B727 APU starting problem

[Bleedvalve]

Morning guys, looking for info please to sort out a problem.
After long flight our APU does not want to start after landing. Left overnight it will start normally. Looks like coldsoaking causing it not to start. What could the problem be?

Thanx

Bleedvalve

[hetfield]

- Does the elec starter kick in (check DC Amps)?
- Does it accelerate?
- Is there an EGT rise?
- If not, what about igniter box?

[Bleedvalve]

Starter kicks in.
Will only go out at about 45 sec, sometimes not.
EGT rise and keep on rising.

It seems that the whole cycle just takes too long. Starter takes too long to kick out and EGT keeps rising.

[Checkboard]

If the battery voltage drops below 18V or so during the start, some of the relays fail to sequence. Select and watch the Batt voltage during the start, and see if the cold has affected the batteries? If the voltage is low the system will prevent the APU from operating due to no fire protection.

(Note: this is (an educated) guess! ;) )

[aviatorhi]

Kind of slim on the details there, how is the start cycle going?

Is it just refusing to even crank or is it not lighting off?

If it's cranking how much time are you giving it before turning it off?

Etc. Etc.

I've seen water get into strange places in 727s, might be some freezing the fuel valve shut in flight. I have this happen often on a crossfeed valve in one plane.

[con-pilot]

Could be the starter itself.

Checkboard has some good ideas on this. A low battery can cause a lot of problems.

[gas path]

What oil does it take? We got around a lot of 'cold soak' issues on the 747 classic by changing the oil from Mobil jet 2 to Castrol 325. The lower viscosity allowed the APU to spin up faster. (Don't think 325 is still available so we use Shell 390 on the 744)

[FlightPathOBN]

You have a 727 that starts? :mad:

[aviatorhi]

There's lots of 727s that start. A few DC-6s 7s even, a bunch of DC-3s too, those old American built planes outlast a lot of the stuff from other places, so don't be so surprised that theyre still running.