APU Start B747/777

[160knots]

Could somebody explain why some pilots flying the above a/c position the APU start switch to the ON position and then after a couple of minutes select START.

While most other pilots position the start switch directly to the start position as per the manual.

Is there a reason for the time delay?

[Shanwick Shanwick]

The ON position opens the inlet door to the APU and some believe that exposure to ambient conditions prior to start prevents damage after prolonged cooling at very low temperatures.

[18-Wheeler]

Yep, as mentioned above it's to give the APU a few minutes to warm up after a long leg in cold air to help it start better. The trick is to make sure to do it below 10,000', or more specifically below ~250kts, so the APU doesn't spin fast enough to begin the auto-start sequence but without the starter engaged.
Some 747 Classics have the APU switch connected to the ground safety relay and so it won't let you open the door before touchdown though.

[vs69]

I was under the impression it's a feature on all -400's that the inlet door won't open in flight due to being wired to the ground safety relay so therefore you wont get a start out of the APU anyway?

[Jet Man]

If they're moving the selector to ON on the ground then it expedites the APU start when START is selected. Therefore, the APU can be started later hence it saves a few kilos of fuel.

[18-Wheeler]

If they're moving the selector to ON on the ground then it expedites the APU start when START is selected. Therefore, the APU can be started later hence it saves a few kilos of fuel.

I think you'll find it's more important to ensure a good APU start, rather than have to go to the gate then wait for a ground cart to plug in so you have power before you shut the last engine down .... while it's burning a good 800kg/hr.

[zlin77]

On the 777, there was a Boeing service bulletin some years ago stating that a slight pause in the ON position was required before selecting START, I think from memory it allowed all the self test pre-start functions to operate correctly.......

[5LY]

On the 777 I'd have said it was to force you to be a bit more delibarate about selecting the switch to start and back to on. A quick flick of the switch to start and back to on was sometimes not having the desired effect, therefore the bulletin which has you pause for a beat at the start position before allowing the switch to return to on. I think it was just to ensure solid contact.

Having said all of that, it's unusual for anyone including me to get a leg up on Zlin. I mostly have to bow to his superior knowledge.

Cheers, 5

[Spooky 2]

There may be some confusion here regarding the start procedure on the 777 APU? Currently there is an operators bulletin TBC 79R1 that talks about the correct method for starting the APU.

"When starting the APU, position the selector to Start and hold for 1 second, then position the Start selector to ON manually. Do not alllow the APU selector switch to spring back to the ON position".

Not aware of any of the other methods described as being a part of any formal Boeing procedures.

[777AV8R]

Yup, the reason for holding the Start switch to 'ON' for one second, was to ensure that the start sequence was initiated. The switch is spring loaded and if the selection isn't deliberate enough, one taxis to the gate and...no APU!

[NSEU]

If they're moving the selector to ON on the ground then it expedites the APU start when START is selected. Therefore, the APU can be started later hence it saves a few kilos of fuel.

Seems illogical to me.

Fuel is not going to be used until the starter motor has cranked the APU to a certain rpm. Cranking won't begin until the door is open. If you went from ON to START or OFF to START at the same time, you would save more fuel with the OFF to START because the APU would begin spooling up 5 or so seconds later.

Switch set to ON (but not start)... distractions occur... Now, did I remember to turn the APU selector to START? (Does the EICAS Memo message APU RUNNING appear prior to 95%rpm?).

Experts: What exactly is cold soak? What parts of the APU are affected by cold soak? (and what parts of the APU are you actually thawing out by leaving the inlet door open during a normal taxi to the gate?).

[main_dog]

The way we do it at my outfit (744) is indeed to first switch it to "ON" during the after landing flow, then select "START" when about one or two minutes from the gate. In this way fuel is "saved" because instead of cranking up the APU right away, it is only started just before it is needed, and by having the inlet door already open it takes under a minute to start (can't remember how long... 36"?)

My (already hazy) recollection of the 742 is that, in a similar fashion, the F/E first opened the APU inlet door and then waited a bit before starting the APU, but that this was specifically to let warmer ambient air in for a while, thus increasing the chances of a successful start after cold-soak in cruise. I suppose this also applies to the 400... and I would have to guess that the parts "warmed up" would be the inlet area (surfaces, inlet screen and inlet guide vanes).

Incidentally, here is an excerpt from the systems volume of our 747-200s:

Air Inlet Anti-ice System

An APU air inlet anti-icing system, which utilizes APU
bleed air, is provided to prevent ice formation on the
inlet guide vanes, inlet duct surfaces and inlet screen.
A thermal switch in the inlet duct opens the anti-ice
valve when icing conditions are encountered. The
APU bleed air valve switch must be open to supply
bleed air to the anti-ice system.



Does any -400 expert here know whether this also applies to the 744's APU too? In other words does the APU bleed valve switch have to be on for the APU to receive anti-ice protection?

[NSEU]

I don't have any 744 APU inlet door opening times yet, but the door only has to open 13 degrees (out of 45degrees) to satisfy the APU start logic. If this takes longer than 10 seconds, I'd be most surprised.

If you can time your APU start to this accuracy to save fuel, hats off to you :)

[FullWings]

We don't have it as an SOP but:

1. If you want to save a tiny amount of fuel by starting the APU quicker and later you can.

2. If you forget to start it, you have to spend less time with an engine running waiting for it to fire up. Significantly more fuel saved.

[gas path]

Does any -400 expert here know whether this also applies to the 744's APU too? In other words does the APU bleed valve switch have to be on for the APU to receive anti-ice protection?
Dunno about the 'expert' bit BUT....
The 744 APU does not have an A/I valve like the classic does. Also the APU bleed air valve is normally left in the 'ON' position but opens and closes as a function of the APU operation.
Same for the 777 too!

[NSEU]

I watched the APU Maintenance Page on a 744 today. The inlet door opened in 8 seconds (after selecting "ON" from "OFF").

Unless the APUC is very slow to run through its required prestart tests, I don't see the point of using the OFF to ON, then pause, then START technique specifically to save fuel.

Cheers.
NSEU

P.S. Note that this 8 seconds may not be for full open, but rather, the time taken to reach the 15 degree open mark which triggers the logic to allow start. On the ground, the 744 APU inlet door opens to 45 degrees.