747-400 Flap problem

[kevinh 409]

Something I only noticed recently by accident, when I had taxied in with the flaps still at 30. I selected flaps up and then shutdown the engines. As soon as the EDPs depressurised the flaps stopped moving (somewhere between 5 and 1). On EICAS the four demand pumps (all in AUTO) were still showing 3000 psi, so the flaps should still be moving, although more slowly. Part of the function of the demand pumps is to boost EDP output for high flow activities like flap and gear deployment, so they certainly should be able to move the flaps. Even alternate flap didn't appear to work. I only got the flaps moving again be restarting engines 1 and 4.Subsequently, with all engines off, I was able to extend the flaps using the alternate system.Anyone got any ideas what the problem might be? Is it a bug or is it a limitation of FS9 (surely not with all the additional modelling in the Queen?).Kevin

[Guest Panman]

If I remember right from the last time I was involved in flap checks (and this is in regards to the 757 at least). If you drive the flaps out on alternate, they have to be driven back up on alternate before you can operate them normally. And vice versa.I'm assuming it's the same for the 747. So, based on that assumption, it's true to life.

[kevinh 409]

You miss the point. I only selected alternate flaps to see if that would drive the flaps in after the normal system became inoperative with no EDP pressure. The point being that demand pump pressure should still have been able to drive the flaps in using the normal system.I've not operated the flaps on a real 747-400, but in a Level C full flight simulator I'm familiar with there is no need to reset the flaps after using the alternate system, just disarm the alternate system and select a new flap detent (I believe any detent will do, on the 747 Classic you had to move the flap lever past the current TE flap positions to "pick them up").On most Boeings you can switch from normal to alternate flaps and back again without any problem. Maybe the 757 is a special case? From what you are saying how would the flaps "know" where they had to be driven back to before the normal flaps can be used? If you mean the flap lever position, well possibly, but as I say you can easily move the lever to re-establish normal control. The 747 has inboard and outboard TE flaps which move at slightly different rates so you would never get them at the same angle as the lever except for 0 and 25 degrees.Kevin

[jon b 2,041]

If the flaps stopped as you say between 1-5 it sounds that its the leading edge flaps thats causing the problem.These as I`m sure you know are not powered by the hydraulic systems but by the pnuematics,could a lack of bleed air be causeing your initial problem?just a thoughtcheersJon

[kevinh 409]

No, the APU was running and the APU bleed valve was open. Plenty of air in the duct (note ADP 1 and 4 both clearly running, also pneumatic).The flaps stopped when the EDPs lost pressure, which happened to be between 5 and 1. As soon as EDP pressure was restored the flaps moved again. No change to pneumatics at all.AFAIK, there is no reason why inoperative LE flaps would stop TE flap motion. That would make the TE flaps entirely dependent on the integrity of the pneumatic system.Kevin

[CF104 0]

>No, the APU was running and the APU bleed valve was open. >Plenty of air in the duct (note ADP 1 and 4 both clearly>running, also pneumatic).>>The flaps stopped when the EDPs lost pressure, which happened>to be between 5 and 1. As soon as EDP pressure was restored>the flaps moved again. No change to pneumatics at all.>>AFAIK, there is no reason why inoperative LE flaps would stop>TE flap motion. That would make the TE flaps entirely>dependent on the integrity of the pneumatic system.>>KevinHi Kevin,Sounds like a small bug you've found. The 744 flap system is a complex beast and this just may be the best that PMDG could do at the time.On the real 744, the TE Flap Control Units(FCU) have two operating modes, Primary and Secondary. In Primary mode, the flaps are moved with hydraulics, if pressurized. If hydraulic pressure is lost, they revert to Secondary mode and will drive to the commanded position by electric motors. Secondary mode is much slower and is automatic in function. Secondary mode is not related to Alternate Flaps mode.Although the above doesn't relate to your problem, the flaps still should have retracted due to hyd. sys 1 and 4 demand pumps working and providing pressure. If the demand pumps were then turned off, the flaps would still retract in Secondary mode.Edited for spelling.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

[kevinh 409]

>>Hi Kevin,>>Sounds like a small bug you've found. The 744 flap system is a>complex beast and this just may be the best that PMDG could do>at the time.>>On the real 744, the TE Flap Control Units(FCU) have two>operating modes, Primary and Secondary. In Primary mode, the>flaps are moved with hydraulics, if pressurized. If hydraulic>pressure is lost, they revert to Secondary mode and will drive>to the commanded position by electric motors. Secondary mode>is much slower and is automatic in function. Secondary mode is>not related to Alternate Flaps mode.>>Although the above doesn't relate to your problem, the flaps>still should have retracted due to hyd. sys 1 and 4 demand>pumps working and providing pressure. If the demand pumps were>then turned off, the flaps would still retract in Secondary>mode.>>Cheers,>>John>>Boeing 727/737 & Lockheed C-130/L-100 Mechanic>http://www.sstsim.com/images/team/JR.jpg>www.SSTSIM.comHi John,I wasn't aware of the secondary system on the 744. Not knowing it was there means I have always manually selected Alternate to get the electric mode when testing. How does secondary mode differ from alternate, as both are electrically driven?I don't think PMDG have attempted to simulate this secondary mode, it would be too confusing. Alternate mode has to run at the same speed as normal mode due to FS9 limitations, so secondary would look just like normal mode, leading to complaints about the flaps still working with no hydraulics.Regarding the problem, I was able to reproduce it only when I first loaded the 744 from the default flight. TE Flaps ground to a halt somewhere around 4 degrees. It was not related to when the EDPs depressurised as I had first thought, just the fact they weren't running. After I got EDP 1 & 4 running again and drove the flaps to 30, I was able to extend and retract the flaps as expected between 0 and 30 on demand pumps alone. The flaps run much slower with a single pump which is a good plus for PMDG. It looks as if a constant or variable is incorrectly initialised, so at start up the demand pumps aren't strong enough to drive the flaps through the slow range between 1 and 5. Once initialised there is enough flap rate to keep them moving. I guess that in modelling the flap rates so well, PMDG have put things on the edge of not running between 1 and 5 if both EDP and DEM pumps are not on line. If it's a bug it's a very minor one. If the maths is fixed point and the numbers are small it's easy to loose resolution and a small number becomes zero, leading to no flap movement.At least the hyd system is operating properly, which would have been more of an issue. Having looked really closely at the system last night, I have to say how impressed I am with the depth of the simulation. It's better than some full flight sims I've seen.My only question to PMDG would be if the flaps can be slowed down like this with only a DEM pump running, why can't alternate electric mode be made to run that slow as well?Thanks for your input, JohnKevin

[CF104 0]

>Hi John,>>I wasn't aware of the secondary system on the 744. Not>knowing it was there means I have always manually selected>Alternate to get the electric mode when testing. How does>secondary mode differ from alternate, as both are electrically>driven?>Hi Kevin,The main difference in Secondary to Alternate is that there is still TE flap assymetry protection in Secondary mode while Alternate has no TE flap assymetry protection.>>Thanks for your input, JohnNo troubles. Good to hear it's not a major bug and has a work-around.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

[Qavion 19]

"How does secondary mode differ from alternate, as both are electrically driven?"Secondary mode (or Primary Electric as it is sometimes known) is still using the Flap Control Units (Computers). Alternate mode bypasses the Flap Control Units and as a result bypasses some of the safeguards, like asymmetry protection.Sequencing of LE and TE flaps varies when using Alternate, also. Displays differ, too. Alternate Mode only extends the flaps to 25 degrees.Switching from one mode to another midstream can cause sequencing problems.Cheers.Q>

[kevinh 409]

Q,Thanks for taking the time to explain. I'll have to try it out next time I'm in the simulator (may be a while though).CheersKevin