AI744LR From Thailand, joined May 2001, 106 posts, RR: 0 Posted (13 years 5 months 3 weeks 5 days 20 hours ago) and read 1136 times:
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.
Me From United States of America, joined Oct 2000, 220 posts, RR: 2
Reply 1, posted (13 years 5 months 3 weeks 5 days 19 hours ago) and read 983 times:
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.
DC-9CAPT From , joined Dec 1969, posts, RR:
Reply 2, posted (13 years 5 months 3 weeks 5 days 19 hours ago) and read 956 times:
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):
Goboeing From United States of America, joined Jun 2000, 2714 posts, RR: 15
Reply 5, posted (13 years 5 months 3 weeks 1 day 14 hours ago) and read 835 times:
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.
RAAFController From Australia, joined May 2001, 125 posts, RR: 0
Reply 6, posted (13 years 5 months 3 weeks 1 day 10 hours ago) and read 819 times:
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 From United States of America, joined Jun 2001, 168 posts, RR: 0
Reply 7, posted (13 years 5 months 3 weeks 1 day 5 hours ago) and read 802 times:
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.