Icing Conditions at up to 16C TAT?

[NSEU]

Last week, I received a Technical Logbook report of the Auto Anti-Ice system (747-400) remaining on at 16C TAT.

When coming through known icing conditions, our flight crews put the Nacelle Anti-Ice switches to ON (out of AUTO). On this particular aircraft, they did this and at 13C TAT, they returned the switches to AUTO, but the Upper EICAS showed that the NAI was still active.

At 16C TAT with the NAI still active, they turned the NAI switches to OFF and shortly afterwards got four NAI Advisory Messages. The crew suspected that all four NAI valves were stuck open, but this is highly unlikely. Looking at the wiring schematics, these messages can appear if the NAI valves are closed and the ice detector probes are still detecting ice.

After cycling the switches again, normal ops finally resumed. All CMC Ground and ops checks were normal.

I suspected a sticky Ice Detector Isolation Relay, but I was just wondering if ice can build up on ice detector probes at relatively high TATs?

Thanks
Rgds
NSEU

[zerozero]

With ice, there are no hard and fast "rules".

Ice, like love, is where you find it.

:)

That said, it just sounds like your crew turned off the NAI a little too early (although perfectly appropriate at 13C). They had probably accumulated so much ice it just took a little longer to melt/sublimate.

I wouldn't think they had any new accretions at 16C, but I'll never say never.

[NSEU]

Thanks, zerozero

They had probably accumulated so much ice it just took a little longer to melt/sublimate.

But an ice detector probe is not measuring the ice on the nacelles or wings but on itself. It should never have a substantial layer of ice on itself because it heats up whenever about 0.5 mm of ice forms on it. If no ice is detected for 3 minutes on both probes, a signal is sent to the NAI system to turn it off.

To the unwary.. don't grasp a detector probe tip. Even on the ground, it will think you are ice and start heating up.

Rgds
NSEU

[zerozero]

Right, I understand that.

I'm only suggesting that, apparently (because I wasn't there) the icing conditions were severe enough the detector was still commanding heat after the crew returned the selectors to AUTO.

I could be wrong, but this is my impression based on your info.

[Mad (Flt) Scientist]

You might also, perhaps, question how accurate the TAT was. If you were in fairly severe or unusual icing - which is seems might have been possible - then perhaps the TAT probe was being affected (despite being heated). if you were getting partial blockage of the TAt probe, then the heating that is trying to disperse the ice on the probe will also potentially be heating up the probe and thus affecting the reading. Maybe it wasn't really as warm as the indication said.

Most if not all heated TAT probes are ventilated so that the heating doesn't bias the reading - at low forward speeds such as on the ground such probes aren't reliable temperature sensors. Perhaps something similar was going on here?

Just a speculation.

(And I agree with zerozero, there's no rules for ice....certainly none that the ice obeys!)

[flyburg]

Same thing happened to me about a Month and a half ago. Descending towards HKG we flew through clouds with a temperature of 5(or so degrees), in line with the new boeing procedure we switched on the AI, WAI and HI flows,when the temperature increased above 10 we switched it back to auto, shortly afterward the AI came on automatically and stayed on to about 16. Couldn't figure it out either.

Never seen that and to my knowledge there was no actual icing( just flimsy clouds).

[NSEU]

Thanks, gentlemen.

If it's a new Boeing procedure, perhaps reports of this nature will become more common (and highlight the fact that there may be icing above 10C).

Re TAT probe icing. Unfortunately, as far as I know, there is no way of comparing the Left TAT probe temperature with the Right TAT probe temperature. The Left seems to be used for displays on the Upper EICAS and on both CDU's. It seems that only a Left TAT probe heat failure or a Left ADC failure will force the displays to use the Right TAT probe.