ramerinianair
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ILS Approaches In Large Jets

Thu May 25, 2006 9:14 am

In IMC, (Instrument Conditions) how far off are the aircraft when they have the runway in sight? Are the pilots flying the approach? When the runway comes in sight, how close are you always within centerline.
Thanks,
SR
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Pilot3033
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RE: ILS Approaches In Large Jets

Thu May 25, 2006 1:46 pm

In large commercail aircraft (767, A330 etc.) the autoland system (which is what I assume you are reffering to) is generally pretty accurate. I've seen autland landing right on the centerline right in the TDZ. Approach "minimums" (the point at which the pilot must see the runway lights) are done based on each airport and/or company SOPs.
Not sure sure about that last part. Someone else may be able to provide more specific information.
As far as consistancy, it would depened on the weather, and if the autoland was working properly.
-Matt
-Matt
 
IAHFLYR
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RE: ILS Approaches In Large Jets

Thu May 25, 2006 9:00 pm

Quoting RamerinianAir (Thread starter):
When the runway comes in sight, how close are you always within centerline.

All equipment on the ground and aircraft working properly, satellites in the future for CAT II/III weather conditions, the runway centerline better be right under the nose gear of the aircraft since an ILS you are flying the localizer course which is the centerline of the runway.
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SlamClick
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RE: ILS Approaches In Large Jets

Fri May 26, 2006 1:56 am

Quoting IAHFLYR (Reply 2):
better be right under the nose gear

Actually the ILS antenna will be on centerline. The nose gear may be left or right, depending on crosswind, so as to cause the antenna to track the centerline. This is part of the reason why there are crosswind limits for categories of ILS approaches and for autoland.

Quoting RamerinianAir (Thread starter):
how far off are the aircraft when they have the runway in sight?

Taking an old Cat I ILS with mins of 200 feet as an example, a 747 might be at 200 feet, radar altitude but if you can picture the plane in the approach attitude at this point, the main landing gear will be dangling somewhere below this height (especially as viewed up a three degree glideslope from the intended touchdown point) and the cockpit might be fifty feet or more above that line - still in the clouds even.

This was one reason for the 3-bar VASI. "Long-body" airplanes have a pilot eyeball location well above the glidepath centerline.

Going to Cat II and Cat III (plus) approaches the 'cone' gets smaller closer in. That is, one-half dot of deviation is quite a bit more lateral distance out at the final approach fix than it is just before touchdown. ("ever smaller corrections until you arrive at the right gate")

At toucdown on a Cat III ILS the pilots are not going to be able to see very much of the world, but one thing they will see is a very short bit of runway centerline lighting/marking directly ahead of them - in the direction they are traveling, if not the direction they are pointing. (Allowing for crabbing here due to crosswind)

One of the hardest things to break a trainee pilot of is the tendency to try to point the plane toward the runway when they break out of the clouds. What is important here is that if the runway is slightly off to the left or right it is because the plane has been holding a crab while tracking down the localizer with a crosswind. If you aim it toward the runway you will take out that drift correction and the next thing you know you are drifting out over the grass, then you are steering back past the centerline - then you are goiing missed approach because you are no longer in position to land. At least ON the runway! Smile
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IAHFLYR
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RE: ILS Approaches In Large Jets

Fri May 26, 2006 2:54 am

Quoting SlamClick (Reply 3):
Actually the ILS antenna will be on centerline. The nose gear may be left or right, depending on crosswind, so as to cause the antenna to track the centerline. This is part of the reason why there are crosswind limits for categories of ILS approaches and for autoland.

Okay, a minor error on my part!!

But the nose gear shouldn't be too far away.  crossfingers 
Any views shared are strictly my own and do not a represent those of any former employer.
 
SlamClick
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RE: ILS Approaches In Large Jets

Fri May 26, 2006 9:38 am

Quoting IAHFLYR (Reply 4):
But the nose gear shouldn't be too far away.

Yeah, viewed up a three degree glideslope and with the plane in an approach attitude which would be another few degrees nose up, it might be fifteen, twenty feet or so on a really large airplane.

Laterally, it should be pretty close to centerline even with some crosswind correction.
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Boeing7E7
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RE: ILS Approaches In Large Jets

Fri May 26, 2006 5:30 pm

It depends on the glideslope for the runway. Standard CAT I is a 200' decision height with a decent rate of 318' per nautical mile (3 degree glideslope) with a threshold crossing height of 50' to 55' depending on the airport design aircraft. In a nut shell, at the DH your just outside the approach lights +/- 3,800' or so from the threshold.
 
rendezvous
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RE: ILS Approaches In Large Jets

Sat May 27, 2006 7:05 am

I've done a couple of ILS approaches pretty close to minimums in a Piper Seneca, hand flown. If the needles are centred you come out in line with the runway. It wouldn't be a precision approach if you didn't!

I beleive the ILS is calibrated from time to time to check it's pointing in the right place.
 
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Jetlagged
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RE: ILS Approaches In Large Jets

Sun May 28, 2006 6:39 am

Quoting SlamClick (Reply 3):
Taking an old Cat I ILS with mins of 200 feet as an example, a 747 might be at 200 feet, radar altitude but if you can picture the plane in the approach attitude at this point, the main landing gear will be dangling somewhere below this height (especially as viewed up a three degree glideslope from the intended touchdown point) and the cockpit might be fifty feet or more above that line - still in the clouds even.

Radar altitude is calibrated to read main gear height above ground on the approach. So at 200 feet RA, the main gear will be 200 feet above ground. On a typical 3 degree glideslope at Vref the 747 pilot's eyepoint is about 40 feet higher than this (i.e. 240 feet AGL).

If this wasn't the case, the GPWS rad alt callouts would never reach zero before the main gear touched. Rad alt should be zero at touchdown, and it might even go negative as the aircraft de-rotates, depending where the rad alt aerial is located.
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