Random 737 questions

[ninja_turtle]

I wanted to know the reason :

1) When the flap lever is selected to flap 25 we get an indication on the speed tape in 700s and not the same in 800s for a flap 30 landing ?

2) Why only the elect hyd pumps have an overheat indication ?
Arent the eng driven pumps subject to overheat conditions as well ?

Our FCOM has a min main tank fuel quantity for the heat exchanger for the elect hyd pumps but none for the eng hyd pumps .

3) Many of our 737s have a (-4) indication on the Radio Altimeter on the ground .
Ive been told that it is an induced error and has something to do get a zero reading upon main gear touchdown and the same error is to cater for compression of the main gear .
Is that correct , can someone explain this better ?

4) If the flaps were extended normally during take off , could you use the Alternate Flap switch to retract the LE devices ?
Ive been given to understand , that if you hadnt extended the flaps using the alternate system , it could be possible to also use that switch to retract the LE devices using the retract option .

5) When the BIAS factor is derived for an aircraft , is it reached upon by only looking at the condition of the engine or does it cater for the condition of the airframe and or other things that can affect performance and fuel consumption with regards to the aircraft in itself and not external conditions .

6) I understand that the min assumed temp that one could use would be the engine flat rating and that if I fed in an assumed lesser than that , the FMC would take it but still give me thrust equal to the flat rating and that the feeding of such lesser min temp served to be of no real purpose really . Is that correct ?

[sellect]

You are correct on your number 3. It is due to compression of the main gear.

[SlowAndSilly]

1) You do get it in the 800 as well. Depends on how large the split is between 25 and 30. Can't remember the precise criteria!

[jet_737ng]

1. answer to that is if the flap speed is within Vref + 4 kts it wont be displayed. So in the 800- almost always (unless the ac is very very light ). while will show up in the 700.

[ImbracableCrunk]

6. I don't think the FMC will allow you to select an assumed temp if it's less than flat rate.

[Pub User]

Q2.

Interesting this one, as our FCOM says the same thing, but the 737 maintenance manual contains this statement:

DO NOT OPERATE THE HYDRAULIC SYSTEMS A
AND B ENGINE-DRIVEN PUMPS FOR MORE THAN
TWO MINUTES WITHOUT FUEL IN THE FUEL
TANKS. THE NO 1. (FOR HYDRAULIC SYSTEM A)
AND NO. 2 (FOR HYDRAULIC SYSTEM B) FUEL
TANKS MUST HAVE A MINIMUM OF 250 GALLONS
(1675 POUNDS/760 KG) OF FUEL IN THEM. IF
THERE IS NOT SUFFICIENT FUEL IN THE FUEL
TANKS, THE ENGINE-DRIVEN PUMPS WILL
BECOME TOO HOT. THIS CAN CAUSE DAMAGE
TO THE ENGINE-DRIVEN PUMPS.

I guess the overheat light would be useless on the EDPs, as you can't stop them turning even if they are too hot.

[Pub User]

Q4.

The alternate system cannot retract the leading-edge devices.

[ninja_turtle]

Pub User , quite agree to what you say , since the eng hyd pumps operate as long as the eng is operating . And theres no way to turn them off with an eng operating , so wouldnt be much of a point having an indication .

Mig15 your explaination is very logical , dont think the main gear would compress 4 feet , thats a bit too much I say too .

Imbracablecrunk , the CFM 56 flat rating is ISA + 15 c = 30 c . I have fed assumed lesser than that ample number of times . Try it out when conditions permit , or may be assume an example just to check it out ..

Jet737ng , about the flap speed , you mean to say that if the actual flap 25 speed is within 4 knots of vref 30 , it wont display , is that correct ?
The only way to figure out flap 25 speed would be vref 40 + 10 , right ?

[BOAC]

Q4.

The alternate system cannot retract the leading-edge devices.- it may have been the 200 or 300, but I remember some really clever TC explaining how you could - convincing, too, with diagrams, but it got filed under the 'interest only' file in my cabinet so I cannot for the life of me remember how it worked. I think it involved shutting down a motor so I filed it.

[flyburg]

Q4 as I recall correctly you can only turn off the fluid supply to the engine driven pumps but not turn them off, hence no sense in an overheat indication. the electric driven pumps can be turned off!

[ImbracableCrunk]

Imbracablecrunk , the CFM 56 flat rating is ISA + 15 c = 30 c . I have fed assumed lesser than that ample number of times . Try it out when conditions permit , or may be assume an example just to check it out ..

Hmmm. Maybe you can't reduce unless the assumed is at least a certain amount more than actual. I know a few times I've tried, and it simply wouldn't accept the assumed temp.

[NSEU]

Q4 as I recall correctly you can only turn off the fluid supply to the engine driven pumps but not turn them off, hence no sense in an overheat indication.

If you turn the EDP off (not pull the fire handle), the pump operates in a "depressurized mode". In this mode, would you expect that the pump is not working as hard, so perhaps would run cooler? According to the Maintenance Manual, even with zero fluid output from the pump (to airplane systems), the pump can still generate a fluid pressure of 2900psi (compared to 3030psi at 3750rpm at 37.5gallons/min in pressurised mode). Whether this slight reduction in pressure indicates a % decrease in work, I can't say.

Note: In depressurized mode, the EDP pump casing is still being cooled with hydraulic fluid. On the other hand, if you pull the fire handle, fluid is shut off entirely to the pump.

It would be interesting to know how much heat (generated by an overheating EDP) is returned to the reservoir and then transferred to the EMDP (where the temperature is monitored).

Rgds
NSEU

[NSEU]

Mig15 your explaination is very logical , dont think the main gear would compress 4 feet , thats a bit too much I say too .

Correct.

I've seen -10 feet displayed on a 747-400 (I don't think this can be explained by strut compression (alone), either ;) )

[Avenger]

Max & Min Temps.
The normal range of assumed temperatures is from 30 to 55C. However the QRH shows that temperatures from 16 to 75C may be used. The maximum temperature is set by the maximum amount of thrust reduction allowable i.e. 25%; and the minimum is where the engine becomes "flat rated" and no further performance gain can be achieved.

The EDP is capable of approx 37GPM flow rate and the Electric pump about 6 GPM. with the higher flow rate the cooling is increased.

No, LE devices cannot be retracted with alternate system, the alternate system requires the hydraulics to be shut off to the LE devices.

[BOAC]

No, LE devices cannot be retracted with alternate system, the alternate system requires the hydraulics to be shut off to the LE devices. - I don't believe everyone agrees with that- it may have been the 200 or 300, but I remember some really clever TC explaining how you could - convincing, too, with diagrams, but it got filed under the 'interest only' file in my cabinet so I cannot for the life of me remember how it worked. I think it involved shutting down a motor so I filed it.

[president]

if the leading edge devices were deployed normally (such as before take off) they CAN be retracted using the alternate system. It should be mentioned in Bill Bufflers book

[Avenger]

Nope! Cant see it myself but am always prepared to be corrected with the appropriate Boeing Refs;

Once the alternate flap master is armed the TE bypass valve is locked out and the LE retract fuse shuts, keeping the LE devices out. THE TE flaps must be, after removing all hydraulic power, electrically cycled, which results in the resetting of the LE fuse through the leading edge control valve.

So, I assume Bill whatsit is suggesting leaving the flap lever in the Take off position, then doing some disabling of system B, then arming the alternate system, How is the LE fuse reset to allow retraction?

Is this a bit like saying we can pull the RUN PWR CB to get the gear up!

[BOAC]

Avenger - I cannot remember the detail, but the TC was convincing! It came up on a recurrent with Hyd B fail, weather forced g/a and long flog to div. In the discussion, guess what came up (no, not the LEDS)?

[fantom]

Dear me,

May I respectfully suggest you dismiss all these distractions by obtaining an Airbus type-rating?

Your miserable lives will, thereby, be fulfilled.

[IFixPlanes]

...2) Why only the elect hyd pumps have an overheat indication ?
Arent the eng driven pumps subject to overheat conditions as well ?...

There is a big difference between the EDP (Engine Driven Pump) and the EMDP (Electric Motor Driven Pump).
The EDP is mechanically driven by the gear box. The heat produced by the mechanical parts are negligible.
The EMDP is driven by a hydraulic-oil-cooled three phase 115 volt ac motor which produce heat like every electric motor does. Hydraulic fluid goes into the electric motor housing to cool the electric motor before it enter the centrifugal pump.
If you do not use the hydraulic power, there is nearly no hydraulic flow to cool the ac motor.
Therefore only the EMDPs are watched by a internal temperature switch. :ok:

[BOAC]

Ifix - that is my understanding too, but how do you explain post #7?

[IFixPlanes]

OK, more details.
If you look closer on the EDP, you see 3 hydraulic lines connected to the pump.
The supply line from the reservoir, the pressure line to the system and ... the case drain.
There is a small flow through the pump housing for lubrication and cooling purpose.
These case drain flows even when there is no system demand/flow.
But the case drain goes not go back directly to the tank. It is routed via 2 heat exchanger (case drain to fuel) in the wing tanks.
Without fuel in the tanks the complete system is getting to hot.

[Avenger]

Without fuel in the tanks the complete system is not getting hot as the engines are not running, Or are you saying w!ith less than 760KG. Do you know if the case drain fliter has a bypass to prevent overheat in case of blockage?

[IFixPlanes]

Sorry for the misleading wording. :(
I meant "less than 760kg of fuel in each" tank. Below this level the heat exchanger are not any more fully surrounded by fuel.

...Do you know if the case drain fliter has a bypass to prevent overheat in case of blockage?...No, there is no bypass.
If the filter starts to produce more resistance due to contamination, you have a increase of the EDP pressure. This is caused by the way the EDP regulates the output pressure.

So, a increasing pressure above the normal range (approx. 2800 - 3200 PSI) could be an indication for a "dying" EDP.

[ninja_turtle]

Imbracablecrunk , there isnt any minimum percentage specified as to assumed being more than actual . You can feed anything higher than actual , the FMC will take it but may actually not reduce any thrust beyond the flat rating of the engine . You are gauranteed to get performance of 30c even with a assumed lesser than that and yes it is possible to feed assumed lesser than 30c , although it may not be of any real use . The only restriction that we use in our company is , not to do an assumed temp method if the difference between the ATOW and the limit weight is less than 500 kgs .

[Denti]

I guess it depends on FMC version. We have quite a few planes that do not allow the pilot to enter any assumed temperature of 30 degrees or lower. I haven't flown enough recently to check if is something about 10.8A or some customization issue.

[vineet.mk]

Hi,

point 5) - The Bias factor on FMC is meant to cater for over all degradation. its a fuel milage offsetting factor. So it caters for engine as well as airframe(and other factors like wt etc).

point 6) I am not sure if the FMC will accept it or not, but thrust will be of the flat rate, and such assumed temp will have no benefit (of thrust reduction)

Hope it helps.

[president]

Bill Buffler: if you lose B sys hydraulics and flaps are extended after take off, the LEDs can be retracted with PTU pressure using the alternate flap system.

[de facto]

You are correct on your number 3. It is due to compression of the main gear.

Thats Funny:E

[framer]

Hydraulic pumps;

The fluid from the entire system is cooled by the heat exchangers in the tanks....so there is your requirement for 760kg (to cover the heat exchangers).

The overheat for the EMDP´s is measuring the temperature of the EMDP casings. They will heat up much quicker than the EDP´s due to the reasons Ifixplanes has said.

If you are tooling around with 700kg a side the fluid temperature will most likely rise. If you turn your EMDP´s off.....it will not rise as much.

Thats my take, happy to be corrected.

[RAT 5]

LED's on ALTN:

IMHO if HYD B is not available, once the LED's are out they stay out. If HYD B is available LED's can be retracted, but the ALTN Flap switch has to be closed. So if you extended the TE flaps & LED's using ALTN Flap switch, and then wish to retract them with HYD B, first close the switch.

[JammedStab]

In the end, there must be a decent amount of heat generated by the mechanical A pumps because there is a heat exchanger in one of the fuel tanks for cooling . And the book says that you will get an overheat if the associated fuel quantity is too low, yet there is no overheat warning for the A system.

But perhaps this is an unusual enough situation that it didn't warrant overheat lights. Why? If your fuel is that low, your engines(and therefore the mechanically driven A pumps) will be shutting down soon enough anyways unlike the B pumps which could be left on all day with the fuel quantity low, perhaps by maintenance or pilots after landing with not much fuel.

Both A and B pumps appear to have the third port for case drain.

[IFixPlanes]

... Both A and B pumps appear to have the third port for case drain. ...:D Already mentioned here (http://www.pprune.org/tech-log/446916-random-737-questions-2.html#post6337120).

... yet there is no overheat warning for the A system ...Use the one from the EMDP.:ok:

[JammedStab]

:D Already mentioned here (http://www.pprune.org/tech-log/446916-random-737-questions-2.html#post6337120).

Use the one from the EMDP.:ok:

I believe you only mentioned about a case drain for the EDP(engine driven pump) as quoted....."OK, more details.
If you look closer on the EDP, you see 3 hydraulic lines connected to the pump.
The supply line from the reservoir, the pressure line to the system and ... the case drain. There is a small flow through the pump housing for lubrication and cooling purpose. These case drain flows even when there is no system demand/flow. But the case drain goes not go back directly to the tank. It is routed via 2 heat exchanger (case drain to fuel) in the wing tanks.
Without fuel in the tanks the complete system is getting to hot. "

I am pointing out that the electric pumps have the same thing(case drain). And why would I use the B pump overheat(EMDP as you call it) to tell if I have an overheat for the A Pump fluid.

[yotty]

JammedStab in each hyd sys there is an EDP and an EMDP.

[framer]

JammedStab, it sounds like you are confused about which pumps provide pressure for which system.
System A has an EDP and an EMDP.
System B has an EDP and an EMDP.

Please excuse me if you have that sorted and I have misinterpreted your posts.

[JammedStab]

I was only on the aircraft type for about 7 months and that was 2 years ago....

That being said, the thread has a generic title for 737. I flew the 737-200 series. I believe that my earlier statement is correct but welcome any corrections if you feel I am wrong that A-system is two engine driven hydraulic pumps, while B system is two electric pumps. No overheat indication for A system pumps. Both types have case drain fluid cooling, both systems have heat exchanger cooling in the fuel tanks and my previous statement was correct.

Can anyone confirm.

Later model 737's. Don't know a thing about them.

[c100driver]

Jammed Stab,

You are correct for the legacy aircraft, incorrect for the classic and NG as they have a different HYD set up from the 100 and 200 series..

[Yeelep]

If you turn the EDP off (not pull the fire handle), the pump operates in a "depressurized mode". In this mode, would you expect that the pump is not working as hard, so perhaps would run cooler?Yes, the pump is just spinning with no output pressure or volume. According to the Maintenance Manual, even with zero fluid output from the pump (to airplane systems), the pump can still generate a fluid pressure of 2900psi (compared to 3030psi at 3750rpm at 37.5gallons/min in pressurised mode). Whether this slight reduction in pressure indicates a % decrease in work, I can't say.In both cases, whether zero fluid output or 37.5 GPM, the EDP is in the pressurized mode. The reference I'm looking at, NG AMM Part 1 quotes 2850psi at 3750rpm at 37.5 gallons/min and 2950psi to 3075psi at zero flow. Either way, there is a lot more work happening when the output is 37.5GPM.

[JammedStab]

It always seemed kind of strange to have near normal hydraulic pressure yet it wasn't enough to operate things properly.

[Yeelep]

It always seemed kind of strange to have near normal hydraulic pressure yet it wasn't enough to operate things properly.

JammedStab,
There should be enough, unless you are referring to running A system from the B system pumps through the ground interconnect. Would you care to elaborate?

[JammedStab]

JammedStab,
There should be enough, unless you are referring to running A system from the B system pumps through the ground interconnect. Would you care to elaborate?

It is a bit vague now but I seem to remember on the 727(I know...different airplane) with two engines inop(the ones with the A pumps), the engines were windmilling and producing a fairly high hydraulic pressure but it was not enough to operate systems properly. Of course we never did similar in the 737-200 as it only had two engines.

Vague memory subject to confirmation.