B737NG IDG thermal disconnect.

[Jerboa]

Guys,

Please can someone confirm for me what indications would be given for an IDG thermal disconnect on a B737NG.
I know that an underfrequency or a low oil pressure fault would cause the 'DRIVE' light to illuminate indicating a disconnect is required.
In the case of a thermal disconnect it will automatically happen so a 'DRIVE' indication would not be required as it has already happened. Is it correct to think this ? And after the disconnect would the 'SOURCE OFF' and 'GEN OFF BUS' illuminate for the engine the disconnect occured on ?
B737NG please !

Thanks.

[Flight Detent]

Hi Jerboa,
As I understand it, the B737NG electrical system, as far as the oil temperature within the IDG rising above 199degC goes, is as follows.
The IDG disconnect is automatic,

The 'Source Off' light only comes on when one of the transfer buses is unpowered, so that won't happen (normally), the other IDG via the transfer bus, will assume the load,

The "Gen off bus' light will come on, since the IDG is now not rotating!

The IDG now has to be removed and dismantled to replace the thermal sensor before getting back into service!!

Hope this helps,

Cheers

[DDG]

Hi ALL,

When a B737NG IDG has a thermal disconnect at 363F(182C),part of the drive system melts causing the disconnect to occur automatically .The amber DRIVE light will illuminate as the IDG now has a LOW OIL PRESSURE AND is UNDERFREQUENCY with the engine still running !

Flight Detent has explained correctly the "Gen Off bus" and "source off" light operation.

Regards, DDG

[Jerboa]

Guys,

Thanks for the replies.

As i understand it the 'SOURCE OFF' light does not mean the transfer bus is not powered. That is done by the 'TRANSFER BUS OFF' light. The 'SOURCE OFF' means a GCB has opened and an AC TRANSFER BUS is powered from a non normal source.
Say IDG 2 is powering both TRANSFER buses then IDG 1 will have a 'SOURCE OFF' light on.

[Flight Detent]

Hi Jerboa,
As I understand the NG system, a SOURCE OFF light indicates the related transfer bus is not powered by the last selected source, and the TRANSFER BUS OFF light indicates the related transfer bus is not powered, confusing eh!!

I think the secret is in the words 'last selected source' here.

your Q, the answer is NO!

Cheers

[alexban]

Gen drive light will illuminate if IDG low oil pressure caused by : IDG failure,engine shutdown ,IDG automatic disconnect due to high oil temp,and IDG disconnected through generator drive DISCONNECT switch. So this light will be ON.


SOURCE OFF light : illuminated when :' no source was manually selected to power the related transfer bus ,OR the MANUALLY selected source has been disconected.' In your case this light will be ON (as FD said,the last manually selected source was disconnected)


Brgds
Alex

[NSEU]

According to the Boeing Maintenance Manual (D&0) 24-00-00....

"The SOURCE OFF light does not indicate that the AC transfer bus is de-energized. In flight, the left SOURCE OFF light comes on when GCG 1 trips. However, the bus transfer function lets IDG 2 power AC transfer bus 1. These are the examples:

-IDG1 fails
-The bus transfer closes the BTBs
-Both AC transfer buses get power from IDG2"

This appears to contradict the earlier responses.

Rgds.
NSEU.

[Flight Detent]

Hi all, again,
So, NSEU, from your example, the left IDG fails, the bus transfer system operates as it should, and now the right IDG powers both AC transfer buses.
And, the source off light will be on for the left IDG, since that was the last selected source for the left transfer bus.

I think thats what I mentioned earlier!!

I think!!!

Cheers

[Jerboa]

NSEU,

Well said. Exactly how i understand it !