744 tech questions

[rhinodriver]

I'm studying for an upcoming interview and have a few questions that I have been unable to find the answers to. I have done searches but unless I didn't type in the key words correctly I have found nothing. Any help would be appreciated.

1. If aft CG is so good why does the 744 start pumping fuel out 30 min after T/O?
2. General overview of pressurization system on 744.
3. Anti/deice system on 744.
4. What is normal differential pressure on 744?

I've looked for tech manuals on the internet but so far have come up empty handed.

Thanks
Rhinodriver

[ConditionLever]

They're good questions, you have put a lot of work in to get that deep into it and should do fine in the interview.

I have never seen anything to say why the fuel in the stab is used as soon as possible (ie as soon as there is space in the CWT to transfer it) But I assumed when I was studying that if you didn't do that the CG would get too far back. Not very technical I know, but someone smarter than me at Boeing has worked out that that is what needs to happen. I think that is enough knowlege actually. A340s have fuel in the stab too apparently.

All I can tell you about the pressurisation of the top of my head is that the 744 is patially pressurised before TO to seal the doors and give that bl...y big fuse from rigidity. The diff press is 8.2psi from memory (better not quote me on that).

De-ice/anti-ice........NFI

Hope that gives you something........Good luck.

[coded_messages]

1) I think this is due to the fact that if you took fuel from the CWT and then the mains you would pretty much soon go out of the C of G range which is around 8.5% - 33% for every ton of fuel in the stab it changes the C of G by 45 units. Also if Fuel gets caught in the Stab it can lead to many problems - Maybe Boeings logic is to get it out asap :)

2) Hard to give you this in a post! General overview is that the cabin is pressurised with conditioned air from 3 packs. 2 Outflow valves at the rear of the aircraft regulate the amount of air discharged. these are controlled by 2 cabin altitude controllers 'a' and 'b' and only one is active at a time.

The Cabin altitude controller maintains pressure - The max being 8.9psi and the structural limit being 9.4psi at 39,000ft with 8.9 psi it generally gives a max cabin altitude of 8000ft.

An overboard dump valve is at the forward lower left hand side of the fuselage. It is normally open a small bit throughout flight to to improve forward cabin ventilation. If the outflow valves close then this will close automatically.

3) There is a single probe on the forward left hand side of the fuselage which senses ice formation.

The aircraft has Nacelle Anti-Ice which is not automatic like the 777. Nacelle Anti-ice is hot bleed air. It is used when the temp is below 10 oC.

Wing Anti-Ice is a valve in the leading edge of the wing, when the valve is opened bleed air flows from the engines into a spray tube in the leading edge. In all my time on the 744 I have yet to see it used!

Obviously the windows are heated too :)

4) 8.9psi

Hope this helps :)

Best of luck :)

[Brian Abraham]

The best place for manuals I've come across is www.esscoaircraft.com

Best of luck Brian

[t6 sparky]

There is a single probe on the forward left hand side of the fuselage which senses ice formation

There is in fact 2 ice detectors, one on either side of the fuselage.

The aircraft has Nacelle Anti-Ice which is not automatic like the 777. Nacelle Anti-ice is hot bleed air. It is used when the temp is below 10 oC.

NAI is automatic, the control switches on the overhead panel are three position off/auto/on. in auto NAI is controlled by the ice detectors.

[BusyB]

"NAI is automatic, the control switches on the overhead panel are three position off/auto/on."

Think it depends on the a/c mod state, none of our a/c have auto.

[Phil Squares]

The standard NAI is not automatic, it's an option just as the auto start.

The wing heat is inop with the LE devices out of the up position.

The fuel from the horizontal stab is pumped into the Ctr Wing to ensure the aft CG limits are not exceeded.

Can't seem to find the reference, but the normal differential is 8.6 psi. (from my greatly degrading memory!)

[rhinodriver]

Thanks to all for your input!

[coded_messages]

Sorry to be perdantic here gents...

Sparky

Direct quote from my Boeing 744 FCOM1 here states " The system consists of a single ice detector probe on the forward left fuselage"

NAI on the fleet I am on is not auto ;) But it is a Boeing option.

Phil

The PSI is 8.9 normal and 9.4 structural, with a minimum 0.11 required for take off :D

[Mister approach]

A quick note on pressurization
Cabin pressurization is controlled by regulating the discharge of conditioned cabin air through the outflow valves.
Two outflow valves are installed at the rear of the cabin. The valves normally operate in parallel.
Cabin altitude and full ventilation rates can be maintained by either valve.
There are two cabin altitude controllers, A and B. Each controller controls both outflow valves.
Positive and negative pressure relief valves protect the fuselage against excessive pressure differential.
The pressurization system has automatic and manual operating modes.
and following on from what has already been mentioned,
For takeoff, the system provides a small positive pressurization prior to rotation to cause a smooth transition to the cabin altitude climb schedule. During climb, cabin altitude increases on a schedule related to airplane climb rate and flight plan cruise altitude. When the FMC climb path has a planned level segment, it is included in the total time required for the airplane to reach the top of climb and cabin altitude continues to increase during the level segment. If the airplane climb flight path is above the FMC climb path and maximum cabin pressure differential is reached during the climb, cabin rate then becomes a function of airplane climb rate so maximum cabin differential pressure is not exceeded.

Hope this helps,

Jason.

[Jerboa]

You might like to add the cabin is also protected by 2 positive pressure relief valves in the event of an overpressure. The 2 valves are located on the fuselage forward of the l/h wing. One will open if diff pressure reaches 9.25 psi and the second will open at 9.75 psi if the first valve fails. If any of these 2 valves ever open due to overpressure they will reset but the butterfly flaps on the fuselage will remain deployed and can only be reset by maintenance.

Correct technical term is Cabin Pressure Controller on this aircraft not Cabin Altitude Controller.

NAI control switches on the overhead panel P5 are optional. Some operators choose square pushbutton switches and others use toggle switches with an AUTO position. In AUTO the NAI valves are controlled by the ice detectors. During Rev thrust the NAI valve will close.
As mentioned earlier the WAI uses bleed air for leading edges, only operates when the leading edge flaps are retracted. The WAI can also have an AUTO position which again uses the ice detectors like the NAI for control.

All -400 aircraft i have worked on have 2 ice detector probes.

[Grunf]

Hello all.

Just to add

structural deltaPi is actually higher then 9.4 psi (probably this value is listed in manuals).

A/C is tested (high blow) for 1.33x the above number (FAR 25 req) so you get 12.5 psi. Hope you'll never see that, I guess

Cheers

[SB4200]

Morning...

Are these the ice detectors visible at the bottom of this shot (http://www.airliners.net/open.file/764293/L/ ).

Thanks in advance

[CV880]

If memory serves me correctly the installation of dual ice detectors goes hand in hand with the automatic nacelle and wing anti-ice optional fit. With this set up you put the anti-ice systems in Auto and if both ice detectors say ICE the anti-ice systems turn on automatically. Most 744's I saw had a single ice detector that simply gave an ICING message on EICAS and everything else was selected manually.
I think the difference in horizontal tail fuel usage between the 744 and Airbus is largely due to the fact that the 744 tail tank was grafted on to an existing structure and fuel system simply to squeeze in more gas whereas Airbus started with a clean sheet and the idea of using fuel movement for CG control from the beginning. I recall that early 744's had a Flight Manual limitation stating you had to land before gross weight reduced to a certain value if there was still fuel in the tail tank. Presumably a structural or CG issue. Not sure if this limitation still applies.

[t6 sparky]

Are these the ice detectors visible at the bottom of this shot.

No they are TAT Probes.

[SMOC]

On the -400 the anti-ice probes are directly under the AOA probe, standard fit is on the left side the right side one is an option. This can be seen in the following pics.

http://www.airliners.net/open.file/863007/L/

http://www.airliners.net/open.file/722028/L/

http://www.airliners.net/open.file/744840/L/

SMOC

[SB4200]

Thanks for hte info SMOC.