Help with understanding a hydraulics issue

[Guest AJ]

Hi Having studied the manual, I need some help in understanding the particulars of a failure of the #4 hydraulics system. In the abnormal procedures manual (pages 6-50 and 6-51) there is a warning about the possible in-flight activation of ground mode after the landing gear has been extended. Regarding this condition, it mentions

[jon b 2,041]

Andrew,What is happening is that the air/ground logic gets confused because of the wing gear not tilting because the system 4 isn`t pressurised.The wheel bogeys are normally tilted in the air as you can see in the sim,but if the hydraulics aren`t pressurised the wheels will be at 90 degrees to the strut,there`s an air ground switch on the wheels that then thinks because the wheels arn`t tilted you must be sat on them on the ground.It will then remove all the inflight protection,as you mentioned the reverse thrust amongst them.In fact all 4 reversers would operate, but only if a pilot tried to deploy them,not on their own uncommanded.The reversers are in fact pnuematically operated and dont require hydrualics.I remember the warnings you mention about activation of the ground mode in the relevant QRH manual,but I don`t think its there any more,I guess Boeing have modified the air/ground logic slightlyhope that helpsJon

[Guest AJ]

Thanks Jon. I really appreciate your input. I was under the impression that the reversers did require hydraulics in order to fundtion. The following is from the PMDG manual:"Each engine has an independantly hydraulicly actuated fan reverser. The presure needed to actuate the reverser comes from the associated engine driven hydraulics system. Loss of the hydraulic system will cause loss of the hydraulics required for reverer opperation." 11-26 Not being an engineer, all this stuff is hard to completely get my head around. Thanks,Andrew

[jon b 2,041]

Andrew,I`d better double check myself on this one then,don`t want to hand out bad info.When I did my inital type course onto the 744,( some nine years ago now,scary thought) I remember we had one of those white boards on the clas room wall.one of the instructors had at one time written "thrust reversers are pnuematically operated" on it but had used a standard marker pen so they couldn`t erase it off the board:-)Everyday we`d go into the classroom and see that same thing written on the board,it eventually sunk in,just about the only thing that did though!regardsJon

[Guest AJ]

Good deal, Jon. I'll be interested to find out what you come up with. Obviously this whole issue is pretty obscure, but I guess that's what makes it interesting to me. Afterall, there have been some pretty bizzare incidents over the years. Since PMDG included the warning (about the reversers unlocking), I just got to thinking about whether there was any reasonable scenario under which this could happen. Obviously, since the pilot would have to pull the reverser levers himself, it seems pretty darned unlikely. Let say the plane is on final approach with this particular hydraulics failure. I suppose if for some reason (traffic avoidance, rapid wind change...) he had to suddenly jerk the throttles back to idle. In the process, might he accidentally pull the reverser levers as well, causing a deployment? Andrew

[Guest Just747-400]

Bleed air is used for:

[Guest AJ]

Thanks for contributing. Thus, as I understand it, both hydraulics (see my quote above) and pneumatics are required for reverser funtionality. Otherwise we have a rather odd contradiction.This once more leads me to think that in this situation (hydraulics #4 failure) that only the reversers on engines 1-3 would actually be in danger of deployment. since the system #4 failure in itself would render the #4 reverser non-functional.From the last sentence, I take it that in the situation we are discussing (the aircraft thninks it is on the ground, when it is not) this protection would not apply.Andrew

[jon b 2,041]

Cheers Alex,Saved me some typing there! just read those exact words in my old favorite book "Operations manual volume 2" a classic read!Andrew,the forward and reverse thrust levers are interlocked so it would be impossible to deploy revers without the main thrust levers being closed,also on approach a good pilot would have his hands on the thrust levers at all times so there wouldn`t be much of a chance of pulling reverse by accident.Something that is a lot easier to do by mistake in the heat of the moment is to push TOGA instead of the A/T disc switch! Thankfully only done it once,but at about 100ft while waiting for a late landing clearance. I was convinced we would go around as an aircraft in front was slow to vacate and was covering the TOGA switch with my finger.After we got clearence I was too busy planning my next move I didn`t my change fingers(1st finger for the TOGA switch,thumb for the A/T switch).luckily I was hand flying and managed to get the A/T out quick before it got too embarasing.regardsJon