Barit 1 said:
I'm not sure this adds up. The "amazingly efficient" Trent with more spools, and more bearings, probably has more heat generated in the oil, and thus any "stagnating" fuel will be exposed to more BTU transfer, leading to LESS probability of icing in the fuel-oil heat exchanger.
Often the most severe design point for the cooler is early in descent, when lube heat rejection is still high, yet fuel flow (the heat sink) is very low.
I agree with Barit1 here,
The solution to an Hi temp light coming on in the decent (which is not uncommon) for many twin-spooled engines is to push up the power and throw out the boards, because those are fuel-cooled oil systems: the increased fuel flow (higher than idle) cools the oil system and the light goes out. No need to write it up since you just fixed the problem.
The point here being:
decent at idle is a high-temp situation; not a freezing situation. I know that sounds backwards, but for most of the seven transport types I flew that is true.
However, the heat exchanger is no doubt regulated by another computer chip on this tripple spooled model. More complexity equals more unreproducible, unpredictable results.
This accident already is shaping up to be another 737-Rudder, 747-center-fuel-tank mystery.
Blaming icing by itself seems unlikely to me. The 747SP flew up to 45,000 feet for 26 hours routinely for thirty years and never had any problems (that I know of.) It even completed a 54 hour flight. KC-135's B-52's and airborne command posts have remained aloft for days without any problems.
Tanimbar said:
On another matter. The AAIB interim report states (Water ice in fuel, p12),
Quote:
As the fuel temperature is further reduced, it reaches the Critical Icing Temperature, which is the temperature at which the ice crystals will start to stick to their surroundings. When the fuel temperature reduces to approximately ‑18°C (0°F), the ice crystals adhere to each other and become larger. Below this temperature little is known about the properties of ice crystals in fuel and further research may be required to enable the aviation industry to more fully understand this behaviour.
I read this with disbelief. The words, "little is known" is, well, shocking.
Until today I had thought that the industry had fully experimented, tested and evaluated the effects of temperature on fuel (at all operating ranges).
By the way, my money is still on stratification ( no, don't respond to this; I need to read the report more carefully and may change my mind).
Regards, Tanimbar
I agree Tanibar and others, this is a shocking statement and makes me wonder how other airframes have stayed aloft for over a quarter of a century with no icing problems. Prist type additives? The 747SP (which I merely jumpseated) has nine tanks and surely runs into this problem up in the 40's. But all of the aircraft I mentioned (747SP, B-52, KC-135, 747-EAC-1) were non-FADEC (before engine upgrades) and their safety records were based on direct-link hydro-mechanical designs.
The FADEC is usually the culprit when you are discussing rollbacks. A pilot with a control cable would keep trying to clear the ice out by trial and error. A FADEC goes back to idle and tries to reboot a start sequence.
JMHO's only.