So I've been flying the Fenix A320 the last few days, and when I switch off the APU at cruise, the plane shudders momentarily and slows down a few knots. Surely this isn't true to life. Is anyone else experiencing this? It's a minor issue, but I thought it was worth mentioning.

10 minutes ago, Jeeper said:

So I've been flying the Fenix A320 the last few days, and when I switch off the APU at cruise, the plane shudders momentarily and slows down a few knots. Surely this isn't true to life. Is anyone else experiencing this? It's a minor issue, but I thought it was worth mentioning.

Out of curiosity, why do you shut down the apu at cruise? 

No real reason to run the APU at cruise unless you’ve got some kind of problem. APU is usually selected off after engine start or after takeoff if needed for performance reasons to run the packs.

1 hour ago, rick celik said:

Out of curiosity, why do you shut down the apu at cruise? 

The first time was out of desperation. I miscalculated fuel for a long flight with a headwind, so I needed to squeeze out every drop to make it. When I switched it off I noticed a slight kick, so on the next few flights I switched it off at cruise just to see if it repeated and it has each time. 

When the APU is running a small flap is opened on the side of the fuselage (the air intake for the APU). Could this be contributing to the slight loss in speed and minor buffet you're experiencing when starting it up?

I mean I don't know, but this is Fenix - I'm sure it's intentional!

Edited July 30, 2025Jul 30 by Tom Wright

38 minutes ago, Jeeper said:

I miscalculated

How is that even possible? When you fill out your flightplan completely, insert the average wind component and then fill out the fuel init page completely it will show you how much fuel you will have left at your destination. 😉

Why would you even takeoff with the APU on in an A320? I don't know of SOPs explicitly recommending it unless for performance reasons, or sometimes high density altitude situations, or MEL situations? 

The most usual is switching the APU off after the engines are started.

Anyway, looks like the Fenix must be really that detailled that it even considers the drag effects from the deployed APU "window"!

Edited July 30, 2025Jul 30 by jcomm

You might use the APU to run the packs if you’re taking off somewhere hot and performance limited and want to keep the pax cool. You might also need it you lose electrical power (think Sully in the Hudson) but otherwise there’s no need to run it after the engines are started.
 

It will use fuel that isn’t accounted for in your flight planning so you may well over a longer flight run into fuel problems. 

I don't know about the A320 but I know for the B737 it's an ETOPS requirement that the APU is kept running throughout the ETOPS segment of the flight, I think because the 737 lacks the redundancy in the electrical and pressurization systems of the larger aircraft when losing an engine.

1 hour ago, Tom Wright said:

When the APU is running a small flap is opened on the side of the fuselage (the air intake for the APU). Could this be contributing to the slight loss in speed and minor buffet you're experiencing when starting it up?

The OP is shutting down the APU though, not starting it up, so if anything, the speed should increase slightly when the flap closes?

The OP is, however, seeing the speed decreasing. I believe this is because, as long as the APU is on, it supplies bleed air, but when the APU is switched off (or, more precisely, when the APU bleed air switch is turned off), bleed air is now taken from the engines, and at a given N1, this leads to a reduction in thrust. (This is the same reason that packs are turned off on performance-critical takeoffs.) I trust this is something that Fenix would have taken into account in their custom engine model - and the OP's observation appears to prove that they have. 👍

2 hours ago, Jeeper said:

The first time was out of desperation. I miscalculated fuel for a long flight with a headwind, so I needed to squeeze out every drop to make it. When I switched it off I noticed a slight kick, so on the next few flights I switched it off at cruise just to see if it repeated and it has each time. 

From the above quote it seems to me that the OP believes that the APU should be running throughout the flight. It shouldn't.

3 hours ago, Jeeper said:

The first time was out of desperation. I miscalculated fuel for a long flight with a headwind, so I needed to squeeze out every drop to make it. When I switched it off I noticed a slight kick, so on the next few flights I switched it off at cruise just to see if it repeated and it has each time. 

You realize having the APU running is burning MORE fuel, right?

Let's clear a few things up... 

I turned the APU off, not on at cruise 

I don't usually leave it on after engine startup, but I had a couple glasses of wine whilst in the cockpit 

I miscalculated the fuel needed because, well, see above. 

But 3 times since (and without the wine) I left the APU on until cruise to see if I was imagining things and, lo and behold, it happened all three times. 

 

Edited July 30, 2025Jul 30 by Jeeper

3 hours ago, Jeeper said:

Let's clear a few things up... 

I turned the APU off, not on at cruise 

I don't usually leave it on after engine startup, but I had a couple glasses of wine whilst in the cockpit 

I miscalculated the fuel needed because, well, see above. 

But 3 times since (and without the wine) I left the APU on until cruise to see if I was imagining things and, lo and behold, it happened all three times. 

 

If you think you've found a bug, then you need to file a ticket with Fenix. No one here can do anything about it except nitpick over your APU usage. 

6 hours ago, Tom Wright said:

I don't know about the A320 but I know for the B737 it's an ETOPS requirement that the APU is kept running throughout the ETOPS segment of the flight, I think because the 737 lacks the redundancy in the electrical and pressurization systems of the larger aircraft when losing an engine.

No it isn’t.  It’s operator dependent.  Air Canada, for instance, does not run their APUs in the ETOPS segments.  There’s just maintenance records required showing that they will start at altitude.