View Full Version : 747-400 Nacelle Anti-Ice During Engine Start
I've heard that several AOMs have a statement which says that NAI is not available during engine start, including those from UAL and Lufthansa.
I can find no system logic (electrical) which prevents the NAI valve from opening on our aircraft, nor do basic pneumatic schematics show a reason for this (same ducts involved on all engine types (RB211, CF6, PW4000)). If duct pressure is, during start, 40psi or so, I assume this will be enough to open both start valve and NAI valve?
From an operational point of view, I can't imagine that NAI is particularly important during engine start, and it may be standard practice to actually switch it off prior to start (?), but I'm still curious.
In a perfect world 40psi would be nice during start but realistically with old APUs and leaky bleed ducts, possibly an ADP and a pack running plus doing a dual engine start, dumping air overboard through the NAI system is going to make the start difficult. Some APUs can barely maintain 25psi during the start on a single engine. I think it's basically written that way to make sure all the bleed air possible is given to the starter to have a successful start in a worst case senario.
I know we used to put NAI on each engine after it was started but once we got the -8 it's now policy to turn NAI on after all engines are started.
The same thing is written about the LE anti ice, saying it's not available (inhibited) with the flaps extended when truthfully it's the fact the LE flaps when extended mask the airflow over the LE and WAI would simply cook the aluminum LE without the airflow to remove the excess heat and the same reason on the ground that it's inhibited. It's easier just to write it's not available than explain why.
Unfortunately manuals are written these days simplifying everything to the lowest common denominator so while not correct get the job done.
Thanks! Makes sense.
Regarding Wing Anti-Ice, I recall seeing a circuit diagram for a European operator which had an actual inhibit circuit tied to flap position. Not on our aircraft though.
At least one European operator selects NAI after each engine start (CF6). Dual start too.
main_dog
24th Apr 2014, 16:45
We used to as well, now on both 400s and -8s we only select N A/I after all start valves are closed...
JammedStab
26th Apr 2014, 15:24
In a perfect world 40psi would be nice during start but realistically with old APUs and leaky bleed ducts, possibly an ADP and a pack running plus doing a dual engine start, dumping air overboard through the NAI system is going to make the start difficult. Some APUs can barely maintain 25psi during the start on a single engine. I think it's basically written that way to make sure all the bleed air possible is given to the starter to have a successful start in a worst case senario.
Perhaps that is why aft cargo heat or Lower Lobe airflow supply(on freighters) can be selected on after start. Keep air for the engines.
I have noticed that when the two Start valve are selected open, duct pressure decreases, frequently below minimum and then climbs back up. Not sure if the minimum duct pressure required is based of the lowest value achieved of where it eventually stabilizes.
APU regulates to around 30psi normally but will attempt to ramp up to 40psi when a start valve is opened which accounts for the initial drop and rise. Some obviously better than others.
JammedStab
27th Apr 2014, 13:13
APU regulates to around 30psi normally but will attempt to ramp up to 40psi when a start valve is opened which accounts for the initial drop and rise. Some obviously better than others.
Sometimes it stabilizes around 28 psi. How many packs do you normally turn off prior to start?
2 off with a good duct pressure during start but if it drops below 25 I'll turn the 3rd pack off. If it's a crap APU I go for all off straight away.