On a recent flight I was at FL190 at -10C for a while. I was IMC but it was very thin cloud. I was watching my wings and there was nothing. Nothing on the windscreen either. All completely clear. Speed was good and there was no need to add power to do anything to keep up a good cruise speed.
After flying along for a while the warning of the auxiliary air inlet came on. The door had opened.
As I was getting closer to my destination I decided to start my descent. At some point the door closed and the warning disappeared.
I should mention as well that during my climb earlier I did pick up some ice. The TKS did its job and after flying a bit in VMC and then entering IMC at FL190 at OAT -10C again it was all gone. Gone from the areas I can see. So I don’t know anything about the underside of the wing and obviously not about the status of my air filter.
Since then I’m thinking about it. I tend to believe that the air filter did ice up. But why was there no other ice anywhere else?
SR22 Checklist for inadvertent icing conditions:
- Pitot heat … ON
- Exit icing conditions
- Cabin Heat … Max
- Windshield Defrost … Full Open
- Alternate Induction Air – ON
Flying into cloud at -10 C you can always expect icing of some kind
The alternate induction air is automatic on newer models.
There were no other signs of icing. Hence the question.
Ah, sorry … on mine it’s manual. What does the POH say? Does it open detecting low temperature? I would say it detects low air pressure and opens. It probably iced up …
Is your’s FIKI?
To ice up a wing, you need super cooled water droplets; if all the water in the cloud is already in the form of frozen ice crystals, nothing sticks.
But ice crystals can clog up the air filter. Most extreme example: You won’t ice up when flying though a (dry) snow shower, but your air filter can will clog up very quickly.
So I would guess you were in a thin, fully frozen cloud. This is plausible in stratiform cloud at -10 degrees.
We did this before in the fuel servo icing thread.
In any practical non turbo installation there is bound to be a temp rise from the air inlet onwards.
That will raise the temp from -10C to something more likely to get structural icing.
The data I posted there is pretty hard to argue with.
I don’t think prop tks helps on a Cirrus due to the location of the air intake.
Peter wrote:
In any practical non turbo installation there is bound to be a temp rise from the air inlet onwards.That will raise the temp from -10C to something more likely to get structural icing.
I don’t think that’s the case. The reason for icing being more likely at e.g. -5C than at -10C is that there is less supercooled water around in the ambient air at -10C than at -5C. More of the water content is glaciated. But you don’t increase the supercooled water content by raising the temperature of a particular parcel of air from -10C to -5C.
But there’s enough supercooled water around at -10C to block an air filter, I think.
Since then I’m thinking about it. I tend to believe that the air filter did ice up. But why was there no other ice anywhere else?
At least in our PA46 that is typical. If the wing ices up in fact it is not even worth bothering with the alternate air. But then there is this very thin cloud which ices up the air filter but leaves no trace on the airframe.
But the details will depend on the aircraft. I just sold an ADL120 to a customer who though convective weather is harmless until both engines of his DA42 iced up badly so he was not able to maintain altitude. From now on he wants to fly around the weather. So the DA42 and the PA46 seem to behave quite differently when it comes to induction icing,
Was that a diesel-engined DA42? Perhaps that’s a silly question, I’m not sure there’s any petrol-burning DA42’s around. But it seems obvious to me that a diesel will be less prone to icing – probably your customer got over-confident from thinking the same. Surely the only delicate point for a diesel can be the air filter? Or perhaps something in the turbo, I know nothing about those.
AStefan, from the SR22 Maintenance Manual (in case it interests you)
In the case of filter blockage, or induction ice, alternate air can enter the engine via the alternate air assem- bly. A tube from each air box leads to the alternate air assembly at the front of the engine. A blast tube con- nected to the heat exchanger provides heat to keep alternate air assembly flap from freezing. Under normal conditions, the alternate air assembly flap is held closed by a strip of magnets. When the air flow decreases, a vacuum is created that overcomes the pull of the magnets and allows the flap to open. After receiving a signal from a switch mounted to the alternate air assembly, the PFD will display “ALT AIR OPEN” in a yellow caution box.
