737NG Radio Altimeter/Autothrottle/Autopilot Interface

[737AvEng]

I've started this thread to discuss only the technical aspects of the 737NG radio altimeter, autothrottle and autopilot interaction and operation. This is due to the Turkish Crash thread and BOAC's request since that thread is quite large.

So, to start off, I wanted to provide some results of some simulation tests I performed on an actual 737-800.

Please note: I could not simulate engines running and all of the actual precise conditions (reactions) of a flight, but what I did note may be of interest to you. In no way am I saying for 100% that this is the way the aircraft is supposed to behave or will behave as I am not Boeing and only the official confirmation of how the aircraft should operate can come from Boeing. I am just noting my observations. :8

Simulation parameters:

A. Aircraft in AIR mode.
B. #1 Radio Altimeter reading -4 feet (on ground)
C. Radio Altimeter reading 1950 feet (test box input)
D. Altitude (barometric) reading 2000 feet.
E. Airspeed reading varied on where I had flaps set, but initially above alpha.
F. On LOC and G/S (APP mode)
G. Autopilot running latest Honeywell FCC OPS (-10)
H. -4 Smiths Autothrottle Computer installed

I was able to verify that the Autothrottle went to flare retard mode while the autopilot maintained the glideslope. I decreased the airspeed and the Alpha warning in the MCP speed window started to flash (A in front of speed). The airspeed amber box appeared and also flashed initially as it is supposed to on both PFD's. The autopilot still tried to maintain the glideslope.

I continued to decrease the airspeed and simulated below glideslope. When I finally got the stick shaker, the autopilot did NOT disengage; it actually still tried to maintain the glideslope. The autothrottle remained in flare retard. After at least 10 seconds the autopilot did disengage, but I do not know the reason (could be pitch trim time limit or no reaction since I still had below the glideslope emitting from the test box).

Next, I started over and failed the #1 Radio Altimeter by pulling the circuit breaker. Now, the autothrottle did NOT go into flare retard. I first believed that this confirmed my suspicion that the autothrottle switched to the #2 radio altimeter. However, when I lowered the #2 radio altimeter to -4 feet, the autothrottle remained in MCP SPEED. I believe this confirms that the autothrottle in fact only does use the #1 radio altimeter for mode changes. Note that with that #1 radio altimeter or both radio altimeters inoperative (circuit breakers pulled), the autothrottle would still engage and remain in MCP SPEED.

In the Turkish crash thread, I had posted that the AMM notes that there is an engage/disengage condition which is "no stick shaker present". This would lead a person to believe that the autopilot will disengage when the stick shaker is active. But, as noted above, CMD B remained engaged and tried to maintain glideslope for at least 10 seconds of stick shaker (in the last test I simulated above and below for the glideslope and the yoke pitched up and down accordingly to the glideslope indications).

Also, some other observations:

A. Autothrottle would not go to flare retard when G/S not captured (this confirms AMM description).

B. When loosing glideslope and localizer signals (turned tester off), F/D remained in LOC and G/S and autothrottle would remain in flare retard.

C. I did not see any nose down trim (speed trim) with the autopilot disengaged and stick shaker active.

D. Once again, TOGA saves the day! Pressing TOGA cancelled flare retard.

The one test I was not able to perform due to time and constraints was a dual channel test simulating a radio altimeter going to ground. I have previously believed that the autopilot would drop out of dual channel if the radio altitudes were not the same. However, I am not so sure of that now based on some other information I am digging into.

[Graybeard]

Good info, thanks. You might clarify parameters B and C. Did you force the #1 radalt to -4 by disconnecting the test box, or was it there the whole time?

Regardless, your test confirmed what I posted on the Turkish thread: you are better off pulling the CB than staring at erroneous data. The airplane is equipped to handle failures better than errors. In that thread I was also amazed there would be supposed professional pilots who are afraid to pull clearly labeled circuit breakers in flight.

GB

[Teddy Robinson]

yep .. very good info ...

so a couple of questions.

The fault on RA # 1 was apparently noted on several occasions prior to the accident flight, I am not aware as to whether it was written up or not ( and I am not going back onto the other thread to find out ! )

IF it were written up, what is the engineering procedure and the operator procedure from the MEL ?

My guess is that the CB for RA#1 would be pulled and collared ?

TR

[Peter H]

[Retired software engineer, no piloting skills.]

It is probably worth reconsidering posting #1544 (http://www.pprune.org/4769568-post1544.html) (permalink (http://www.pprune.org/rumours-news/363645-turkish-airliner-crashes-schiphol-78.html#post4769568)) on the original thread. In which cargun (http://www.pprune.org/members/293995-cargun) stated that:

... back as in September 2007, I read in local forums that THY 737 pilots were complaining about negative left RA readings. They were told and believed this to be harmless and explained with "yet another cell phone activated". Due to such a repetitive error, resolved with a stupid explication, crew may have ceased to report this "harmless error". That might be the reason the previous errors are there on the FDR, yet not on the airplane's maintenance log book.

[Rainboe]

From Bulfer:

With a RA inop, do not use associated Flight Control Computer for appr or landing, and do not use associated A/P for appr.
If No1RA system is inop, A/P A should not be used for appr- A/T automatic retard during landing flare is inop.
If No2RA system is inop, A/P B should not be used for appr.

(reason is No2RA controls Flare program- No1RA does not control Flare. They act totally independently- as you would want!). This should be standard knowledge. So no great discovery! I think most pilots would remove a RA display that was indicating a false figure. I would expect a display indicating -7' would be amber and would be removed by most pilots- with time available. As for 2000' on final approach? Not a time to be messing about with the CB panel!

[bobcat4]

Good info, 737AvEng!

I suppose you're an engineer, not a pilot. Were you (or any pilot nearby) able to recover for a stall at 500' with all that back trim? Yes, I know SOP implies trimming as part of the stall recovery procedure, but you have to know that. Not much time to look it up...

[Rananim]

Interesting data.Thanks

[Graybeard]

Radio altimeter indications rarely go to 0', valid, on the approach at 2000' or whatever arbitrary altitude.

As the ground return signal diminishes on aircraft climbout, and if there is a strong enough leakage path, the indication will drop back to -8 or so, instead of indicating correct altitude, and will remain at -8 the entire flight until the ground return is strong enough again on the approach. The crossover from correct to incorrect indication can be at any altitude up to about 5,000 feet, depending on strength of leakage path, and altitude gain programming of the radalt receiver.

There should be plenty of time to pull a CB during cruise. Not knowing the downstream effects of a false indication of radio altitude will lull a crew to ignore the false indication rather than kill it. Moreover, if the radalt indication has shown on prior flights to start being correct again at 500' or 300' or whatever, on the approach, maybe the crews preferred that.

At any rate, belly corrosion needs to be addressed, and some, if not most, maintenance organizations need training on radio altimeter troubleshooting and corrective actions.

GB

[Rainboe]

The RA only indicates below 2500' AGL. Above that you have no indication and no scale. I don't know if they had a false indication above that. It would leave you minimal time to troubleshoot.

[HalinTexas]

Were the pilot's arms broke? Was their vision impaired? Were they retarded?

[Jetdoc]

It's very difficult to troubleshoot a problem that is not in the logbook. It's also usually frowned upon when you start taking apart serviceable airplanes in scheduled service.
If these pilots were so concerned about the previous rad alt problems, they should have documented the faults in the logbook. At that point, it would require someone to take some action that would have to be documented. Everything would be very clear then.
It's actually amazing how much troubleshooting on rad alts can be done with the aircraft on the ground.
I would personally prefer that pilots not troubleshoot faults during the approach. It's obviously a critical time which requires the pilots undivided attention. Systems like A/Ps and A/Ts only require monitoring during the approach phase. There are switches on the MCP that turn off systems that are not functioning correctly. No need to search for circuit breakers. After the aircraft lands, any faults can be written up in the logbook.

[Graybeard]

When the signal leakage path leads a radalt to believe the plane is at -8 feet, it will do that at all altitudes above the signal strength crossover altitude. Yes, it will indicate -8 feet at cruise, and all altitudes above signal crossover.

Sounds like the THY pilots were convinced by MX that the cause of the erroneous altitude indication was EMI from laptops or something. In that case, they would cease writing it up, as reportedly happened a couple of years ago.

I would like one of you experts to point out the radalt on/off switch on the MCP.

AAL had a radalt indicator on/off switch on their DC10-10, to reduce indicator failures, but the radalt transceiver was still alive and feeding the other systems.

No, a radalt error is not easy to troubleshoot on the ground, as it will indicate perfectly with such strong ground return. There are ways, however...

GB

[Jetdoc]

Unless the rad alt problem was fleetwide, I don't see how anyone could be convinced not to write it up. It only takes a minute. Let any engineer sign it off by a statement like 'the problem is cause by a laptop or something.' Now it is documented. I knowthat there are a few who may try to do that but many more will either placard the system inop or troubleshoot if time permits.
There are 3 main components in the system. 2 antennae and an R/T. Replacing an antenna will be the perfect time to check for corrosion. If these fail to cure the fault, the cables can be looked at. Most of the modern aircraft now do not have independent indicators but are displayed as part of the EFIS system.
There are no switches for the rad alt. My comments referred to the A/T and A/P. The rad alt primary function is to supply information to those systems. Pilots are not just flying by the rad alt. They fly by the A/P and A/T. When these systems do not respond correctly, they can be turned off.
Pilots need to monitor the aircraft performance as well as system operation at all times and not focus on one item. In this case it seems that they didn't focus on anything.
The A/P and A/T monitor rad alt information. They are looking for specific altitude data. As the data received was not flagged as faulty, these systems unfortunately responded in a correct manner. If the aircraft was monitored correctly, we would not be discussing this at all.

[BOAC]

737 - apologies for missing your PM yesterday and indeed thanks for getting this going - very useful info.

Just to clarify RB's (reason is No2RA controls Flare program- No1RA does not control Flare. - which is confusing! As I understand it, BOTH RAs are needed for the FLARE to be active. The bar on use of A/P with FAILED RA is because of the lack of RA control smoothing at the very least and possibly also some 'other buried' F/D- A/P functions are missing too

Personally I doubt any pilot would attempt to "remove a RA display that was indicating a false figure" but in the time-honoured RAF fashion would probably hang their flying glove over it (ie ignore it:)).

I come back to the comment I made on R&N (no doubt 'usefully' modded out of existence:mad:) that we need to expand the training input on RA functions for starters so that more crews are aware of the implications of the involvement of the RA in the approach modes, and as I also said I'm sure B will be looking at some sort of comparator.

As I mentioned elsewhere ( I note 'Safety Concerns' has declined my inviation to discuss:hmm:) unless there is a non-volatile interrogatable ROM, the likelihood (NB - no criticism of LAEs here!) is that a reported 'blippette' of RA mis-read would be bite checked and 'PRF' ('Please report further')? That is another mini-campaign I have failed on, namely getting 'PRF' into a more formal Tech Log entry so it was easily visible to successive crews rather than relying on trawling back when 'time available' onto faded copy sheets in the log.

Your efforts on the A/P disconnect confirm my belief that it does not 'disconnect' at the stick shake but probably as you said,at a trim limit as I think happened to the BA 777 at LHR..

Thanks for your efforts.

[737AvEng]

Good info, thanks. You might clarify parameters B and C. Did you force the #1 radalt to -4 by disconnecting the test box, or was it there the whole time?It was there the whole time. I had only one radio altimeter test box, so I could not control both radio altimeters at the same time using different altitudes.

With a RA inop, do not use associated Flight Control Computer for appr or landing, and do not use associated A/P for appr.
If No1RA system is inop, A/P A should not be used for appr- A/T automatic retard during landing flare is inop.
If No2RA system is inop, A/P B should not be used for appr.Here is the official wording straight from the Boeing 737 DDG (non-customized) for the -600/-700/-800/-900 Radio Altimeter MEL:

MAINTENANCE (M)

Do these steps (AMM 34-00-00/901):

Open and collar associated P6-1/P18-1 panel Radio Altimeter circuit breaker to deactivate the inoperative radio altimeter.

For airplanes without FCC Operational Program Software (OPS) 2212-HNP-03B-05 or later installed, re-initialize the Flight Control Computer (FCC) associated with the inoperative radio altimeter by momentarily opening and then closing the applicable P6-2/P18-1 panel FCC circuit breaker.

NOTE: Rockwell Collins FCC Operational Program Software (OPS) part numbers are considered to be equivalent to Honeywell FCC OPS part number 2212-HNP-03B-05 and later.

Refer to MMEL Item 32-17. An invalid radio altimeter signal will generate a dispatchable PSEU fault.


OPERATIONS (O)

NOTE: For airplanes with -1, -2, or -3 SMYD, an invalid signal from radio altimeter number 1 will result in failure of both stick shakers to self test.

1. Ensure that weather minimums or operating procedures are not dependent upon its use.

2. With radio altimeter(s) inoperative, do not use the associated autopilot or autothrottle for approach and landing.

3. For airplanes with FCC Operational Program Software (OPS) 2212-HNP-03B-05 or later installed, if the remaining radio altimeter fails:

NOTE: Rockwell Collins FCC Operational Program Software (OPS) part numbers are considered to be equivalent to Honeywell FCC OPS part number 2212-HNP-03B-05 and later.A. AFDS (both sides) will limit the bank angle to a maximum of 8 degrees in all roll modes.
B. Use of the Autopilot/Flight Director System (AFDS) is at the discretion of the flight crew. AFDS may not:
1) Command sufficient bank angle to execute proper departure and/or approach maneuvers.
2) Make enroute course changes within airspace limitations.
4. For airplanes without FCC Operational Program Software (OPS) 2212-HNP-03B-05 or later installed, dispatch with an inoperative radio altimeter:

NOTE: Rockwell Collins FCC Operational Program Software (OPS) part numbers are considered to be equivalent to Honeywell FCC OPS part number 2212-HNP-03B-05 and later.A. Results in the same side Autopilot/Flight Director System (AFDS) limiting the bank angle to 8 degrees in LNAV mode.
B. The opposite side AFDS (operative radio altimeter) is not affected.
C. Failure of the remaining radio altimeter can result in:
1) AFDS (both sides) limiting the bank angle to a maximum of 8 degrees (all roll modes) when flaps are extended, 8 degrees in LNAV with flaps retracted.
2) Use of the Autopilot/Flight Director System (AFDS) is at the discretion of the flight crew. AFDS may not:
a. Command sufficient bank angle to execute proper departure and/or approach maneuvers.
b. Make enroute course changes within airspace limitations.
- which is confusing! As I understand it, BOTH RAs are needed for the FLARE to be active. The bar on use of A/P with FAILED RA is because of the lack of RA control smoothing at the very least and possibly also some 'other buried' F/D- A/P functions are missing tooThere definitely needs to be clarification on this subject. When entering dual channel operation, the first FCC to be engaged becomes the master. If it is the right, I am assuming that the right radio altitude will be used for flare. If it is the left, then I would assume the left radio altitude is used. I can not say with 100% confidence that an erroneous radio altitude will cause the autopilot to drop out of dual channel.

Boeing's philosophy from the best of my knowledge has always been to compare external systems only when there are 3 or more installed. In certain systems on the 737NG, if the left radio altitude fails, the system will switch to using the right. However, it appears that the autothrottle is not one of those systems (even though it is receiving right radio altitude).

[Jetdoc]

ASFKAP

Quote:
Let any engineer sign it off by a statement like 'the problem is cause by a laptop or something.
No engineer in their right mind would sign off a defective Rad Alt like this without something written in black and white (like a Boeing service letter or Airbus TFU etc) telling him that a faulty Rad Alt might be caused by this)

Let me clarify what I wrote. I was suggesting the crew write the problem in the logbook. This way, an engineer has to commit to put something in writing. He is going to think twice about writing nonsense.

[NSEU]

Rad Alt systemsreally are very simple and reliable as mentioned above, the T/R (the box) is the heart of the system and nine times out of ten is always the cause of the problem.

This has not been my experience. Perhaps this is an aircraft type/manufacturer thing? I haven't changed a Rad Alt box for 20 years, but have had lots of bonding problems with antennae.
Some aircraft rely on bonding through the screws (and countersunk holes on the antenna). The mating surface of the antenna and the fuselage don't always provide a good bonding surface, especially if the corrosion inhibitor is forming a barrier.
We started to find some of the countersunk, metal surfaced holes on the antenna painted over with white paint (not sure if this is ex-manufacturer or ex-workshop). This was creating lots of problems with bonding.

Rgds.
NSEU

[NSEU]

I suppose you're an engineer, not a pilot. Were you (or any pilot nearby) able to recover for a stall at 500' with all that back trim? Yes, I know SOP implies trimming as part of the stall recovery procedure, but you have to know that. Not much time to look it up...

Bobcat4...
In case you haven't already guessed, the tests were run using a real aircraft (on the ground) ;) This was not in a simulator

[bobcat4]

NSEU

In case you haven't already guessed, the tests were run using a real aircraft (on the ground) This was not in a simulator

Wow! I thought it was in the sim. :)

Well, that would make my question very silly... :ouch:

[4dogs]

Graybeard,

Your advice about pulling CBs in flight to remove erroneous displays would seem to be counter to much of the current operating wisdom.

737AvEng provides some insight into the dangers of pulling CBs when not required by the QRH - you are embarking on a voyage of discovery about what systems are dependent upon the radalt signal and what systems share the CB as a power source. The DDG gives a big hint - there are specific AMM procedures to be followed to allow despatch with a disabled RadAlt which, even if you knew what they were and understood what they achieved, on most occasions cannot be achieved in flight because you cannot access the equipment bays. The associated Ops procedures are meaningless if the maintenance procedures have not been completed.

While your nom de plume indicates that you have been around for a while, your advice lacks the necessary gap between old and bold - there are a few of us in our career twilight who have learnt that CBs are no longer simple on/off switches....

Stay Alive

[Graybeard]

Circuit breakers are intentionally placed in the overhead and back wall of the cockpit, while the devices they control are all over the airplane. They put CBs in reach of the pilots, and say, "Don't touch?"

It would save a lot of wiring and cockpit real estate if the the CBs were near the devices, or near the power source. The radio altimeter transceivers in the MD-11, for example, are in the Center Accessory Compartment, right near the Remote Control Circuit Breakers (power source), yet their power is routed to a clearly labeled "Radio Altimeter (1,2)" CB on the fright deck, and back to the CAC.

I rank fear of CBs up there with fear of hand flying that is revealed in the THY thread.

The radalt was almost certainly locked on -8 feet well before THY started the approach, but only became evident on the voice recorder with first selection of approach flaps.

It's better to have a sensor reporting "Fail" than reporting itself healthy, with erroneous output.

GB

[bubbers44]

GB,

I agree, #1 RA probably was reading -8 for some time before starting the approach and it would have helped if a failure was annunciated. The retard action probably required glide slope intercept to begin.

[Peter H]

from post #1302 on original thread ...Dutch Safety Board: Malfunctioning altimeter caused THY 737 autopilot to decelerate
The cockpit voice and flight data recorders show that at 1,950 ft. the "left radio altimeter suddenly indicated a change in altitude, from 1,950 ft. to -8 ft., and passed this on to the automatic pilot. . .

... I assume that this means that the RA was giving correct readings from its upper limit of 2,500 [2,499?] feet down to 1,950.

[bubbers44]

Intermitent failures come and go as they wish. We had so many TCAS inop indications on the early models we ignored them because they were a nuisance and constant distraction. If they had a corroded RA altimiter antenna, it might have worked a lot of the time then failed whenever it felt like it. It is hard to troubleshoot because on the ground the temps are warm and impossible to duplicate. I can see the crews prior to this flight not writing up the fault if it happened a lot.

[Jetdoc]

It is hard to troubleshoot because on the ground the temps are warm and impossible to duplicate. I can see the crews prior to this flight not writing up the fault if it happened a lot?????????????

I can see it being hard to troubleshoot because it is not in the logbook. Aircraft maintenance is performed in all sorts of weather conditions and generally speaking problems will get solved. It may not occur on the first attempt but it will get solved. The most important thing is that the problem is documented. That is the starting point.


Circuit breakers are intentionally placed in the overhead and back wall of the cockpit, while the devices they control are all over the airplane. They put CBs in reach of the pilots, and say, "Don't touch?"

It would save a lot of wiring and cockpit real estate if the the CBs were near the devices, or near the power source?????????????

Greybeard. You might have noticed that the circuit breaker panels contain information as to which bus the circuit breakers are connected to. That is because the busses are right there at the circuit breaker panels. Putting the circuit breakers near the devices they power would not be of much help to the crew.

[john_tullamarine]

circuit breaker panels contain information as to which bus the circuit breakers are connected to

Therein lies a problem ..

(a) I have no major concern tripping a breaker IAW some prescribed abnormal action .. presumably, the OEM's (or other originator's) electrical design folk and FT/flight ops, etc., have been all over the ramifications of the action

however,

(b) I have seen some (apparently, and to me) strange protections and multiple power leads in the C/B sandpit. What one reads on the decal may not tell you more than a small part of the story ... In the absence of adequate design information (and the WDM is not much use to the average line pilot) the pilot MIGHT be putting him/herself in a stickier situation by tripping off both the intended and some other unknown services ?

Not being an electrical type of chap, (all I can recall from my training days was that it hurt if you put your finger on the wrong spot) I sometimes wonder at the innovation displayed when it comes to finding a convenient C/B to pick up a convenient voltage source ..

[Jetdoc]

It is true that some circuit breakers may supply more than one system. More important is to consider the time it takes to find the circuit breaker. On an approach, I don't think I want the pilots looking behind them at the CB panels...............

[Swedish Steve]

On an approach, I don't think I want the pilots looking behind them at the CB panels

Well they solved that on B777 and A330 by putting most of thrm downstairs. Now this may be a good idea as it stops the crew touching them, but I find it very inconvenient in my advancing years having to squeeze up and down the hatch all the time!

Talking about CB labeling, there was a problem with B767 zone trim last week. It was deferred in the MEL, but the crew asked me why the zone trim switch was OFF, when they had found a CB to control it. Anyway after a scan of the Schematics (I still have the old hard copies!!!) I advised them that it was correct. The CB was only for the Overheat protection close circuit. (not what it said on the label). So leave it alone.

[Port Strobe]

I rank fear of CBs up there with fear of hand flying that is revealed in the THY thread

Which is easier to tackle in this scenario?

Crew notice erroneous indication, arrive at their own conclusion as to the cause, and more importantly the effect of their solution, and decide to remedy it on the job, or;

Crew notice erroneous indication, think I'll keep an eye on that and if that's wrong then something else may be wrong too, so then when thrust doesn't given the correct response override the thrust lever servo motors for a kick off and if they still won't play ball disconnect the autothrottle and do it yourself?

As for Boeing's advice on CBs
Flight crew cycling (pulling and resetting) of a circuit breaker to clear a non-normal condition is not recommended, unless directed by a non-normal checklist. (granted it's not cycling you're talking about but the spirit of Boeing's advice is pretty clear to me)

Just a thought that fear of hand flying may be more fundamental in this profession and ought to be tackled first?

[KristianNorway]

Ok.. back to basic. Let's see if I can wrap my head around this.
I'm interested from a completely technical point of view to understand the interaction of these systems.

The RA (1) gives input to FCC (1) and RA (2) gives input to FCC (2). In a dual autopilot approach the bus tie breaker separates the two DC buses upon G/S capture. So.. with erroneous RA readings on the side of the initial master F/D coupled A/P, as in the case with the Schipol accident, the A/T would go into RETARD even though the other RA indicates e.g. 1950'.
Here I then assume the A/T is automatically coupled to the master FCC. Please do correct me if I am wrong.

Engaging both A/P's on approach has in other words nothing to do with one A/P being a comparator?

Any clarification would be of great interest. A posting from CaptainSandL, Graybeard or BOAC seems often to contain in depth knowledge, so please give your take on the subject if you find the time.

Thanks in advance.

Cheers from chilly latitudes! :ok:

[737AvEng]

The FCC's do send some discretes to the A/T computer, but the A/T computer is independent in regards to using radio altitude. The A/T computer has both the left and right radio altimeter buses wired to it, but it appears it only uses the left for functionality changes. (Note that this is functionality for the Honeywell FCC / Smiths Autothrottle Computer installation on 737NG line numbers 1 through 1269, 1271 and 1273 through 1277. This is the system that was intalled in the accident aircraft and the aircraft I performed my testing on. The remaining 737NG's have the Rockwell Collins Enhanced DFCS in which the autothrottle computer was integrated into the FCC).

With the A/P, the first FCC engaged becomes the master for dual channel. This is why there is now concern that with an erroneous radio altitude on the master side, the master FCC may command a flare in dual channel operation. However, I do believe that if there is an actual failure of either radio altimeter, the master FCC will command an end to dual channel operation (reverts back to single channel).

In the accident airplane, from what we have been told by Boeing, only the right FCC (CMD B) was engaged.

We are patiently waiting for Boeing for more clarification on all of the questions that have arisen.

[Vertical Speed]

To the best of my knowledge (as a 737NG Instructor and ex-pilot) the only NNC in the QRH which still suggests pulling a CB is that for the "Gear Lever will not move UP after Takeoff" There is really no other time when Boeing recomend touching CBs for the reasons suggested earlier-- one cannot be sure of the global effects of these actions! What I fail to understand though -- having tried similar scenarios on the simulator- whats wrong with manual thrust control? With 3 pilots in the cockpit how did no-one notice the low speed until too late??

[Graybeard]

If one cannot be sure of "the global effects" of pulling a CB, how can one be sure of the global effects of erroneous output of said sensor? Erroneous data is a worse situation than a failed state.

I never suggested pulling the CB at below 2K feet on the approach, although there was a jumpseat guy who was practically staring at the overhead CB panel. In spite of the allusions in the THY prelim report, I find it highly unlikely the #1 radalt suddenly went from 2K feet to -8. It's possible, but unlikely.

Boeing might not be recommending pulling CBs in the QRH, but neither do they recommend dispatching a plane with known chronic defects.

Yes, there are downstream effects of pulling any CB, but having unrelated systems tied to a CB, that is not so labeled, should be cause for grounding the plane. You're not likely to find Boeing doing that, and you won't find a good retrofit outfit doing that.

GB

[Nightrider]

Also I do not recommend to pull a CB if not explicitly asked for, exactly the one mentioned here I had to pull today, using the "non exhaustive" terms of the QRH thaqt I have to decide for more or appropriate actions. The box screwed up and beside the quick changing altitudes, everything between 0 and 2500 was displayed, the nuisance GPWS warnings called for the CB to be pulled. "Too low, pull up" during a transmission to ATC while cruising at FL330 raised some eyebrows at the other end, and the system kept going with all kind of warnings about flaps and gear....There was no cockpit communication nor normal ATC com possible, thus the CB had to go.

During the approach, after the loc was intercepted, there was no lateral guidance on the left flight director and of course no GPWS warnings. However, the 2500 ft call and all subsequent, correct altitude, calls were present.

This all just to add to the discussion.

[Jetdoc]

I could be wrong but I dont recall an overhead CB panel on the B737. I seem to recall the Rad Alt Cbs to be on the upper panels behind the pilot and copilot.
It still remains to be said that monitoring system operation and aircraft performance is critical. Switching off malfunctioning systems from the cockpit control panels is the most effective way of ensuring that there will be no further surprises.

[Nightrider]

Jetdoc: I could be wrong but I dont recall an overhead CB panel on the B737. I seem to recall the Rad Alt Cbs to be on the upper panels behind the pilot and copilot.


Very much correct, L/H RA CB on P18-1 and R/H RA CB on P6-1.

[Graybeard]

OK, back wall, like a McDouglas twin. It's been awhile. Still, it is no big deal for a 737 jumpseater to find and pull a radalt CB if the pilots are too busy. Like many other systems, you won't find a radio altimeter OFF switch, other than the CB.

Thanks, Nightrider, for relating the recent firsthand experience.

GB

[Jetdoc]

Agreed Greybeard....there is no off switch for a rad alt....no arguement there.....but its not required. What was required was better monitoring of the systems it ties into. How can the crew let the aircraft get to the state it was in?
Monitoring the aircraft performance and system operation is mandatory. Switching off systems like the autothrottle, which was not performing correctly is important. Finding out why it wasn't performing correctly is best left until the aircraft is on the ground.
Anytime I fly, I want the pilots on the aircraft flying the airplane. That is their number one job. Take off and landings are critical phases of flight. No need to troubleshoot. The focus is on flying.

[Rainboe]

In the time available when the fault might have been noticed (below 2500' AGL only), I'm not surprised the CB was untouched. That is positively not the time to hunt down a CB amongst the hundreds available, even for a jumpseater. However, all 737 pilots are aware of the interconnection between systems. If noticed, the failure should instill a heightened sense of 'something is up with this plane, double attention to what is going on!'. If not noticed, the gear warning should instill a similar suspicion. However, it sounds so simple to say 'pull the CB'. Because of the interconnection between systems, a simple 'pulling CBs' may cause other problems. Boeing doesn't recommend such troubleshooting at all by pilots in the air. It boils down to airmanship, and not letting a simple failure or a blown 25cent filament lead to disaster.

We seem to have had an awful lot of perfectly serviceable aeroplanes simply flown under control into the ground. Straight off the top of my head, Turkish, Eastern Tristar, Egyptian Sharm 737, Kenyan 737 and line upon line of more incidents.

[BOAC]

Of topic, as this thread is about electronic interaction between with RAs and A/T etc, but I cannot let this pass here!
Still, it is no big deal for a 737 jumpseater to find and pull a radalt CB if the pilots are too busy - I'm not sure which era of cockpit managenent you hail from, GB, but there is no way anyone I know would allow a 'jumpseater' to fiddle with a panel of 100+CBs when I am 'too busy', especially one not reportedly qualified on type.

[Graybeard]

From what I read of the Turkish accident, the jumpseater was an F/O, which surely meant he had passed ground school, and could read English: "Radio Altimeter -1" on the Left (or #1) Radio bus.

I suppose pilots now are not taught systems architecture, either.

GB

[BOAC]

To repeat:
"there is no way anyone I know would allow a 'jumpseater' to fiddle with a panel of 100+CBs when I am 'too busy',"

Now-a-days the norm is to monitor/agree switch/etc activation and not just 'have it done behind our backs', especially when the problem is not significant - and just flying the aeroplane would sort it out. Things have changed, I suspect.

[Rainboe]

Graybeard, it's totally unreal to even consider pulling CBs on final approach when the problem has probably only been identified at 2000'RA! I wouldn't even consider it without consulting the MEL and the QRH first, and no modern Boeing pilot would or should. It's a daft suggestion, especially getting a jump seater to start pulling CBs unmonitored! I really think it is not the time for a non-big jet pilot to tell us how to fly them!

[Jetdoc]

I’m not sure what some people are thinking here. There is a bigger picture to consider. If the crew fixated on the rad alt as much as it has been here, the aircraft would have been a smoldering wreck on the ground and many more would have died.
Autopilots and autothrottles are aids to the pilot. They are dependant on several sensors. Autopilot sensors like rad alts are not interconnected to each other so that each autopilot remains an independent system. One of the requirements for autoland systems is that autopilots remain independent of each others sensors electrics, and hydraulics.
Although aircraft components are highly reliable they still can fail at anytime. Additionally, when autopilots are used for approach, they rely on external signals beyond the control of the crew. These are not set and forget systems. At no time should the crew turn their attention away from making sure the aircraft is performing properly.
This aircraft was allowed to carry on until it had no capability to stay in the air. I don’t know what was going on in that cockpit but I imagine it was far from what would be considered a textbook approach.

[Nigd3]

Graybeard

From your posts you sound an experienced chap but I have to disagree with you when you state that pilots should be pulling CBs for an erroneous indication. The CBs are there to protect the wiring not the equipment and not normally to be used as switches.
They can be used as part of an emergency procedure to try and isolate equipment for something like fumes in the cockpit but these CB's tend to be the "generic" ones that when pulled isolate whole busbars.
Erroneous data should be flagged and then the subsequent action taken to isolate the specific sensor, ie switch to opposite side/disconnect AT and/or AP.

Most guidance material these days states that CBs that pop are to be reset once only, if they are required for continued safe flight and landing. If not critical then the crew should leave them popped, they did so for a reason. This is not directly related to you suggesting that they should be pulled on purpose during flight but it gives an indication of how cert authorities view the use of CBs.