Because water is in the fuel
Is that a fact, established for these Aztec and other incidents? AFAICT nobody has the slightest clue – other than by elimination of other causes. After my 2014 incident I had the fuel tested by a lab which cost me £500. They found a well out of spec gum content but not a lot else.
But let’s assume it is water. The water has to come from somewhere, however. Avgas isn’t supposed to contain it.
I have been searching for the max permitted water content of 100LL, without much luck. AC20-125 suggests 30ppm (0.003%).
Up to 1000ppm (0.1%) can be dissolved in avgas; any more will come out as droplets.
The Exxon avgas data sheet says nothing about water content (I would think it must be close to zero, given the distillation process?) but suggests up to 0.2% volume increase is possible due to water absorption.
Aztec has bladder tanks and these can trap water?
The DA42 weeping wing system seems to leave the outer wing un-protected which isn’t ideal for aileron control.
I had never heard about that Aztec issue and I would love to know more. There are no limitations in the POH, even for aircraft with no ice protection whatsoever. The section about icing is actually missing from the original POH, it was added by an AD in 1999, see here local copy
I’ve flown in air below -20C a couple of times (FL190 across Canada, FL160 across the rockies) and I didn’t run into any issue (actually I did, I lost the heater at FL160 once. In shorts & t-shirt.). The only ice-related issue I had was the air filters icing up in IMC causing both engines to cough which scared me quite a bit. Personally I’m out of the “flying into icing conditions” game.
On a side note, I’m wondering how someone trains for flying in icing conditions. I got my IR without entering a single cloud so the first ice encounter was a little less that pleasant.
A few years ago there were a few instances of a dual engine failure in Beechjet400A aircraft, in the cruise in the high 30’s low 40’s.
It was dealt with by changing SOPs on Anti Ice usage local copy
A fold in the bladder tank could store some water which is not picked up in the drain of the Aztec. I suppose it could just be an issue with one tank but if I remember correctly Timothy had the starboard tank issue as well as me so unless it is something specific to the installation on that side it seems unlikely. I suppose it could just be a strange coincidence.
I recall tales of Partenavia twins icing up on the air inlet and making a few people sweat a bit. Alternate air is provided to get over this.
It is not an alternate air issue otherwise it would not resolve itself when changing tanks.
The scenario where alternate air protects is AFAIK applicable only where the fuel servo ices up.
And that would not resolve itself by changing tanks.
On the TB20 layout, the alternate air (data posted at my link above) is plenty warm enough to ensure the fuel servo inlet is above 0C, with an OAT down to about -35C (IIRC).
@Timothy did your aztec have the tip tanks? Mine has them and the drain valve isn’t at the lowest point of the tank, so water could get trapped in the tip tanks and make its way to the outboards once the plane starts moving, or at least that’s what I’m thinking.
Back in winter 2018 the airplane was sitting outside and I think that someone once confused my aircraft with a gas station, as evidenced by the high evaporation rate in 30L increments and the fact that some of the fuel caps weren’t quite close. Of course it had rained and as a result I had water in all tanks. I drained it all and I’m 100% positive that I was getting pure 100LL from the quick drains after doing so. Still, the right engine stumbled right after takeoff; after landing I drained another half liter from the right outboard tank.
My SOP is to takeoff on the inboards as a result.
DA42 (at least ng and newer) requires alternate air open anytime in IMC. Failures could be owners disregarding this (as I’ve witnessed firsthand). Or disregarding temp limitations when using diesel.
