View Full Version : B-737NG F/D dissappear at G/S intercept

15th Aug 2002, 08:40
On approach you lose both engine driven generators and follow the abnormal procedure. You fail to get any generators back but manage to get the APU going. Once it becomes available, you power both AC transfer busses (B-737NG) with it and re-engage A/P "A" in command.

You are ready for the ILS approach and you get vectors to intercept the localizer. You intercept it and once established inbound, you have the G/S armed with the APP mode engaged. At the precise moment the G/S is captured, both F/D command bars dissappear and you are left with raw data information to complete the approach.

We know that the cross tie relay opens at the G/S intercept point isolating DC bus1 from DC bus 2. We also know that when the APU is the only power source, the galley busses and if needed, the main busses are automatically shed.

My question is, why do we lose both F/D at the G/S intercept point?

15th Aug 2002, 14:19
Dunno! But yet another reason to ensure your raw data skills are equal to -or better than, your button pressing skills...

15th Aug 2002, 21:36

However it rreminds me of a charted 757 we had for few weeks. Always around 1500' AGL on autopilot the flight directors dissapeared. As soon as autopilot was disengaged the came back in.

The reason for this, I am told, is the reason that somewhere (in the UK?) there is a rule that the flight director can not use the same information as the flying autopilot, i.e. from the same computer.

At 1500' AGL on a 757 all the autpilots kick in for LAND 3 and therefore, the flightdirectors dissapear.

16th Aug 2002, 02:19
As Fresca previously mentioned... Do your 737NG's have the option of biasing the FD's out of view during a normal LAND 3 ? (some 747-400's also have this pin-programmed option)

I'm hoping here that you didn't have a dual gen failure in real life and that this was just a sim exercise (Do you know for sure that this reflects real life ops? i.e. with regard to DC Bus Tie ops and FD disappearance).

I was also wondering what would happen to a 767 in this particular electrical config. As far as I can see (from my training notes/Boeing Wiring Schematics), the FCC's will send a Centre Bus isolation request to the Bus Power Control Unit, but the configuration of the L & R BTB's and GCB's will not allow Center Bus Isolation to take place (Land 3 requirement). Because of this, the control of the DC Tie is still under the control of the DC Tie Control Unit. Under single gen (APU ops), the DC Tie unit will not change the state of the DC Tie (i.e. it will remain open).
In this config, I would presume LAND2 a possibility. By the way, can you recall what A/L Status was annunciated (on the 737NG)?

I would be interested to know the reponse from Boeing on this one. I also hear that some (real) 737NG's have a disturbing habit of biasing the FD's out of view for a short period during takeoff.


16th Aug 2002, 08:14
Actually, this ocurred on a sim check ride and the check pilot confirmed he did not fail anything else on the IOS at that time. The 737 NG only has two A/P's for its Autoland capability. And at the time only one was engaged, meaning single channel operation. After the F/D dissappearance act, A/P was disengeged and the approach continued with manual flight. Any ideas?

16th Aug 2002, 22:19
Between loss of generators and APU online you will only have battery power. This means loss of one (right) ADIRU and one DEU. I know out of experience that in both cases the FD will disappear as soon as you change from ALT HLD or VNAV to a descend mode (GS, V/S, LVL CHG or VNAV PTH/SPD). My guess would be that the FD needs two independent and valid signals from both the display computers and ADIRU's.

17th Aug 2002, 15:02
I am still looking into this one, since no engineer can give me anything valuable yet. Hooked on the DC Bus 1 is the AFCS system "A" F.C.C. and on DC Bus 2, the AFCS system "B" F.C.C.
When the APU Generator is the only source of AC power it supplies both AC transfer busses through the closed BTB's. Since all of the required busses are powered at that time, the only new event on the approach is the opening of the cross bus tie at G/S intercept which separates DC Bus 1 from DC Bus 2. It may very well be, that since there is only one Generator supplying all of the load that the effected F/D are biased out of view because of the lack of redundancy, as Track mentions.
The right ADIRU does lose power momentarily when the loss of both generators occured, but it is not lost per se, since it can stay on DC power for the next 5 minutes. The APU start and the subsequent APU Gen powering both AC transfer busses happens before that time limit, thus the right ADIRU is not lost in this situation.
I shall be climbing back in the box on wednesday and I hope to find out more about this enigma. Stay tuned and keep your coments coming!

seat 0A
20th Aug 2002, 12:00
The problem is known within my company and put before boeing.
Until the magic word comes out of Seattle, we are told to switch off the FD switch at the pilot not flying side. This brings back the FD on the PF`s side!
Again, I don`t know why, but it works!

Bobby Johnson
24th Aug 2002, 06:25
Here goes my attempt at explaining this situation.

The whole rationale behind the disconnect is based around 'dual channel' redundancy operation. It would appear that the operation on single channel was not considered worth
isolating as we can all fly manually down to CAT1 limits.

Why do I think this?


FCC - Flight Flight Control Computer
IFSAU - Intgrated Flight System Accessory Unit

It is all down to the DFCS Power Transfer Interface,this unit uses the following components:

FCC A and B
Bus power control unit
Standby power control unit.

These in turn control the Bus Tie Transfer.

During a dual channel operation, each FCC must be on an isolated power source. FCC A gets dc power from DC bus 1 and FCC B gets dc power
from DC bus 2. Normally, the two dc buses are connected with the bus tie relay. If either FCC is in the active G/S mode (ie when glides
slope captures),it will cause the bus tie relay to open. This will separate the DC power sources to the two FCCs.

What happens when the Bus Tie Relay Opens and why does it open?

When EITHER the 'A' FCC or the 'B'FCC is in the G/S active mode it tells the IFSAU (just a control and comparison computer)
to isolate the DC Buses. The IFSAU needs confirmation from the Standby Power Control Unit that the power is normal by providing a ground
to the IFSAU.This causes the G/S engaged position to be activated in the IFSAU and cause the ground to be removed from the bus tie relay.

If the Bus Tie Relay has a ground from the IFSAU then the relay closes and the DC Buses are connected together, when the ground is
removed the rely opens and the buses are isolated.

The Bus Power Control Unit which comprises of the TR's usually get their power from different 115v ac buses (AC Bus 1 & 2).If the TR's
suddenly get their power from one source ie when a transfer occurs the Bus Transfer Control Unit knows where the source of power is from
(either the generators or the APU generator).
It therefore knows that even though both buses are powered it is from a single source. It therefore sends a signal to both FCC's that
the DC Buses cannot be isolated. Both FCC's now removes the flight director as it does not have its required redundancy for the Dual Channel approach.


To answer your question, why does it happen?
The Bus Power Control Unit knows where the power source is from that is supplying both buses, it knows it is a single source from the APU
generator so it tells BOTH FCC's that the buses cannot be isolated. The important point here is that it tells both, so even on a single channel approach the FCC fails the
Flight Director.

And that is why I think it happens.

29th Aug 2002, 04:11
Right on! I just got back from a trip where I was able to get all the information Bobby provided from the Maintenance Manual. That is exactly the reason of the F/D dissappearance and I wonder why as pilots, we have to do so much digging for the information? No where in the Boeing Ops. manuals is it mentioned that one of the Flight Director functions is the BOV (F/D commands bias out of view). The sole existence of the IFSAU was unknown to me and what it does, as well. I remember the famous "chapter five" we had to learn back and forth when we took the B-727 initial training. Everything was in there, but now a days in the era of only caution ranges and red lines, much needed information is not provided to the pilots anymore. Thanx again for that explanation Bobby, great job!

Bobby Johnson
29th Aug 2002, 17:14
I agree it should be mentioned in the ops manual so you can plan on this event, during any briefing.

I will shortly be doing my instructors rating on the BBJ so if you have anymore questions feel free to ask. It will help me gen up on the books, ie make it appear to be worthwhile.

That scenario will be a good instructional technique item.

For dedicated BBJ and NG stuff try my forum at www.thebbj.com