Commercial Landings

Lots of reasons.

You are descending from 30k ft. Have you ever experienced a 5k/min descent?

 

Good point. I have experienced a rapid descent in a Boeing 720B on a test flight. Main gear deployed, spoilers up, and all hell broke lose from the vibrations of the spoiler turbulence hitting the horizontal stabilizer. Overhead panels in the cabin started to drop and we had a good ride to 11k.

 

it's about keeping a managable forward speed vs the decent vs not alarming the pax... not to overspeed and stay with other traffic ( not overtake it) under normal conditions it is not unusual to start a decent 100 miles or more out
TCAs have high performance arrivals for capable aircraft and traffic permitting... in my busy TCA the controllers knew that I liked to use them, which would lead to vectors to land from 15000, it didn't matter whether it was downwind, base of straight it... it would cut down a lot of sequencing and time...TCAs also have performance departures available, it clears a lot traffic out that does not need to be sequenced...transitioning large TCAs is a pain

 

In the 747,we plan 3 miles distance for each 1000 ft of altitude to lose (idle descent). So a descent from 35000 ft is 105 miles. We also need about 10 miles to slowdown from a descent speed of 300 kts to approach speed. So total distance is 115 miles (no wind). And that takes about 20 mins or so to cover.


As far as descent rates, a 1000 fpm or 5000fpm descent should feel the same other than the deck angle. Once stabilized, you shouldn't feel the descent. If I remember correctly, the 4 forces of flight (thrust,drag,lift,weight) are equal in level flight, and also equal when descending at 5,000fpm. Or climbing at 5,000fpm. When I used to fly Brasilias, we would routinely descend at 6,000 fpm on the LAX Palm Springs flight. We'd cross the mountains at 11,000 ft and be cleared for the visual approach. We'd put out one notch of flaps, drop the gear, and push the props up to max RPM (flat pitch). Then slowly push the nose over to keep 200kts. That would give us around 6,000 fpm. It was a great approach....

 

Tonight LAX approach control had us down to 210 kts while we were still 75 miles out and at 12000' so you can't do too much in these kind of situations. Also remember there is a 250 speed limit below 10000'. With no ATC issues and if the 250 below 10000 rule was eliminated we could get the jets down a little quicker but the bottom line is there is a large amount of potential energy when the airplane is at altitude and the energy needs to be reduced/eliminated prior to landing. All airplanes are a little different but I can say the 757 is really slippery and difficult to bleed off energy.