APU Overhaul

[ppppilot]

Once, an engineer told me that the APU goes to overhaul, usually due to cycles not to hours running. He also told me, that one cycle it is considered to be every time EGT changes, not every start and shutdown. That means a cycle is when the apu is on and you connect the bleed to the packs.
Does anybody know if that it is true?
Tailwinds

[Dream Land]

When first learning to operate turbine engines I was always taught that it was the starting process that the hurt the turbine engine, otherwise it could virtually run forever, but it seems to fly in the face of what my companies policy is by shutting down the APU even if the leg is only 30 minutes. :confused:

I think generally the less thermal cycles the better, i.e. trying to accomplish a complete descent at thrust idle, hopefully an engineer will speak up.

[javelin]

Also consider this - most of the Spanish and Canaries airfields insist on you shutting the APU down 5 mins after chocks.

This doubles the starts and shortens the life of the APU.

We can't guarantee that the fixed power will either :-

a. Be there

b. Work !

They place all these constraints upon us without thinking of the financial repercussions :uhoh:

[hetfield]

I'm afraid there are enough beancounters around in various companies who tell us pilots when to start the APU by SOP:\

[VinRouge]

3 factors here:

Thermal Shock (which affects fatigue life of the blade and ultimately leads to cracking due to thermal stresses generated). A blade will ultimately fail when cracks are created in the blade due to plastic hardening. Due to high cycle fatigue in turbines, the crack can propigate QUICKLY!

Creep (generated by temperature and centripetal forces on the blade, the first a function of load on the APU, the second a function of speed which is typically constant in APUs). Failure mode is Creep bursting or the tips wearing out the frangiable material leading to decrease in turbine efficiency and ultimately siezure.

At the end of the day, you have to do what your SOP/Local procedures specify! I would imagine the design point for apus are quite forgiving, as due to the nature of their operation. Many have centrifugal compressors, thus at least from the cold end, not too many worries. I would IMAGINE (not 100% on this) that the hot end would be designed more for longevity rather than fuel efficiency, meaning that the blade designer could get extra material in there to maximise vibration damping, whilst lower operating temperatures would minimise thermal stress cycles whilst perhaps not offering the best fuel efficiency.

[Brian Abraham]

It seems every turbine manufacturer has its own policy. On the Turbomeca (helicopter) you counted cycles by each start cycle and an increment was then added for each time the N1 dropped below a certain figure. Forget the exact numbers but for example <70% add .1, <80% add .05. In our operation not uncommon to have a cycle count >2 for a flight. Aircraft was fitted with automatic counters but always had troubles reconciling the number given and think engineering gave up on them.

[gas-chamber]

Also to consider whether the APU is on a Power by the Hour lease or fixed maintenance contract and fuel burn in combination which could have the hours driving the dollars more than the cycles.

[lomapaseo]

Typical cycles are what the manufacturers recommendation for maintenance are based on. Cycle limitations consider the following:

For the highest stressed parts under centrifugal loading (the rotor disks) the cycle considers the greatest change in stress which includes not only speed effects but thermal effects. Often a cold start to takeoff is worst then a warmed up engine from flight idle to full power in flight.

Just to add complexity to this, it is not unusual for the combinations to actually drive the lowest stresses into compression (negatives) due to thermal fights and this significantly lowers the (start-stop) fatigue life.

For the hot section parts, considerations of time at max temperature become important. This is often statistically assumed based on typical start stop cycles and useage. However, for some applications it might take into consideration time spent between typical start and stop.

And then there are applications where time at max conditions are more critical than all others. This might be a condition where to meet a mission requirement you push the turbine inlet temperature right up to a max for only a minute or two in one cycle out of a 100 start and stops, and yet after accumulating only a half hour at that condition, your engine hot section needs to be overhauled.

The above are just examples and as always the maintenance manual reflects the actual experience of the manufacturer for that application.

[Dream Land]

Also to consider whether the APU is on a Power by the Hour lease or fixed maintenance contract and fuel burn in combination which could have the hours driving the dollars more than the cycles by gas-chamber Good point, that's probably what drives the strategy at my company. :ok:

[SNS3Guppy]

What constitutes a cycle varies with the powerplant manufacturer. It's inaccurate to say a start and a stop. Cycles are measured in part cycles, and complete cycles, depending on percentage of power or temperature changes; specifics are spelled out in the particular maintenance publication.

[ppppilot]

Thank you all.:ok:
I have also heard that 90% APUs are Honeywell, who bought Garret. The B757 APU use the Fairchild Metro II engine (Garret TPE331 now APU Honeywell 331). When I was said that, I was riding the CRJ200. I don't remember the CRJ APU manufacturer but every time packs was switched on, the APU EGT indication went into the red side for an instant. That probably was due to not having a system such as in bigger crafts with variable angle vanes for the bleed output. That also could be the reason for the cycle consideration of such a thermal shock.
Tailwinds