Random 737 questions
 

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 ?

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

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!

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.

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

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.

Q4.

The alternate system cannot retract the leading-edge devices.

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 ?

- 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.

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!

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.

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

Correct.

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

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.

- I don't believe everyone agrees with that

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

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!

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)?

Dear me,

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

Your miserable lives will, thereby, be fulfilled.

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: