Can´t open any Maindeck Cargo Door

[downscc 1,327]

 

 

therefore using the single GEN that is switched to ON and thus ignoring the second GEN not being utilised?

 

Yup, that's my conclusion after reading the electrical systems descriptions and on line sources combined with my quite old BSEE degree.

[phil747fan 14]

In flight - it's no longer an issue as you're no longer connected to Ground Power or APU. It has to do with variable voltage vagaries (fluctuating voltages) of GPU's/ APU's and (over) sensitive sensors on the cargo door mechanisms, etc.

APU can still run when you take off ....

[Qavion 19]

I really don't think cargo sensor sensitivity is the issue here. Isolating the cargo drive systems and main deck cargo doors from the rest of the aircraft means that when you use heavy duty door and cargo drive motors, it doesn't upset the rest of the aircraft. You don't want large voltage fluctuations or power drop outs when you're programming the FMC or aligning the IRUs.

The main deck cargo system can only be powered by APU Gen #2 or External Power #2. Both of these are automatically deactivated when 3 or more engine generators reach 6100rpm (this equates to a certain engine N2 or N3 depending on engine type). So, at well below takeoff thrust, it's not possible to get power on the cargo systems. Also, the power is controlled by air/ground relays triggered by main gear strut compression.

On the real aircraft, when you reach takeoff thrust and are barrelling down the runway, even though you may see the APU AVAIL lights still illuminated, the aircraft electric system logic won't let you use APU electrical power.

The system is almost idiot proof :wink:

Cheers

John H Watson

[rsrandazzo 6,353]

Gents,

 

Has nothing whatever to do with power quality, etc.

 

John's explanation is the valid reason.

 

If you'd like to learn more on this kind of design decision, Google "UAL Flight 811 cargo door"

 

The industry learns things the hard way...

[VS0 23]

I really don't think cargo sensor sensitivity is the issue here. Isolating the cargo drive systems and main deck cargo doors from the rest of the aircraft means that when you use heavy duty door and cargo drive motors, it doesn't upset the rest of the aircraft. You don't want large voltage fluctuations or power drop outs when you're programming the FMC or aligning the IRUs.

The main deck cargo system can only be powered by APU Gen #2 or External Power #2. Both of these are automatically deactivated when 3 or more engine generators reach 6100rpm (this equates to a certain engine N2 or N3 depending on engine type). So, at well below takeoff thrust, it's not possible to get power on the cargo systems. Also, the power is controlled by air/ground relays triggered by main gear strut compression.

On the real aircraft, when you reach takeoff thrust and are barrelling down the runway, even though you may see the APU AVAIL lights still illuminated, the aircraft electric system logic won't let you use APU electrical power.

The system is almost idiot proof :wink:

Cheers

John H Watson

 

Wow thank you John, so just to clarify the Door will run off ONLY APU GEN 2 or GPU GEN 2 when AVAIL and thus not in use? Meaning that essentially the cargo system is using this generator in the "background" so to speak as it is technically not in use. I would assume then that with APU GEN 2 ON and GEN 1 AVAIL the door would not operate?

 

Time for be to purchase the product and test it myself I guess .

 

Thanks again.

[Qavion 19]

I would assume then that with APU GEN 2 ON and GEN 1 AVAIL the door would not operate?

 

 

ah.. As I understand it, if you have two sources of power available on the right hand side (i.e. APU 2 and EXT 2) EXT #2 takes priority (but EXT 2 must remain in AVAIL mode). APU #2 can help the power source on the left hand side of the ship to power all the normal stuff. Earlier, I was thinking more along the lines of only one type of power source being available.

 

Note that if you only have one type of power on the aircraft (e.g. EXT #1 and EXT #2) EXT #1 will have to handle all the normal (in air) stuff but it will also have to power all the cargo loading equipment for the lower lobe cargo (including the lower lobe cargo doors). This loading equipment is lower power stuff... and, of course, it's not like a passenger aircraft where you have to power lots of galley equipment and inflight entertainment stuff.

 

I always have trouble finding a book which tells me that this is the way it should be... However, I recall, at least one time, a freighter pilot, during his preflights, switching APU #2 to ON during cargo loading... and freaking me out... but then I realised EXT 2 was also plugged in and in AVAIL mode (and there were no complaints from the cargo loaders).

 

Hope this helps

Cheers

John H Watson

[VS0 23]

ah.. As I understand it, if you have two sources of power available on the right hand side (i.e. APU 2 and EXT 2) EXT #2 takes priority (but EXT 2 must remain in AVAIL mode). APU #2 can help the power source on the left hand side of the ship to power all the normal stuff. Earlier, I was thinking more along the lines of only one type of power source being available.

 

Note that if you only have one type of power on the aircraft (e.g. EXT #1 and EXT #2) EXT #1 will have to handle all the normal (in air) stuff but it will also have to power all the cargo loading equipment for the lower lobe cargo (including the lower lobe cargo doors). This loading equipment is lower power stuff... and, of course, it's not like a passenger aircraft where you have to power lots of galley equipment and inflight entertainment stuff.

 

I always have trouble finding a book which tells me that this is the way it should be... However, I recall, at least one time, a freighter pilot, during his preflights, switching APU #2 to ON during cargo loading... and freaking me out... but then I realised EXT 2 was also plugged in and in AVAIL mode (and there were no complaints from the cargo loaders).

 

Hope this helps

Cheers

John H Watson

Thanks John it's very interesting. When I get round to purchasing her I'll have a play around and see what does and does not work, I'll assume PMDG have got it spot on

 

Thanks again

[Nixon 30]

Posted in wrong thread sorry.