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Prolonged flight in IMC on alternate air?

I’m wondering about the downside of prolonged flight in IMC on alternate air.

If the OAT is cold enough everything around is already frozen but the air filter may clog up (had that already) and then one switches to alternate air or it happens automatically. So far so good. But the question is about deliberately planning and performing a flight with that as to be expected.

I’m looking for opinions.

Frequent travels around Europe

There have been reports of alternate air doors that have frozen shut. So opening alternate air as a preventive measure when flying in icing conditions (and even rain) may be a good idea, and is recommended by some AFMs.

One Cirrus SR22T pilot on this forum reported having lost engine power due to air intake icing and the alternate air door (automatic) not opening until he reached warmer temperatures (OAT -13℃).

I have myself experienced loss of engine power caused by loss of turbo pressure, most probably from air intake icing. And although being in IMC I had no signs of icing on windshield or wings.

Away from the ground I do not see what risk you may incur from flying with open alternate air.

Last Edited by Aviathor at 02 Mar 18:38

Having alternate air open in visible moisture below 4 oC OAT is SOP? There shouldn’t be a problem, possibly a small loss of MP.

Oxford (EGTK)

This is a very good topic.

The automatic (spring loaded) alternate air doors are extremely dubious IMHO (because they can freeze solid and then won’t open) and as the SR22T event shows could kill somebody. That plane was sold immediately afterwards.

The drawback of flying with alt air open is that you lose a fair bit of power. I posted on this before, following a quick and dirty test. IIRC, my TB20 might reach FL200 but only FL170 with the alt air open. Once at say FL180 you would probably manage to just about hold it there. Probably much less of an issue if you have a turbo.

This is relevant too. The graphical data posted there shows that an inlet duct which has a significant reduction in cross-section from the input to the servo inlet hole is going to warm the air up by some 10C which is why I got really nice icing at -15C but the airframe was totally clean. I have had two engine stoppages in these cases. I believe prop TKS protects against this, but the data in that thread shows that alt air would definitely protect against it.

Shoreham EGKA, United Kingdom

Bush pilots I think preferred carburretted engines over injected (180 vs 185), because, I thought, they were easier to fix. But the other thread suggests there maybe ice protection reasons, assuming you do not use carb heat when the air is too cold and you warm it into carb icing territory.

I flew normally aspirated injected Lycoming 540s with MEAs at F150 in the winter. The engine analyser never presented problems in IMC, however the Janitrol heater was pretty useless. How the Cheyenne coped with a Janitrol, or the turbo piston versions was never clear to me.

Oxford (EGTK)

I would always select alternate air if IMC at possible icing temperatures. Losing a small amount of power is better than the alternative. But I guess you might have to “risk it” and leave closed in a non-turbo aircraft where marginal on climb performance, not that you should really be there anyway.

What I find amazing is that in spite of the laws of physics and the lack of evidence the sr22 lack of manifold pressure incident is still being rehashed over and over as a “frozen alternate door incident”.

The problem with icing incidents or accidents is that the evidence is removed from the crime scene by the rise of temperature closer to the ground and therefore not available to the investigators And after the engine is recovered from the wreckage, it runs just fine. Therefore we can never get anything else than “probable cause”.

If the alternate air door suddenly opens at a lower altitude and the engine comes back to life, chances are that the alternate air door was indeed blocked by icing. As George would say, “What else?”

Last Edited by Aviathor at 02 Mar 21:44

What I find amazing is that in spite of the laws of physics and the lack of evidence the sr22 lack of manifold pressure incident is still being rehashed over and over as a “frozen alternate door incident”.

Nobody I know knows what happened and that is simply a reasonable guess. It was obviously icing related, and being avgas was unlikely to be fuel icing so would have been induction icing.

And, to put it crudely, the plane was sold very quickly indeed afterwards and the data read out of the engine log never came out anywhere. Obviously I don’t think that is something that should have been done, but lots of people sell planes in similar circumstances… it’s a consistent pattern in GA.

Shoreham EGKA, United Kingdom

Okay, so let’s study the spots where induction icing can happen on a Continental engine. The CMI injection doesn’t have a venturi to meter fuel – so it won’t be there. Suppose the CMI injection actually had a metering venturi, I would think the temperature of the charge air would be above icing zone anyway, even after passing the intercoolers.The other culprits would be the alternate box, and filters, right?

. What do we know about the alternate box? We know that it is located under the sump, ahead of the two air filters that are positioned one on each side of the cowl cheeks (for lack of a better term). Now for a water particle to get to the box, it would have to cross through the filter, turn 180 degrees, and move not only against the airflow / turbo suction but also against gravity as the box sits somewhat higher than the filters. One would also have to assume that the box would be around the 0C mark, despite being enclosed in a warm cowling and bolted to a hot oil sump. Once all that would happen, both filters would ice up (if filters iced up before the box froze solid, the box flap would open and no manifold pressure drop would happen). I am not good at physics but I don’t think that is reasonable – much less obvious.

One failure mode that I would call obvious, and which would show the exact symptoms blamed in this case on supposed induction icing (i.e. manifold pressure dropping to the exact level of the ambient pressure) is a failure anywhere in the upper deck pressure circuit.

Last Edited by Shorrick_Mk2 at 02 Mar 22:28
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