionate tasks are the highest priority tasks outside of life-saving, so I agreed to give it a go.
The lollipop effect: Chinook pilots overcoming icing conditions
We generated the aircraft as rapidly as possible and got airborne within an hour of the call. We had six hours of fuel on board, so could do the task and still divert to Rome if necessary.
Given the weather and darkness, the only viable option was to play airliners and transit above the mountains IFR and then shoot an instrument approach into Sarajevo.
We broke out at about 600 feet and could see some cultural lighting but no airfield. I was about to go around when my navigator said “Good picture on the goggles.”
18 Sqn Chinooks working as part of Aviation Task Force 3 operating in Estonia
I looked through the goggles and visible through the snow was the glow of the runway lights. I quickly re-briefed the crew that this would be a night snow landing with heavy recirculation and nominated my approach and landing reference point, which was one of the glowing lights halfway down the runway.
We slowed and passed through our 100 ft/30 knots gate and started the approach. All three of the crew were now giving me the commentary on height, speed and position of the snow cloud.
We touched down and sank belly deep into the snow. I checked the radar altimeter, attitude indicator and compass to confirm we were down and not moving as the windscreen was still opaque.
I was getting nervous when more than 30 seconds later I could still see very little through the windscreen.
When I mentioned this, the crewman at the ramp invited me to look in the mirror. The ramp was cracked open, and I could see a snowmobile approaching the aircraft with our passenger and his escort.
We looked at each other in the cockpit and then the snow that had hit our super-cooled aircraft on landing started to melt off the windscreen with the windscreen heating just starting to have an effect.
In the words of one of my crewmen, the aircraft had turned into a massive lollipop! The recirculating snow had stuck fast to our super-cooled metal aircraft.
Take-off with ice: power of the Chinook helicopter
With passengers on board, we reviewed the departure. I had no idea how heavy we were with all the ice, but we couldn’t shut down and I was very confident about the power reserves we had remaining.
We also needed to get up through the band of cold air as soon as possible and above the clouds, where the temperature was just above zero. So we did a hybrid snow take-off into an instrument departure.
Suffice to say, the Chinook took it in its stride, and we were climbing at almost 3,000 feet per minute when we broke out on top of the clouds.
The 40-minute transit back to Split was uneventful apart from the disconcerting noise of the rotor blades shedding the ice, which was hitting the aircraft.
As we approached Split the clouds cleared and we were treated to a magnificent view of the Croatian coast at night.
We made a visual approach into Split Airport and taxied to our parking spot to drop off the passengers. I then noticed the airport handling staff pointing at us and a bit of a crowd developing. We were attracting attention.
I asked one of the crewmen to go and see what the issue was. When he approached the crowd and looked back at us, I could see the look on his face. Apparently, the aircraft looked like something from a Christmas card and was still covered in ice!
I still thought the best option was to reposition back to our base, which was less than a mile away, and then think about how we were going to shut it down.
Dealing with droop stop icing
On landing at Divulje we taxied in, and I could now see the lumps of ice falling off the aircraft. We all knew it was the aft droop stops that were going to be the problem and it was obvious that they were stuck with ice.
Members of 430 Tactical Helicopter Squadron from Canadian Forces Base Valcartier transport personnel and equipment to the Grant High Arctic Data Communications System (HADCS)
It was then that the crew room conversation from the Falklands 10 years before came back to me. The Canadian technique was to use a high-pressure water hose from a fire truck to blow the ice off the rotor
heads and clear the droop stops.
When I suggested this as an option to the crew there was silence on the intercom until I recounted where I had learnt this and who from.
The most senior of the two crewmen, who had known and flown with Captain Dan, then endorsed the plan, commenting that if Dan said it would work then it should be fine.
So we briefed the fire crew and positioned the truck. We also made sure that the surrounding area was clear as I did not want anyone injured by flying shards of ice.
After what felt like five minutes of being in a car wash, we reinspected the rotor heads – which were now completely clear of ice. It worked perfectly.
Hard-won experience in helicopter icing prevention
I have reflected on that night a number of times since and have shared the story with colleagues over the years.
It was a high priority task that was well executed by a highly trained crew in challenging conditions and in an aircraft that we all knew and trusted.
Even after more than 10 winters of flying in Norway, I had never come across katabatic drainage like we experienced on that approach and the effect that the snow had on a supercooled aircraft.
But my real takeaway was about that conversation that we had all those years ago in the safe learning environment of the crew room. The curiosity about how others had operated the aircraft and how to use some hard-won experience from a sage operator to complete the task safely.
Train for certainty but educate for the unknown.
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