Turbofan stall on engine start?

[bluesky74656]

An interesting thing happened on a recent commercial flight. We taxied out from the gate and had to be deiced. I assume we taxied out on one engine.

After driving, as we taxied toward the runway I heard them trying to start the second engine. We were on an A320 and you can hear the "wah wah, wah wah" as the engines start. First question, what is that?

Anyway, after a minute or so that sound gets weak and dies out. We come to an abrupt stop and both engines are off. Out of the silence I hear the APU start up,followed by one engine and then the other.

The captain told us they had to "reboot the computers". But it sure sounded to me like they tried to start the second engine by cross bleeding from the first, and managed to stall the first engine in the process. Does that sound right? Or what else do you think could have been going on?

 

[Velocity173]

I would think if it were a compressor stall, you'd hear a popping sound??? Turbine helos can sometimes get them on start up with bleed valve issued or high altitude starts with worn blades / stagnant heat in the compressor. It's pretty loud too.

 

[Kritchlow]

An interesting thing happened on a recent commercial flight. We taxied out from the gate and had to be deiced. I assume we taxied out on one engine.

After driving, as we taxied toward the runway I heard them trying to start the second engine. We were on an A320 and you can hear the "wah wah, wah wah" as the engines start. First question, what is that?

Anyway, after a minute or so that sound gets weak and dies out. We come to an abrupt stop and both engines are off. Out of the silence I hear the APU start up,followed by one engine and then the other.

The captain told us they had to "reboot the computers". But it sure sounded to me like they tried to start the second engine by cross bleeding from the first, and managed to stall the first engine in the process. Does that sound right? Or what else do you think could have been going on?

If they were starting the second engine off the gate, without the apu running, it was definitely a cross bleed start. You need to add some power on the running engine to maintain a minimum psi of air. I'm surprised it stalled though. I can understand why it wouldn't start the second engine, but I figured it wouldn't bleed enough air to actually make the engine quit.

As far as the wah-wah on start up, that is just the harmonics of the engine. Sort of like a twin with the props out of phase.

 

[Kritchlow]

I would think if it were a compressor stall, you'd hear a popping sound??? Turbine helos can sometimes get them on start up with bleed valve issued or high altitude starts with worn blades / stagnant heat in the compressor. It's pretty loud too.

Yeah, that doesn't sound like a compressor stall..

 

[kayoh190]

As far as the wah-wah on start up, that is just the harmonics of the engine. Sort of like a twin with the props out of phase.

I'll defer to you Airbus guys, but is it possible the wah-wah sound was the PTU? That thing can bark pretty loud when you're firing up the other engine.

 

[Greg Bockelman]

After driving, as we taxied toward the runway I heard them trying to start the second engine. We were on an A320 and you can hear the "wah wah, wah wah" as the engines start. First question, what is that?

Did it sound sort of like a dog barking? If so that was a unit called a Power Transfer Unit. It is used to pressurize one hydraulic system with another hydraulic system. That was over simplified, of course.

Anyway, after a minute or so that sound gets weak and dies out. We come to an abrupt stop and both engines are off.

Did the airplane go dark?

The captain told us they had to "reboot the computers".

That is a very real possibility. There are a bunch of potential faults that require powering down the airplane completely and starting from scratch. I have had do do that on more than one occasion.

But it sure sounded to me like they tried to start the second engine by cross bleeding from the first, and managed to stall the first engine in the process.

I suppose that anything is possible, but I find that scenario highly unlikely, based on 2 years and 1,500 hours on the plane.

Does that sound right? Or what else do you think could have been going on?

I think my scenario is what went on.

 

[Kritchlow]

I'll defer to you Airbus guys, but is it possible the wah-wah sound was the PTU? That thing can bark pretty loud when you're firing up the other engine.

Absolutely it could be... The PTU sounds like a "sawing" noise. The slower the engine rpm the more so.
I interpreted the wah-wah to be a different noise, but he could indeed be talking about the PTU.

 

[Ted]

I think my scenario is what went on. After all, I was left seat.

 

[Greg Bockelman]

You know how to hurt a guy, Ted. I was not Captain at the time. That was 18 years or so ago.

 

[bluesky74656]

Did it sound sort of like a dog barking? If so that was a unit called a Power Transfer Unit. It is used to pressurize one hydraulic system with another hydraulic system. That was over simplified, of course.



Did the airplane go dark?



That is a very real possibility. There are a bunch of potential faults that require powering down the airplane completely and starting from scratch. I have had do do that on more than one occasion.



I suppose that anything is possible, but I find that scenario highly unlikely, based on 2 years and 1,500 hours on the plane.



I think my scenario is what went on.

I hear the sound every time an Airbus starts the engines. It almost sounds like a noisy windshield wiper, but more of an electric motor type sound. It progressively got weaker as the engine noise faded out.

The plane didn't immediately go dark, but the power flashed, I assume after the APU started.

If shutting everything down and restarting is a plausible solution to known problems that makes sense.

 

[Kritchlow]

I hear the sound every time an Airbus starts the engines. It almost sounds like a noisy windshield wiper, but more of an electric motor type sound. It progressively got weaker as the engine noise faded out.

The plane didn't immediately go dark, but the power flashed, I assume after the APU started.

If shutting everything down and restarting is a plausible solution to known problems that makes sense.

Did it sound like a saw cutting metal? If so, that was indeed the PTU.

 

[Kritchlow]

The PTU is a means for the pressure in one hydraulic system to power another hydraulic system (three on the Bus but only two are connected by the PTU). So, with one engine only powered, that side is powering the hydraulic system on the other.

 

[Vf6cruiser]

The "Atari Ferrari" is 500 computers flying in close formation........and they did indeed have some scenarios where they had to be reset and required power down. So you pushed off the gate with the engine running, they hit start on engine 2 and shortly after get a large fault that requires they shut down and re-start. If it was a cross-bleed start the APU would not be working at all and you would have had to return to the gate and get a re-light from an air cart. The grinding noise is the hydraulic system in operation.

 

[evapilotaz]

Reboot the computer. Must be a Microsoft OS flying that airplane.