AI744LR
Topic Author
Posts: 103
Joined: Fri May 11, 2001 6:14 am

Landings

Sat Jun 02, 2001 12:59 am

When a plane is given clearance to land, what happens next? I mean, is the landing an estimated "glide" down to the runway with enigines at low thrust or is the plane actually "powered" down.

What I really wanted to know is, how does the plane maintain that level altitude during the "flare" or the final moments before touchdown. Just before touchdown, planes tend to "hang" for a few seconds (I guess to adjust the nose gear up for landing). I want to know how this is achieved IF the final stretch is actually a glide. According to me, a glide is a powerless descent and therefore, I don't see where the power "to hang" comes from in the final moments when there is no emphasis on thrust.

Please educate.

Thanks.
 
me
Posts: 220
Joined: Sat Oct 21, 2000 4:57 am

RE: Landings

Sat Jun 02, 2001 1:36 am

All US airliners are required to maintain a flight path on or above the glide slope when using a runway served by a ILS or VASI. So if the runway has an ILS approach, that frequency will be selected in the nav radios and the crew will use the localizer and glideslope, even during visual approaches with 100 miles visibility.

Once fully configured for landing, power is required to maintain the standard 3 degree glideslope. On the DC-9 1.3 EPR or aprox 70%N1 power settings are a good ballpark figure to start with.

Once the aircraft gets within one wingspan of the runway, ground effect takes place. Ground effect is a reduction in induced drag caused by the ground interfeering with the airflow around the wing. Less power is required and the aircraft tends to float, which combined with the flare, slows the sink rate prior to touchdown (hopefully).

All that's left is to control your airspeed by adjusting pitch attitude and power settings, compensate for crosswinds and touchdown within the touchdown zone.



 
Guest

RE: Landings

Sat Jun 02, 2001 2:24 am

It's also important to mention that you must hold Vref. Vref is determined by your landing weight and flap settings. In windshear situations, 1/2 the windspeed and all the gust may be added to compensate.

Here's some memory Vref speeds (at flaps 30 for the 73):


Weight Vref 30

88,000lbs 120kts ( in error by plus 1 kt )

99,000lbs 127kts ( exact )

110,000lbs 134kts ( exact )

121,000lbs 141kts ( exact )

132,000 148kts ( in error by plus 1 kt )

143,000 155kts ( in error by plus 1 kt )
 
AI744LR
Topic Author
Posts: 103
Joined: Fri May 11, 2001 6:14 am

RE: Landings

Sat Jun 02, 2001 4:31 am

WOW! Thanks guys! That's amazing information for me. I never ever considered the "ground effect." Now I get a better idea of why the plane kinda "hangs" prior to touching down. That's really amazing!!

Thanks folks!
 
Soku39
Posts: 1731
Joined: Fri Nov 03, 2000 7:16 am

RE: Landings

Sat Jun 02, 2001 11:41 am

What do you mean adjust the nose gear during the flare, its either up or down.  Confused
The Ohio Player
 
goboeing
Posts: 2429
Joined: Mon Jun 05, 2000 5:31 am

RE: Soku39

Wed Jun 06, 2001 7:05 am

I think what AI744LR meant by adjust nose gear before touchdown was to make sure it is straightened out pointing down the runway, because if it wasn't it would probably make a nasty landing. Just a guess, but that's probably what he meant.
Nick
 
RAAFController
Posts: 124
Joined: Wed May 30, 2001 9:50 am

RE: Landings

Wed Jun 06, 2001 11:27 am

AI744LR,

Just another point.......I agree with the statement about ground effect, but the aircraft could also appear to "hang" due to the actual flare.

Also, just in case you don't know, the idea of the flare is partially to take away excess speed, and actually stall the wings, so that in a "perfect landing", the wheels will take the weight of the a/c just as the wings stall. You don't need power for this. Physics says that broadly there are two engergy types (well this is simle anyway).Forward momentum....ie given by thrust etc, is Kinetic energy. Your height above the ground is Potential energy. Hence why if you drop something, it gets faster as it falls.........potential energy turns into kinetic. Well in an a/c, when you come in to land, yiou want to slow down, one way of doing this is to get higher, withouit adding any more energy (thrust) from the engines. By pulling the nose back, tha a/c starts to climb, thus 'washing off' (slowing) the speed, as Kinetic energy becomes potential. Since the wing then stalls, and becomes less effective at producing lift with a slower speed, the aircarft does not reall climb, but hangs, and settles onto the runway.

Other effects are obviously the wing stalling, meaning that the aircraft ceases to 'fly', it also takes the weight of the nose gear and a whole host of other good effects.

Hope this helps.....i tried to expalin real simply, and i am sure others will correct anything i get wrong, or will maybe explain a bit better.
 
TimT
Posts: 168
Joined: Sun Jun 03, 2001 1:38 pm

RE: Landings

Wed Jun 06, 2001 4:28 pm

All heavy aircraft have "centering cams" built into the nose strut. When it's fully extended (off the ground) it's forced into a straight ahead position. Unless it's hung up it won't turn. If it's hung up you're promised an interesting touchdown.
Find a pilot that can land a DC-9-10 gently everytime and you'll win the gold. This a/c does not have leading edge devices (slats) and it does have a slick wing. It flies in ground effect until it can't fly anymore. Then it sort of plops on the runway.
 
Guest

RE: Landings

Wed Jun 06, 2001 10:14 pm

Find a pilot that can land a DC-9-10 gently everytime and you'll win the gold.

Tim T, this is Tim T....amen to that bro! Do you work for USAJet?
 
me
Posts: 220
Joined: Sat Oct 21, 2000 4:57 am

RE: Landings

Thu Jun 07, 2001 1:26 am

DC-9 Capt,
Isn't it about time for you to change your user name now that you're a big time major airline pilot?
 
tt737fo
Posts: 468
Joined: Thu Jun 07, 2001 2:13 am

RE: Landings

Thu Jun 07, 2001 2:33 am

Here it is...my new username!

TT737FO
Formerly known as DC-9CAPT

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