SJC-Alien
Topic Author
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True Or False..I Need The Final Answer

Mon May 22, 2000 5:00 am

I have heard in the past, that a pilot of an aircraft isn't allowed to deploy the thrust diverters while the front wheel is still in the air, to avoid causing a 'Slam Down' of the front gear. It was indicated that MD-80 types were prone to the 'Slam Down' effect if the thrust diverters were activated too early. I don't think it's a regulation, and jets with the engines on wings probably wouldn't matter. Does anybody have the final answer?

Thanks
SJC Alien
 
Guest

RE: True Or False..I Need The Final Answer

Mon May 22, 2000 6:05 am

It makes no difference whether the engines are wing or tail mounted. The effect of rapid de-acceleration will still cause the nose wheel to contact terra firma a little harder than desired.

The effect on MD80's is probably more intense due to the distance between the NLG and Main gear.

 
calpilot
Posts: 881
Joined: Sun Oct 17, 1999 5:16 am

False..

Mon May 22, 2000 7:52 am

Final Answer: False. You may deploy T/R's with the nose gear is still in the air. On the MD80 we at CO do not deploy until the nose wheel is on the ground to pervent the buckets from hitting the ground. However it is allowed if need be. When I deploy the T/R's on the B757 prior to nosewheel touchdown I increase back pressure on the control column as airspeed decreases, and there is a slight pitch-up moment.
 
Buff
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RE: True Or False..I Need The Final Answer

Mon May 22, 2000 7:56 am

To the best of my knowledge, there are no restrictions in the Boeing fleet. But sound judgment must be used if deploying the reverse thrust prior to nose wheel contact. If the thrust levers are not at idle, interlocks will prevent the reverse levers from working; if the aircraft bounces, a go-around will probably be necessary; if there is a strong crosswind on a slippery runway, too early or assymetric reverse deployment may cause loss of control.

Additionally, the reverser sleeves may be opened without accelerating the engine. Common practice from my experience is to deploy the reversers after touchdown when a go-around is not required, but not to accelerate the engine in reverse thrust until positive nose gear contact.

Reverse thrust is most effective at high forward speeds and is usually brought back to idle below 80 knots. Wheel braking at all speeds is most effective though in slowing and stopping the airplane.

Hope that helps,

Best Regards,

Buff
 
Guest

Buff

Mon May 22, 2000 10:26 am

Very nice answer Buff.

Best Regards,
JFL.
 
B727-200
Posts: 1008
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RE: True Or False..I Need The Final Answer

Mon May 22, 2000 12:10 pm


It is funny this has come up as a question, as it is something I am noticing more and more. In Australia, particularly SYD and MEL, I don't think you would see a B737 (300 or 400) put the nosewheel down before engaging reverse thrust. It does not appear to happen as much on the B767's and A320's that frequent these ports.

As a hypothesis (and argue this please if I am way off), Australia B737 pilots increased approach speeds by about 20-30knts some time ago as an additional deterant against possible rudder failure. Is it possible that this increase in approach speed has made it essential to engage reverse thrust earlier to still make the same high-speed taxi turn-off's as the B767's and A320's?

Your thoughts will be appreciated.

B727-200.
 
AAR90
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RE: True Or False..I Need The Final Answer

Mon May 22, 2000 12:25 pm

If there is a specific manufacturer requirement, you have to ask the manufacturer. AA requires MD80 pilots to put nosewheel on runway prior to reverser deployment. Was not a requirement on F100 when I flew it and I've never seen it as a requirement for DC10, B757 or B767 either.

From AA DC-9 OpMan:

"On toughdown, it is important to lower the nosewheel to the runway
and hold forward pressure on the control column. When this is done, the
wing angle of attack is decreased, resulting in minimum lift, placing more
weight on the main gear sooner; this not only increases rolling friction
but increases braking effectiveness as well. This procedure is five times
more effective in decelerating the airplane than that achieved from
holding the nosewheel off in an attempt to use 'aerodynamic' braking."

*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
Guest

RE: True Or False..I Need The Final Answer

Mon May 22, 2000 12:52 pm

hmm, that is a really strange operation from the DC-9 manual, to put forward pressure on the control column ?
Until what i have learned now on short field landings, after touching down, brakes are applied very steady and full back pressure on the yoke or stick to transfer weight to the main landing gear. I have been thinking about it and to my own conclusions, if the center gravity is ahead of the main gears, when applying fwd pressure (nose down) will tend to raise the main gears, if CG is behind main gears, logically tail will kiss earth. Hmmm, unless the reverse thrust will tend to pitch up the airplane and to counteract this fwd elevator is applied meanwhile thrust reversers are engaged. I haven't sat on a first officers seat on large aircraft to get the actual feeling on how and when the airplane responds to different inputs ,but hope to do someday. Please correct me anyone.  
 
SJC-Alien
Topic Author
Posts: 870
Joined: Wed Jul 14, 1999 1:15 pm

The Final Answer

Mon May 22, 2000 2:21 pm

Most excellent answers, everybody...

Thank you...

I thought maybe runway length had something to do with it, as at SJC, it seems MD-80 pilots seem to land with a high nose attitude, as the runway 30L, I thinks is a paved 10,200, but not all of the pavement is available?
But LAX, SEA and SFO,,there seems to be a difference in the nose high attitude, meaning lower than when they land a SJC.

Buff, CALPilot, AA90 and HP-873- you went beyond the
call of duty, explaining the system.
Thanks for the intelligent replies....everybody.

SJC Alien

 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: True Or False..I Need The Final Answer

Mon May 22, 2000 2:51 pm

On MD80, AA says _slight_ forward yoke pressure to put weight on wheels and provide greater nosewheel friction. This increases nosewheel steering capability and wheel brake effectiveness. Too much forward pressure will tend to lift the tail thereby decreasing wheel brake effectiveness.

As to differing nose up attitudes, I haven't noticed any difference on my landings, but I don't have as good an aspect as an LSO (or spotter) would have.  
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: True Or False..I Need The Final Answer

Mon May 22, 2000 5:18 pm

Buff mentions cancelling the Rev Thrust @ 80 Kts due to it's ineffectiveness at low speed. Another factor that reqs Reverser cancellation at low speed is at these low speeds, the fan blades (on Hi-bypass engines) tend to dig into the Fan Case abraideable shround . If this shroud wears away too much it sets up vibration.
 
dnalor
Posts: 346
Joined: Fri Mar 10, 2000 7:58 pm

RE: True Or False..I Need The Final Answer

Mon May 22, 2000 6:21 pm

In addition to Buff and VC-10 I have read an article by Captain Len Morgan in Flying Magazine that some engines will start to pop loudly with compression stalls below 80 knots with advanced reverse throttle. Something to do with sucking in there own hot exhaust.

I have been on a Qantas 737 300/400 and experienced this myself but didnt understand what was happening till I read about it.

It made sense as the pilot was aiming to turn off the runway at a taxiway closer to the terminal rather than rolling out.

Hard to say but I estimate we were well under 80 knots with heaps of reverse on 
 
SJC-Alien
Topic Author
Posts: 870
Joined: Wed Jul 14, 1999 1:15 pm

Answer

Mon May 22, 2000 10:18 pm

Ok,,
people who have seen my video's at the Airline Collectible show's have made comments on the landings at SJC as the MD series seem to have a rather high nose attitude as they cross the piano bars.

In comparison, the SFO video, they don't seem to be so high in the front end,,but maybe it's the line of sight from the photographers point of view,,,,,maybe?

AA90 response seems correct...but as you can tell, I'm no pilot... 

Thanks
SJC Alien
 
Guest

RE: Answer

Tue May 23, 2000 1:11 am

By the way, on a book I have, says that on DC-9s i suppose not all of them, it is possible to deploy thrust reversers any time after the thrust levers are at idle. And on some DC-8s, reversers may be used in flight as speed brakes. Anyone with experience if please could tell me which models dc-8s and-9s are those.

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