B767 APU bleed on for take off

[winglit]

I'm looking after a couple of B767's in Quito, Ecuador at the moment. Although it seems that these PW4000 powered jets aren't suited to this operation due to it's performance limitations.

I'm the engineer on these jets but I'm also a PPL, and I have a question that an experienced tech pilot might know:

Quito (UIO) is 9,300 msl. There are runways 17 and 35. On our performance charts there is almost a 20 tonne difference on MTOW for either runway. Primarily due the the terrain and single engine performance off 17.

The way that the operator loads these flights, we are almost always at MTOW and sometimes we have to wait for the wind to change or at least drop below a 10kt tailwind so that we can take RW35.

Sometimes a tonne extra payload can make a huge difference and we occasionally have to offload bags.

With a packs off T/O we could gain an extra 1000KGs, but it's not recommended at Quito as you would be above 10,000ft in only a few seconds, and the crew should really be wearing O2 masks.

Why can't you take off with the eng bleeds off, but with the APU bleed on, powering the packs? I've asked the same question to our pilots and they just reply, "Because it's not in the book!"

Does anyone have an explanation?

[westhawk]

I would think Boeing must have high elevation airport operations options available for a price. Have you consulted with Boeing about this? Might be worth a phone call if you haven't done so already.

[plain-plane]

one further question is: what is max allowed altitude with only apu feeding the packs on a 767... any other apu limitations?

[763 jock]

The reason that you cannot do a "bleeds off" on the (GE CF6) 767 is because the ADP is on the centre hydraulic system. If you attempt a "bleeds off", the ADP will not run sufficiently on APU air only. I've no knowledge of the PW version though.

Gear and flap retraction is very slow, shortly followed by "gear disagree" and "flap disagree" EICAS cautions.

Colleague of mine found this out after an engineering request.

[dusk2dawn]

"My" 767 AFM states 8400' pressure altitude as max for t/o and landing so you must have some kind of "paid-supplement" from Boeing already ! or ?

[grounded27]

With a packs off T/O we could gain an extra 1000KGs, but it's not recommended at Quito as you would be above 10,000ft in only a few seconds, and the crew should really be wearing O2 masks.


I had a cpc failure out of Quito for Guayaquil in an MD-11, we were 12k to 14k in the time it took us to don oxy and go manual on cabin pressure. That approach and T/O is a BLAST!

[Pontius]

I, personally, can see no reason why you shouldn't be able to do it (once it has been approved by Boeing etc, of course). We used to do a similar thing on the 737 (3/4/5) but I appreciate the differences between the aircraft types. 763 jock has some very good points but the centre hydraulic system's primary source of power is from 2 electric pumps, with the air-driven demand pumps coming from the centre manifold (supplied, in this case, by the APU). The demand pumps are, of course, there to help out the struggling electric pumps during periods of high hyd press demand and I have absolutely no doubt whatsoever that using APU air to power both packs and the hyd demand press pumps will cause wheezy performance of the hyd demand pumps and lead to slow gear retraction and, possibly, flap retraction. At the end of the, however, the systems will still all work and the THEORY works.

A possible solution to this could be to isolate a pack, thereby leaving more APU air to help out the hyd demand pumps. One pack is good to 35000', so you could reconfigure to normal operations before that becomes a problem.

In answer to Plain Plane's question, the APU is supposedly okay supplying air to the packs up to 20000'.

[8che]

Pontius, how long do you think it will take for the centre system with a power starved ADP to raise the gear and flap ?

Thats the point. It may do it but your engine out profile to 1500ft is busted with gear doors etc blowing in the wind.

Its the ADP thats the stopper here.

I would also be interested to see the approval for 10000ft T/Oand Landing ops from Boeing.

[Pontius]

You're right, 8che. I was only considering the theoretical case behind whether or not it would be possible. I'm pretty certain it would be possible but I admit to having no idea of how long the wheezing demand pumps would take to assist the electric pumps to do their job.

Most important is the engine failure case that you state. I, stupidly, did not think of that and, for obvious performance reasons, there's no way Boeing would certify the procedure; if not for any other reasons, then definitely for this.

I've never seen one but I've heard of a few South American approvals for operating above 8400'. I believe these are issued individually to airlines and clearly have specific procedures applicable to the airports they're operating from. I once spoke to a pilot from this region who operated a 757 and he considered me a sea level type of pilot as I'd only made it up as high as Bogota :)

[763 jock]

I seem to recall the the ADP is automatically signalled to run when the take off roll is commenced. Whilst it is a "demand pump", the MEL limitations are huge if it is inop. There is a penalty of about 18000 kgs if the ADP is inop, whereas the number two electric is allowed with no penalty if inop. No 1 electric is not allowed U/S (reserve brakes & steering). Basically, the ADP provides the muscle for gear and flap retraction. It needs engine bleed air for power, hence no supplementary procedure for "bleeds off". Packs off is OK, commonly used.

The 757 is a different bit of kit. You can do a bleeds off as there is no ADP.

As for high altitude ops, LBA is very challenging. :}

[None]

I support 763 jock's perspcetive.

If you go to Boeing's Volume I, you will see that the APU-to-Pack procedure is only valid for the 757 due to its different Pneumatic System. Any use of the APU-to-Pack procedure for the 767 would be contrary to Boeing procedure.

Think of it this way....the reason you are doing a Pack-off takeoff is because you are performance limited. Now try running the APU during the takeoff, and you have a few problems for the 767. First, like 763 jock says, there is not enough pneumatic air to power both the pack and the ADP. Boeing tells us this. If you lose an engine, then you now have a very slow or even no gear retraction in a performance limited scenario. Not good. In reality, it is actualy worse than what your performance says because you have the APU door open. On the 757 that is a 500 pound hit on performance. I don't recall what it is for the 767.

I fly both the GE and P&W version of the 767. They are similar with this particular issue.

Also, as 763 jock states, the Performance hit for an ADP inop is huge for the 767...on the order of 40,000 pounds.

I'm with 763 jock. Since it is not an approved procedure in the Boeing manuals, and it also is clear to me why it is not approved, I will never do it in a 767.