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APU Inflight start on Battery

Posted: Mon May 17, 2021 5:39 pm
by StereoTechque
APU installed on the A320 has a FL restriction of FL250 when only starting with batteries. However normally if TRU assisted then it can be started upto service ceiling.

What is the reason behind this?

Re: APU Inflight start on Battery

Posted: Mon May 17, 2021 9:27 pm
by Lpbri
I would guess that starting an APU in thinner air takes more juice.

Re: APU Inflight start on Battery

Posted: Mon May 17, 2021 10:40 pm
by acmx
Just a guess...but I would bet it because it’s harder to start an apu the higher you go. A battery is depleting it’s energy throughout the start so having a height ceiling makes sense to make sure the battery can last through the start. A tru is converting ac to dc so as long as everything is working it can supply power for any length of time. So it could supply power for longer to help with a high altitude start.

Re: APU Inflight start on Battery

Posted: Tue May 18, 2021 1:09 am
by Starlionblue
The A330 has the same limitation. The operational envelope diagram has a note about EMER ELEC CONFIG with the batteries only altitude limit. Emergency electrical configuration would indeed be the only time you start the APU in flight on batteries.

As mentioned above, probably a function of the power of the battery vs the engine-driven generators.

Re: APU Inflight start on Battery

Posted: Tue May 18, 2021 2:07 am
by StereoTechque
Alright..So in that case the self sustaining speed of APU will increase as we go higher.

Re: APU Inflight start on Battery

Posted: Tue May 18, 2021 2:36 am
by e38
StereoTechque wrote:
APU installed on the A320 has a FL restriction of FL250 when only starting with batteries. However normally if TRU assisted then it can be started upto service ceiling.

What is the reason behind this?


The issue is the decrease in air density as altitude increases.

The APU is a gas turbine engine which requires sufficient airflow to start properly. At higher altitudes, if using battery only to start the APU, the energy required to achieve sufficient rotation to achieve ignition can deplete the battery power. The FL250 lmitation is designed to conserve battery power when operating in the emergency electrical power configuration.

I’ve also been told by Airbus engineers that the colder temperatures at higher altitudes can initially result in excessive APU oil pressure due to greater thickness of the oil at the colder temps. The FL250 limitation also helps to mitigate this issue and reduces stress on the APU as well as the batteries.

e38

Re: APU Inflight start on Battery

Posted: Tue May 18, 2021 9:02 am
by StereoTechque
Thank you all. Doubt cleared.

Re: APU Inflight start on Battery

Posted: Wed May 19, 2021 4:13 pm
by Florianopolis
I mean it makes sense the air is thinner so the APU has to spin faster, but doesn't that also mean that there's less air to squeeze to the same pressures? The same power going in results in a higher rotation speed because the air is thinner for the same compression, in other words? Could it be because the battery has less juice at lower temperatures?

Re: APU Inflight start on Battery

Posted: Wed May 19, 2021 7:23 pm
by StereoTechque
Florianopolis wrote:
Could it be because the battery has less juice at lower temperatures?


Normally batteries are located in the Avionics bay which is temperature and pressure controlled. It is not exposed to ambient air. However they are ventilated to ambient by a small vent hole.