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Fuel icing and precautions

Agreed 100%, but BA038 is one of many examples...

I don't think they ever managed to re-create the conditions, despite spending millions removing and rebuilding the entire 777 fuel system in some hangar, but they think they know what happened, and it was a gradual build up of ice particles which stuck to the inside of a fuel/oil heat exchanger.

A very rough calculation, assuming an internal radius of 2mm and a 500mm length of pipe, with a 12L/hr (or 2L/hr per cylinder) fuel flow leads to a transit time of less than 2 minutes from the distribution spider to the injectors

2 minutes (I am not doubting your calculation) is a very long time - loads of time for the fuel to cool down to the 150kt airflow temperature.

Administrator
Shoreham EGKA, United Kingdom

I don't see what turbine fuel systems and avgas have in common. Definitely not comparable here.

Perhaps some injected engine in some extreme humidity/temperature condition is susceptible to icing in the fuel lines beyond the filter. But even that is easily addressed by using IPA. Don't know about UK, but it's very available in Scandinavia. Gas stations, car part stores and even general grocery stores sell it.

EFHF

I don't see what turbine fuel systems and avgas have in common. Definitely not comparable here.

In that case how exactly would you say avgas fuel systems ice up (which they definitely do)?

I agree IPA can be used, but it would be useful to have an idea of when it is needed.

That is why I started the thread.

It's not very useful to just throw it in for every flight which might go to say -20C, not least because IPA needs to be carried in quite a quantity on any long trip (say 1%). Let's say you fly down to Greece (20hrs), you need to carry 5-10 litres of the stuff, and it's not great to carry such a lot of a highly flammable liquid inside the aircraft.

That is why ferry pilots mostly use PRIST; it's nasty toxic carcinogenic stuff but you need far less of it.

I buy IPA on Ebay, and use it for various cleaning jobs also.

Administrator
Shoreham EGKA, United Kingdom

How cold will those pipes actually get, even if the OAT is -30, given that they are quite close to a toasty engine and the airflow will be well mixed as it passes over the engine block and cylinders?

I don't think they ever managed to re-create the conditions, despite spending millions removing and rebuilding the entire 777 fuel system in some hangar, but they think they know what happened, and it was a gradual build up of ice particles which stuck to the inside of a fuel/oil heat exchanger.

They actually did, IIRC. I can recommend the Air Crash Investigation episode covering BA38, found on youtube.



It says "this video contains content from Channel 5 who have blocked it from your country on copyright grounds"

IIRC they did manage to reproduce the FOHE icing but not the engine stoppage at just the right time.

How cold will those pipes actually get, even if the OAT is -30, given that they are quite close to a toasty engine and the airflow will be well mixed as it passes over the engine block and cylinders?

That's the big question

Administrator
Shoreham EGKA, United Kingdom

It says "this video contains content from Channel 5 who have blocked it from your country on copyright grounds"

Typical, works great from Sweden. Guess you have to find an open proxy to view it :-)

Has anyone wondered why the pipes running from the fuel distribution spider (on top of the engine) to the fuel injectors are so thin, and unprotected?

I would say: For cooling. A major problem with some larger piston engines that I have been operating (mostly GTSIO-520 in C421 and 404s) was fuel vaporising inside the fuel lines. (Re-)Starting a hot engine on a warm day could be very difficult or next to impossible as you have to operate the primers long enough to blow the vapour out of the system but not as much as to flood the engine - which will happen almost at the same moment. Also when the engine is idling on the ground in summer it will occasionally miss a beat or two due to vapour bubbles. So for me it makes perfect sense to expose the fuel lines to the airflow in the interest of keeping the fuel in the liquid state!

I have never before heard of AVGAS freezing to be honest. If there is water present when operating at low temperatures, then it will be frozen already on the ground. My coldest AVGAS flying day I can remember was during a ferry flight with a Cessna 421. It was parked overnight at Goose Bay in February and was cold soaked in the morning at nearly -40 degrees C. After heating the engines (only the engines, nothing else!) for about an hour with a ground heater, we finally could get them started. During flight at FL 250 towards Greenland, the OAT indicator was off it's scale all the way (but it only indicates down to -45 deg C or so). No sign of fuel freezing.

EDDS - Stuttgart

Somewhat off-topic, but I vividly remember my PPL check ride on a beautiful, brisk winter day. It was -29C when we set off. I usually never wear "long johns" but preflighting in an open airfield was too cold even for me without them. Then I ended up sweating my butt off. Also will never forget the thermal bumps over the yet-to-freeze sea.

IPA was used. Before and after the flight.

EFHF

Fuel icing with avgas is usually a problem where there is a large temperature difference between the surface and cruise altitude because the degree to which avgas is hydroscopic varies with temperature. It also varies with length of the exposure of the fuel pre-flight to humid air. So if the aircraft is stored at -30deg then icing probably won't be a problem. But (an extreme case) leave a tank less than half full (large exposed surface area) for a month at +25deg in say Florida and then fly in -30deg temp you are likely to get ice crystals blocking the filter - and then the distributor screen when the filter goes into bypass. The ice crystals are possibly too small to block an injector, but not too small to block the screen or the filter.

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