Resolved Majestic Dash 8

[brett 2,283]

[stu7708 244]

You have sharp eyes Brett.

If I remember correctly In the first one the pilots had indications that the left gear hadn't locked in place, and in the second it failed to extend at all.

[allardjd 1,853]

I was pretty sure that they had prior indications of problems in the first one - the crash truck must have been staged at the threshold and ready to go. He appears behind the AC chasing it very soon after it passes the threshold - too soon for it to have responded to an unexpected accident.

John

[brett 2,283]

I can't believe I hijacked my own thread.

 

I agree John, they were ready for trouble. Either knowing or not knowing things are not right with your ride, it's a scary experience.

 

I have read up some more on the AC/DC electrical system in the manual and am getting my head around it. It seems like the checklist does not mention the AC generators at all, the advanced tutorial mentions putting them on but not off. There was also a mentioned that the AC generators should not be turned on until the engines are running or when attached to the external power if needed otherwise DC power is good enough while on the ground.(APU or external power for run up) 

[allardjd 1,853]

That implies that the AC generators are not directly engine driven. They are either DC electric motor driven or are not really "generators" at all but rather inverters that convert DC to AC in a black box.

John

[brett 2,283]

From the manual;

 

AC Generators

The AC generators produce the variable frequency AC power during the flight, and are linked to the NL turbine. Therefore, the output voltage and frequency of these generators are directly dependent upon the actual propeller rpm. It is important that the AC generators will not come online unless the propellers are unfeathered.

[allardjd 1,853]

Hmmm - that seems like a strange way to do it. I wonder what they do with variable voltage and frequency AC to make it useful?

My curiosity is piqued now and I'd really like to see a schematic of the electrical system.

John

[brett 2,283]

Hope they don't come after me but here's part of it, check out the top pdf on this list  for more info.

 

[brett 2,283]

I can't believe I hijacked my own thread.

 

I agree John, they were ready for trouble. Either knowing or not knowing things are not right with your ride, it's a scary experience.

 

I have read up some more on the AC/DC electrical system in the manual and am getting my head around it. It seems like the checklist does not mention the AC generators at all, the advanced tutorial mentions putting them on but not off. There was also a mentioned that the AC generators should not be turned on until the engines are running or when attached to the external power if needed otherwise DC power is good enough while on the ground.(APU or external power for run up) 

 

I wanted to note that the original manual states not to switch on the AC generators until after engine startup while the advanced real pilot tutorial says to turn them on before engine startup and instead during the planes power up procedures. 

[allardjd 1,853]

OK, got it - it's a totally DC airplane. All the AC, either ground power or from the AC generators goes through the TRUs (transformer-rectifier units) which converts it to DC. Apparently all the aircraft loads are DC. I suspect the AC generators are the high-capacity units. The DC generators and the APU generator are probably a good deal smaller. If the power is going through TRUs, the variable voltage and frequency are not a factor as long as the minimum voltage is met.

Good post. Hope you don't get in trouble...

John

[brett 2,283]

Thanks John, sounds like you got it figured.    I am going to keep reading up on it because it's an interesting subject.

 

All good to learn something new but my original question still stands,

 

'What is the proper procedure for switching on/off the AC generators?'  

[allardjd 1,853]

Lacking anything better, I'd shut them off after parking and setting the brakes but before engine shutdown. You don't want a generator under load acting as a brake on an engine at shutdown - (tough on both engine and generator), though there would probably be some kind of relay scheme that would trip the generator automatically when the engine shutdown is initiated if the crew failed to do so manually. That may even be the preferred method, which might explain why the AC generator shutdown is not addressed in the check lists. Just guessing, but it kind of makes sense.

John

[allardjd 1,853]

There's more - the pdf you linked is a gold mine, though it lacks check lists or detailed operational procedures.

There is, as I speculated earlier, a pair of 400 Hz 115 V AC busses and also a pair of 26 Volt 400 Hz AC busses. Those are used for avionics stuff and are a pretty common voltage/frequency service in big AC. These busses are not powered directly from the AC generators because the generator's output voltage and frequency are variable. Instead, those 400 Hz busses are powered through inverters fed by the 28 VDC power.

You can think of the engine-driven AC generators as just one more source of DC power for aircraft services, provided through the transformer-rectifier units (TRUs). They are no more complicated than that - just black boxes that make 28 VDC - you can pretty much forget about the fact they are AC. It's just done by engineers to confuse pilots and it generally works.

John

[brett 2,283]

I think you are right or at least it sounds like a very credible deduction and was kinda swaying that way. Since the original manual and pilots tutorial are in conflict I will go with the original manual and turn them on after engine start and before shut down. Maybe over time I will see something in the planes electrical panel that will make me change that or get a more exact answer. It still bothers me that the real world pilot switched them on before engine start, maybe just a company procedure that is done for some unknown reason even though it went against manufactures recommendation. Maybe I am reading it all wrong.   

 

I will consider this solved for now, thanks for all your wisdom and time John.