Busted APU On 737?

radelow · Fri Jul 30, 2004 1:45 am

I flew PHX-SAN last night on Southwest. It was an older 737 as the cockpit had the old analoge gauges. Anyway, the flight was running late and the pilot came on and told us we were waiting for the equipment to start up the engines. Huh? Don't normally APU's do that? Anyway the cart eventually came and the engines started on up. As we left the plane I asked the pilot why they used the remote starter and he said the APU was MIL or MAL or something like that which I assumed means busted??? Is a plane allowed to fly with a busted APU?

Mark

OPNLguy · Fri Jul 30, 2004 1:50 am

It was the "MEL" or minimum equipment list that he was referring to. It's perfectly safe, legal, and otherwise acceptable to operate a 737 without an operative APU, as long as any restrictive MEL provisos are complied with. The only one that comes to mind is that both engine-driven generators have to operative, as I'm sure they were. If one had not been, either it, or the inop APU would have had to have been fixed prior to departure, since you need 2 operative generators (the APU drives one too) to launch...

broke · Fri Jul 30, 2004 1:59 am

Each transport category airplane has what is known as an MEL or minimum equipment list. It is indexed by the ATA Chapter and lists what equipment can be inoperative and what restrictions, if any, have to be observed.
Generally the APU can be inop unless the flight is going into CAT III weather conditions or is on an ETOPS flight.
For a CAT III approach, the APU has to be operating with its generator on standby to pick up the load if an engine driven generator fails. Nothing like losing a lot of your electrical power on short final in nasty weather to raise your level of concern.
The APU on an ETOPS flight doesn't have to be operating, but it has to be operational in case of some system failure occurs requiring the APU to provide electrical power.
This is only one of many possible conditions that the MEL covers.
There is another list called the CDL, configuration deviation list, which lists what parts may be missing from a plane and what restrictions need to be observed. Most of these parts are aerodynamic fairings and the restrictions take into account the added drag that can be incurred when one is missing.

radelow · Fri Jul 30, 2004 2:06 am

Well the weather, as usual was beautiful and well PHX-SAN isn't ETOPS....so that's what it must be! Busted APU. Thanks guys!

Sure sucked sitting on the stand with no A/C in PHX.

744lover · Fri Jul 30, 2004 9:01 am

Hi Broke,

"For a CAT III approach, the APU has to be operating with its generator on standby".

Don't know about what airplane are you referring to pal, 'cause on the 747-400 (and I think that on the classics too) the APU cannot be started in flight. They have all 4 engine electrical generators running in parallel.

At the specific case of the 747-400, the APU can be left on after departure until 15000 feet providing bleed air, or 20000 feet running unloaded.

See ya,
GHN

LineMechQX · Fri Jul 30, 2004 9:22 am

744lover- Having never worked on a 744 before I wouldn't know for sure. But if you or someone else who has worked on them, could confirm you statement that would be cool. I know CRJ apu's can be started enroute, depending on altitude. And it seems the usefulness of an apu would be highly reduced if an airstart wasn't possible. ie Q400 apu cannot run in flight, therefore we call it ballast. Though single engine cat III's are approved. But that's another story.

Late
PC

modesto2 · Fri Jul 30, 2004 10:19 am

Radelow, generally, if the APU is inop, maintenance will MEL it. They'll bring out an air start cart to start one engine (I think no. 2 away from the jetbridge), push the aircraft away from the gate and then do a cross-bleed start. At OAK, I've heard pilots requesting a cross-bleed start from the tower because they usually need to rev the engines above idle...make sure nothing's behind you!

As stated, each airline has a list of MELs. Basically, if "this" happens...then you can do "that." Essentially, the MEL charts illustrate when an aircraft can still operate with an inop component(s).

L-188 · Fri Jul 30, 2004 10:42 am

And inop APU isn't a groundable item.

As mentioned they brought out the cart to supply air for the starters.

Starlionblue · Fri Jul 30, 2004 11:18 am

The APU on an ETOPS flight doesn't have to be operating, but it has to be operational in case of some system failure occurs requiring the APU to provide electrical power.

I've flown an AA763 JFK->CDG with an inoperative APU. Also, the first start cart could not provide power. All the lights started flickering, then it went dark. Several times. Isn't that ETOPS?

MD11Engineer · Fri Jul 30, 2004 1:23 pm

The air cart will only provide compressed air. Electrical power has to come from the GPU until one engine is running. Sometimes clowns of rampers pull the plug before the engine is running. It can be very expensive, at least f*cking up the start valve, if not the whole engine...

Jan

HAWK21M · Fri Jul 30, 2004 5:23 pm

Whenever we have an APU on MEL,We supply Electricals thru GPU cart & Pneumatic supply thru Pneumatic Jet Starter.Get one engine started up usually #2.Get the Aircraft pushed back with clearence at the back & the crew cross start the other engine using Cross bleed air from the 1st operating at around 80%.
What the pilot was reffering to was that the APU was MAL [Malfunctioning] or APU was on MEL.
About getting hot inside the aircraft with inop APU,A ground AC cart is available.I wonder why it was not used.

regds
HAWK

broke · Fri Jul 30, 2004 9:10 pm

When I was referring to the restrictions on operations with an inoperative APU, I was referring to twin engined airplanes not 3 or 4 engined airplanes.

radelow · Sat Jul 31, 2004 12:15 am

Why wasn't the A/C used...Hrm...maybe because the turn-around was like 20 minutes and WN is cheap. Smile

It never got all that comfortable on the plane. I wouldn't be surprised if one of the A/C packs was broken as well. This plane was *old*.

SlamClick · Sat Jul 31, 2004 3:39 am

Hi 744lover. To clear some things up, it is quite common to be able to start the APU in flight. I'm surprised that it is not on the 744. The A-330 has a dedicated APU battery and it can be started in flight after the loss of all engines. (At least you don't have to crash in the dark.)

Some earlier designs had made it possible to start it inflight so long as normal electrical power was available, but it could not be started on battery if the battery was the only source of electrics.

On the 727, the APU could not be run in flight at all.

MD11Engineer · Sat Jul 31, 2004 3:45 am

SlamClick,

On the older 747s I worked on the APU can´t be started in flight, AFAIK due to the design of the APU inlet door.
I assume they considered 4 engines/generators enough redundancy.

Jan

CVG777 · Sat Jul 31, 2004 5:29 am

How ironic, I had had the exact same question becuase yesterday, July 29th, I was flying ATL-FLL on a Delta 763 and the pilot told us that we had to have the engines cranked due to the busted APU, "which we'd be flying without today." Big grin

I am far from being any type of mechanic, but I take it that this is something that can be fixed in overnight maintenance? Is a busted APU a relatively common occurence?

SlamClick · Sat Jul 31, 2004 6:01 am

Well, CVG777 there are a couple of sides to the answer for your question. As noted in posts above, airliners have a Minimum Equipment List (MEL) for components which specifies how many are installed, how many must be operative for a (passenger)(ferry)(unpressurized)(but not into known icing etc.) flight, depending on what is broken. It also defines how long they have to make repairs (24 hours, three days, ten days, 120 days) what restrictions the condition might impose, and special procedures to be used.

What you will not find in the MEL is "wing." Two wings are installed, two wings must be operative. On the other hand, if a given fuel tank has two electric fuel pumps, one may be inoperative provided . . .

So, unlike a wing, an APU is a redundant, non-essential component so of course you are more likely to be flying with an APU inoperative. You will never be flying with a wing inoperative. Summing up: Non-essential means it is more likely that it might be inop on any given flight.

Now an APU is a very complex item. It is a small jet engine that runs a generator and either supplies its own bleed air, or runs a compressor for pneumatic service. It has self-test gear, automatic fire supression system and a host of other features. Machines can break at any point and so APUs are, historically somewhat trouble-prone. For example, if a fire detection loop fails, you cannot start the APU even though it works fine. It just could not detect a fire, so we are not permitted to run it.

Long answer, but I hope it explains it for you.

Starlionblue · Sat Jul 31, 2004 6:29 am

The A-330 has a dedicated APU battery and it can be started in flight after the loss of all engines. (At least you don't have to crash in the dark.)

Thx for making me spill my tea Big grin

To clarify my AA763 JFK->CDG experience:
- The first 763 was frelled. We did not board. They brought out a new one after about an hour.
- After boarding, the Captain said the APU was inoperative and we would get power + air from the ground to start an engine.
- Wait wait wait.
- Presumably connections were made. The lights went off, then on as power was provided by the cart. One engine started spooling up, then after about 4-5 secs all the lights started flickering on-off-on-off-on, then finally off. Emergency lighting came on (nice to know it works).
- This was tried a few more times. Same result.
- Captain came on and said the cart was not providing the right amount of power and some circuit breakers kept popping out. We had to get a new cart. At this point, some pax were pale and wanting to get off. I tried to explain but got nowhere. The Captain and the head F/A came on and said nothing was wrong with the plane. It was the cart ("which we will not be bringing with us").
- Wait wait wait.
- New cart. Startup went fine.
- 4 hours delay but I did get 8000 AAdvantage miles as compensation. Wheeee!

My question: JFK->CDG is ETOPS, right? So how could we fly with an inoperative APU?

[Edited 2004-07-30 23:30:07]

MD11Engineer · Sat Jul 31, 2004 7:29 am

They probaly rerouted the flight further north as to stay out of ETOPS limitations.

Jan

737doctor · Sat Jul 31, 2004 9:04 am

From the SWA MEL concerning APU's

May be inoperative provided:

A. All TR units operate normally.

B. Procedures or other deferred items of equipment does not require its use.

C. Visual confirmation is made that no damage has occurred to the APU exhaust area. The check should include the tailcone area and adjacent control surfaces to confirm that there is no evidence of heat damage or delamination.

D. Install placard near APU panel per MEL SP #1--Placarding.

Auxiliary Power Unit Pneumatic Power System may be inoperative provided:

A. May be totally inoperative.

B. May be inoperative with a maintenance limit "FOR GROUND USE ONLY".

C. May be inoperative with a maintenance limit "FOR ENGINE START ONLY".

D. Install placard and limit near APU panel per MEL SP #1--Placarding.

Note: For Auxiliary Power Unit Electrical Power System refer to 24-2 APU Generator System.

Radelow, as for whether or not the packs worked:

One may be inoperative provided:

A. Flight Altitude is limited to FL 250 or below.

B. Refer to Appendix 10-1 Pressurized/Unpressurized Flight with Pack(s) INOP.

C. Install placard on pack switch per MEL SP #1--Placarding.

D. Note: This (M) procedure may be accomplished by the flight crew.

(-200)(-300)(-500) Figure 02-21A

May be inoperative provided:

A. Limit altitude to 10,000 ft for passenger flights. Refer to the appropriate sections of the Flight Operations Manual.

B. Maintain outflow valve full OPEN.

C. Refer to Appendix 10-1 Pressurized/Unpressurized Flight with Pack(s) INOP, Two Packs INOP.

D. (-300)(-500) Lower cargo compartments remain empty.

E. (-300)(-500) Ramp personnel to verify cargo compartments are empty before departure.

F. Install placard on pack switches per MEL SP #1--Placarding.

Note: This (M) procedure may be accomplished by the flight crew.

And how would you know if they indeed worked or how old the aircraft was? APU's do indeed break down and as SlamClick put it, they can be MEL'd for a variety of reasons. Don't be dissing my employer over things you don't understand.

JetMechMD80 · Sat Jul 31, 2004 10:23 am

CVG777,
As for it being able to be fixed overnight, yes it most likely could be. However, how do we know that the APU was not placed on MEL that same day? Maintenace would not yet have a chance to look at it. AND maybe a part that is needed is not on hand. You can not keep every part, you might need at every station. Thats why we have MEL's.

LMP737 · Tue Aug 03, 2004 4:48 am

The beauty of the APU on the 737NG is that it's not only relaible but easy to change as well. In the first year of service AA only changed one 737NG APU. They had budgeted for much more but were pleasantly suprised when they did not have to.

Changing the APU on a 737NG can be accomplished by two people in one shift quite easily. In fact if you are proficient at it it can be done in between 4-6 hours. It seems like every generation of aircraft gets easier to work on.

B757A320 · Tue Aug 03, 2004 10:33 pm

ETOPS is only for two engine aircraft that are certified for this flights. (md-11, l-1011 or B-747 are not etops).

an APU is designed for electrical power and bleed air power supply.
If the malfunction is in the APU bleed system the APU can still be used for electrical power supply.

an inoperative bleed system of APU doesn't have to affect an ETOPS flight because the APU is required for it's electrical pwr back-up.

on B-757 and A-320 the APU is able to start in-flight but is not always able to deliver bleed air, because of the air thickness on 10 km height.

the cause of an inop apu bleed system can also be the apu bleed air valve installed in the bleed air duct (ATA 36)

a BUSTED APU isn't always the right term. (unless the whole APU inop)

PhilSquares · Tue Aug 03, 2004 10:53 pm

Having been on the 744 and 747 classic for a number of years, I can confirm the statements about the APU not being started in flight.

On some models it is certified for inflight operation, but again not inflight start. The basic reason is the APU inlet door. To certify it for inflight starting would require a complete redesign.

For CATII/III operations you don't need the APU.

radelow · Wed Aug 04, 2004 2:38 am

I wasn't dissing them but they did not run the A/C on the ground because the APU was busted. The A/C being busted was a joke. In flight we had full A/C as the engines provided bleed air.

The plane was OLD. What's wrong with saying that? Still got me to SAN safely. Some people are so seriously here. Sheesh.

Mark

L-188 · Wed Aug 04, 2004 5:27 am

Radelow, just wanted to add the obsevation that until the 1960's and the 727 most jet aircraft did not even have APU's, but required ground power and pneumatic power to get the lights running and get the engines started on the ground.

NKP S2 · Wed Aug 04, 2004 5:46 am

The plane was OLD. What's wrong with saying that? Still got me to SAN safely. Some people are so seriously here. Sheesh.

Well, this is the serious forum Smile

Really though, I doubt the connection can be made from an inop APU to the airplane's age. Would "old" be an issue if the APU was inop on a 2-3 year old plane? What if you had a 25 y/o A/C with a low-time APU...or...a 6 year old A/C with a high time APU? It's not like a 737 flies around for 25 years with the same engines/APU/landing gear assys it had when it rolled out the door brand new.

XFSUgimpLB41X · Wed Aug 04, 2004 9:35 am

Guaranteed on a hot summer day, or a realy really cold day.. the APU is gonna take a big stinky dump in the CRJ- just to tick us off and make us work harder. Smile

Brand new airplanes, but that doesnt mean shat. New airplanes tend to have plenty of problems as the bugs are being worked out. APU's are finicky pieces of machinery anyways. If one of the peripherals dies on them, they just differ the whole thing and the fun begins.

It's quite the hassle doing the huffer start, pushing back, doing the crossbleed start.... then in summer time sweating yourself silly because the engines barely put out any air at idle thrust.... or in wintertime having to do an unpressurized takeoff and landing. Of course it always happens on the last leg of the trip too. Smile

greasespot · Wed Aug 04, 2004 10:35 am

You can change a 732 APU in 4 hours with 2 people. I have done it. The 727 is even faster providing the APU is in the cocoon and ready to go.

Greasespot

shark · Thu Aug 05, 2004 2:01 am

For a CAT III approach, the APU has to be operating with its generator on standby.

I was referring ot twin engined airplanes not 3 or 4 engined airplanes.

Broke,
Where did you get this information. I've worked on MD80's and 737's for the last 15 years as a line mech. An APU on MEL is not reason for a CAT downgrade.

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