The fact that ice is a problem in the jet engines of an airplane, even in relatively warm weather can be very difficult for most people to understand, even for pilots and others who should know. This is why Oluf Husted has made this attempt to explain it, so that even his grandmother would be able to get the point.

Dear Granny

13. of april 2005

As you’ve heard by now, I’ve been fired from Scandinavian Airlines Systems (SAS) after 20 years. They even managed to have my pilots license revoked by making accusations of insanity against me. Well, at least the story is insane. Here it goes:

It’s all about condensation which freezes, and about the ability to read word for word. Last things first:
You remember when I read a bedtime story to my son, and he knew the story. He would say to me: “Daddy, you have to read exactly what it says.” I guess I told you the same thing 60 years earlier.

You have had a lot of experiences with condensation yourself: It is your glasses, which fog up when you enter a room with a higher temperature, and why you rushed to put a coaster on the table when grandpa set down his iced drink. He never learned. It was always worse when humidity was high, and you knew that.

Grandpa often had problems with his camera lens, and it irritated him when you told him to wipe the dew off the lens before he started filming. And do you remember when you were an hour late for the first birthday of your first grand child? You recall how amused the mechanic became when he simply started up the car with the little bit of electrical power that was left, after grandpa had almost emptied the battery. Your car had stalled 5 minutes away from home on a sunny morning with just a little light fog. But it was 30 minutes before the mechanic showed up. When he did, he started up the car immediately, added a quarter gallon of carburator alcohol in the tank, and told you that you had had ice in the carburator, although the sun was shining and the temperature was + 5 degrees Celcius. By the time he got there the ice had melted, and the engine started with a purr. You’ve bought a lot of carburator alcohol since then.

It does seem strange too, that you can have icing problems, when the weather isn’t freezing. But on the bottles of carburator alcohol it often says: “Should be added to the gasoline, when the temperature is below +5 degrees celsius. If you don’t believe me just go take a look – I’m sure there’s an empty bottle in the garage.

“But how can ice build up, when the weather isn’t freezing?” you’re probably wondering. Well, it doesn’t right away. It was the metal in the carburator that had been sub cooled to maybe -3 degrees celsius, but it takes a few minutes after the car has been started, and that’s why you had gotten about 5 kilometers away from home, before the engine started coughing and eventually stalled.

”But why is the metal sub cooled when the weather isn’t freezing?” Well, do you remember how you water the flowers in the back of the garden when the hose isn’t long enough? That’s right! You squeeze the hose, and then it sprays about six or seven meters. The water has to run faster, when you make the hole smaller, and that’s why it can spray further. The same thing happens in a cars carburator and in the jet engine of an airplane.

But in a jet engine it isn’t water, but air, which is forced to run faster. Do you remember how cold your fingers got, when you squeezed the hose? Soon you learned to use gardening gloves although the weather was warm.

You’ve probably also seen, that ice can build up at the valve of a bottle of gas when it’s in use, for instance when you go camping. Most times it happens when the air is moist early in the morning. Remember when the fire extinguisher was emptied because the date of use had expired. On the valve of the fire extinguisher there would be a big chunk of ice, but we kids,  never really got a good explanation for it. Well, you’re getting it now.

So, grandma: When water or air is forced to run much faster – like in your garden hose or in the carburator of the car (about 1600 gallons of air has to be sucked through the carburator every minute), then pressure drops, exactly as it does in a pressurized bottle with gas or foam, when it is emptied. When pressure drops, so does the temperature. When it’s the other way around, when pressure rises, so does the temperature – think for instance of how hot the bicycle pump will get at the valve, when you’ve pumped the tire hose full of air.

The temperature typically drops 6-10 degrees celsius – both in grandpa’s carburator and in a jet engine, and it’s worst right where the intake is the narrowest. In a car this is where the gasoline is mixed with the air, and that’s why it helps to add alcohol to the gasoline. The alcohol will remove the ice just as fast as it builds up. Some car manufacturers have even figured out, how to keep the metal in the carburator above freezing temperature by use of electrical power. Then the condensation and the humidity will simply continue into the engine, where it actually helps the compression.

The coldest spot in a jet engine is right where the “propellar blades”, we call them compressor and stator blades curve the most, and that’s on the back side. That means that the ice, which is very dangerous, unfortunately can’t be seen from the front, and that’s why we have to feel the back of the blades with our bare hands. And since some engines on some planes are placed at the tail part about 4 meters off the ground, this requires a special ladder, which can be quite difficult to get a hold of. It sounds really complicated, and as a matter of fact it is. But it’s important, because it’s very hazardous to take off with ice in the engines. If there is a thick layer of ice, which breakes off and gets sucked into the engine, once the plane is moving, it can simply destroy the engine completely. It’s like throwing rocks into the engine, and I’m sure you can imagine that it can’t stand that.

This coldest spot in a jet engine is not where we mix fuel with air, so it wouldn’t help adding alcohol to the fuel. Instead we use hot air to keep the ice prone metal parts above freezing temperature. When the jet engine was still “young” (like you in the late 1940s) this hot air was plenty to keep the engine free of ice. As pilots, we just had to remember to turn the system on, just like grandpa who “just” had to add alcohol to the gasoline, when he filled up the car.

But later, as the engines got thicker and more powerfull, we had to lower the idle power. Otherwise the plane would simply run too fast on the ground, and the brakes would get too hot, since we have to use them to keep the speed down.

This meant that ice could now settle on the most ice prone parts of the engine, if we hadn’t done engine run-ups for a while. The low idle power meant that less hot air was circulated in the engine anti-ice system, compared to earlier when we taxied out for take off or in for parking. We’ve learned by now, and we do a hands on check of the back side of the blades, before we start up the engines. But obviously we can’t do that right before take off, so now we do engine run-ups regularly with the brakes on, while we’re still on the ground and always just before we start rolling for take off.

When the engines really started getting thicker around 1970 the airlines’ expenses for engine parts rose. It often happened that the blades were dented or worn due to ice, and when this happens, the blades have to be replaced in pairs. Otherwise the engine won’t be in balance, and it simply has to be. And the blades cannot be repaired. They have to be replaced, and those now cost more than $ 18.000 a pair. It was ice that caused the increasing number of damages to the blades, and the airlines started to suggest the pilots to consider doing engine run-ups regularly, as I just explained, if we thought the weather so required. But no one in the whole airline industry ever agreed on, "when the weather so required", and how often you should do run-ups. Some manuals said that you “should” do such and such. Others said that you “must” or “could”. It was very confusing. And believe it or not, it still is.

The engine and aircraft manufacturers weren’t keen on admitting that their thick engines couldn’t stay free of ice at idle power. You see this is a requirement made by the authorities that a jet engine can stay ice-free with just the engine anti ice system on. Oh yes, and of course the engine manufacturers make good money selling parts for an engine that holds up for approximately 30 years. Actually, they virtually make nothing selling new engines. The competition in that market is to hard. So they have to make their income selling amongst other things “propellar blades”, when the old ones are wrecked – for instance due to ice.

When an airline buys new aircraft, it can chose from a variety of engine types. So it is understandable that the engine manufacturers don’t want to state that their engine doesn’t quite meet the standards set by the authorities or that it is extra difficult to handle in the winter.

Scandinavian Airlines System (SAS) had huge extra expences, more than $ 18 million a year for new titanium compressor blades (the ones that cost more than $ 18.000 a pair) after buying the type of aircraft that I flew (the MD-80). So SAS put pressure on both the engine and the aircraft manufacturers to state that when temperatures were + 2 degrees celsius or below and the air was moist, pilots had to do engine run-ups regularly, and always at least once before take off. SAS were among the first to use the term “shall do” and not “could do”. But our busy management forgot to correct that part of the manuals which said that an SAS airplane always does a rolling take off, so that more planes can get airborne. It’s simply faster, and this is good especially in the rush hours of the airports. So the new procedure hadn’t quite caught on amongst the pilots. We weren’t even blamed if we happened to forget the new procedure during the tests which we all had to pass every 6 months.

You probably remember that I was an instructor when I flew DC-8. This had focused my attention on new procedures, and I had started to do engine run-ups. I often just had to do one right before the start, because we rarely have more than a 5 minute taxi to the runway. I hadn’t really given it much thought that my various co-pilots didn´t understand why I was being so cautious. At least not until Stefan Rasmussen crashed due to ice on the wings of his aircraft and perhaps in the engines. This was on the third day of Christmas 1991.

But I did think about it when I spoke to a few colleagues who hadn’t really started respecting the new “have to” in the manuals, or hadn’t even noticed that the procedures had been tightened in November of 1990. This even included people in management. So when I told them: “Read exacly what it says” and make sure that all about 1000 DC-9 and MD-80 pilots remember to do engine run-ups, they got very tired of me. This “little captain” who basically told them very loudly that they couldn’t read. That didn’t make me popular. But I hollered, because I know that it is extremly dangerous, when a pilot doesn’t do what he’s supposed to do. We do mind you have the responsibility for a lot of peoples lives every time we board our plane.

80 days after Stefan Rasmussen’s crash SAS finally aquired an even better procedure, but by then it was too embarrasing to admit that the “little captain” had in fact been right.

Six years later SAS was declared world champion in low engine maintenance costs. We were about 65 percent below the world average. We were now the best airline in the world to do engine run-ups, so that ice didn’t build up in the engines and later broke off to damage or destroyed them. But by 1996 we went back to the poorer procedures in order to avoid slowing down traffic to much during rush hours in the busiest airports. It was the Frankfurt Airport that demanded this.

A couple of years later it became evident how bad things can get when you don’t do enough to keep your engines free of ice. On December 14th 1998 there was freezing fog and freezing rain at Gardermoen Airport in Oslo, Norway. Here SAS and 4 other airlines had the engines of at least 15 planes either damaged or destroyed. This was the worst series of accidents and incidents in aviation history! Still the Norwegian authorities weren’t very keen on investigating what had happened. The authorities quickly concluded that the weather was so unusual on that particular day that it would be at least 100 years before something like that would ever happen again.

Therefore there was no sense in investigating it.

But occurences like that must be investigated – according to law, and so it was by the Norwegian Aircraft Accident Investgation Board. But only after I spent a lot of effort convincing the Danish Secretary of Transportation, Sonja Mikkelsen, and MP Margrethe Auken of the Transportations Committee that they should demand an investigation. Then it was done. The Norwegians just never published a report, as it is also required by law. Maybe this is due to the fact that in the report, which SAS always draft themselves after such an occurence, it said that the weather was actually completely normal for that area on that day. SAS has however tightened the run-up procedure a little since then, but it is nowhere near what it was, when SAS was world champion. (click here to see the ad from Flight International)

That was quite a story, but I know you women catch on quickly. I wish it was just as well with the world’s airlines.

Love, your oldest grand son, Luffe