Loss of Both Engine Driven Generators 737 NG

[Don Gato]

Hi! I would appreciate help for this single technical question: In the condition that leads to the Loss of Both Engine Driven Generators Checklist, are the DRIVE lights ON? Or only TRANSFER BUS OFF - SOURCE OFF - GEN OFF BUS? As always, thanks in advance for your replies. DG

[B737900er]

When it comes to Electrical system diagnoses. Always start at the top of the electrical panel on the overhead (AC/DC meter and downwards....). Before you initiate any QRH check the CB's.
Drive failures usually start with a Source off and works its way back up. As the drive slows down the other lights start to appear.

[RAT 5]

Check FCOM 2 for what causes each of the lights. I'm not sure 'Loss of Both Gens' is relevant to the question; they are independent its, so the relationship between their warning lights is the same L & R. I can see that Transfer Bus Off would relate to Loss of Both.
DRIVE lights monitor the oil pressure, temp and rotation of the Gen. Source monitors the electrical output of the Gen. The former is a mechanical issue, the latter electrical. A Gen could fail for various reasons. An electrical failure doesn't necessarily mean a mechanical one, but a mechanical one will cause an electrical one. Checking the lights may give you an insight into which type of failure you have.

[Don Gato]

Thanks for the replies. Probably I should have added that the issue came up while discussing which checklist to run (DRIVE) or (LOSS OF BOTH ENG DRIVEN GEN) should all lights -FOR BOTH ENGINES- (DRIVE - TRANSFER BUS OFF - SOURCE OFF - GEN OFF BUS) illuminate. Not sure if that scenario of lights is realistic, though. Thks

[tdracer]

Don, the absolute first thing you should check if you loose both engine generators is look down and make sure the engines are still running...
It may sound obvious, but off the top of my head I can think of multiple cases where a dual engine flameout or thrust loss was initially diagnosed as a loss of electrical generation. In one case the pilots never did figure it out (the engines self recovered) - we didn't know the engines had quit until we looked at the DFDR...

[flyburg]

Go to this web page and look for the loss of both engine driven generators! Excellent briefing on the subject.

http://www.smartcockpit.com/plane/BOEING/B737NG.html

[RAT 5]

the absolute first thing you should check if you loose both engine generators is look down and make sure the engines are still running...

Indeed true of any Gen OFF BUS scenario. In a low thrust scenario, even idle descent, an engine flameout will throw up a GEN OFF BUS first and the CWS will divert your eyes overhead to the SOURCE off light. It is very easy to rush, and there will be little yaw at idle thrust, especially if A/P in CMD. A little aileron displacement may give a clue, but on a dark night the bright CWS ELEC can be very captivating.

[Zaphod Beblebrox]

tdracer is absolutely correct. We did this exercise in the Sim for initial training on the 737 Classic -400. I cannot speak to the NG and how the following issues relate but in a simulator at night, as most sim sessions are, the lights suddenly go out and there is a very quick way to determine you have generator failure or engine failure. Look at the top of the instrument stack and see if you have oil pressure. The Low Oil pressure lights are powered off the battery bus, if they are out then chances are you have a black cockpit due to a generator failure. If you are staring at two illuminated amber low oil pressure lights you have a dual flame out.

I don't know about the configuration of the NG. However it easy, at least initially, to mistake a dual generator failure for a dual flame out.

[RAT 5]

NG: The subtle scenario is idle descent flameout. The CWS flashes ELEC and the heads go up to the overhead panel. On the upper DU engine gauges you will see 'ENG FAIL'. Depending on windmilling RPM the oil pressure light may stay out. ENG FAIL is the clue, but you need to look for it. It is easy to rush into the first thing you see, i.e. ELEC. Remember the forward instrument panel is not monitored by CWS. That is for MK. 1 eyeball.