B737 NG No leading edge ext lights
 

On approach today we start selecting flaps but as we select flaps one needle moves but no green light. Flaps five no green light but the flaps have moved and the leading edge devices have extended.
Question is what is happening system wise and there is no adequate check list. The closest that comes is LEADING EDGE FLAPS TRANSIT

What did the overhead show?

Both bulbs burned out....


:8

( hehehehehe he)

Yes this ! :D

Have you tested the light ?

All lights were in working condition

Nothing! And yes the lights were working

If there was confusion why did you continue beyond F1 before assessing the true configuration? If the overhead LED's showed correct combination with the TE flaps, then proceed, as it seems you did. It would suggest a failure in the connection to the 'green light'. The overhead LED display would have priority as to truth. What configuration did you choose to land in?

I had a similar fault once on a -300. We cycled the flaps and watched the overhead lights. The amber ‘in transit’ light for one of the slats was not illuminating, although the green ‘extended’ light was. This caused the light by the main flap indicator to remain amber.

The chances of the Green light/ Transit light and Leading Edge Devices Annunciator Panel lights all not working are minuscule as they feed from different sensors. If Amber transit light is not illuminated, they are not in transit. If the indication needle shows Flap 1 with no transit light and the green F1 bug shows, they are at flap 1 etc. LEDS can be seen out of the window. If you think the hydraulic fuse has locked them out, then check manual extension ( alternate ) "test" button on Leading Edge Devices Annunciator Panel will illuminate all lights.

green F1 bug shows, they are at flap 1

The F bug comes from the flap lever not the flap position: at least in the sim.

The thing was there were no amber lights or green lights (overhead or otherwise) but we can see that they were extended. We landed flaps 15

Interesting. As well as no green light, you didn’t have any amber light either leading towards sensor issue. You can further diagnose through the overhead panel, looking out the cabin windows and also on you PFD placard limit indication to see if the devices are being deployed as they should. However the QRH covers you with the first condition statement of the checklist you did as you are unsure of the devices position. This has occurred before with a sensor fault in my case and skew in another. Happy landings!

RAT, you are correct, thats' why we set the speed according to the lowest "indicated" to give the safety margin. The point is system diagnosis. If the F1 bug appears it means the signal has been sent from the lever to the actuator and the bug appears. If the pitch changes in level flight with fixed power it means "something has moved" . If there is no "transit light" it means they are in "commanded position" . Of course there is always possibility of multiple complex failures, but unlikely and the QRH has "flaps up landing" if there is any doubt. Once the "eyebrows appear" we have another clue.. as Mr Boeing advises..time spent on diagnosis is rarely wasted and the QRH assumes the flight controls are in the normal positions for the phase of flight. The alternate flap extension system is completely independent and locks out the hydraulic fuse as a fail safe. There is something missing if we can't diagnose how much flap we have..But in cloud, at night, with NO light indications it would be worth levelling off and spending 10 mins investigating.

There is something missing if we can't diagnose how much flap we have..But in cloud, at night, with NO light indications it would be worth levelling off and spending 10 mins investigating.

:ok:

The flap position indicator (F1 bug) does not indicate info from the flap handle. It comes from the flap position transmitters at the #1 and #8 flap transmissions. The flap handle position sensor is used for the trailing edge uncommanded motion function.

The one component that is between all the sensors and the L.E. indication is the FSEU (flap/slat electronics unit).

True the FSEU uses the flap handle for UCM situation, however The FSEU is using both flap lever handle position and actual flap position ( as sensed) to give you the indication, if they dont agree the indicator needles will stop and you will have a TE disagree warning. Although, referring to OP if no lights are working..?

My comment was from observations in the sim in a realistic working environment. Thanks for the info on 'behind the panels' electrical connections. The F bug is displayed on the speed tape immediately the flap lever is extended; i.e. before the TE flaps move. I now understand that there is also a sensor on the flaps to detect any 'disagreement'. I wondered if that might have been simplified by using the TE gauge, but that would not work in a total AC failure when the gauge is not powered. Thanks for the info.
The gotcha was demonstrated by crews who did not monitor the TE gauge and continued to extend the flap lever and bug the F speed without noticing the TE flaps were frozen. That is where I used to hammer the point that F bug is from the lever not the flaps themselves.

Excellent discussion. Thanks to all contributors.