jb747
Enthusiast
- Joined
- Mar 9, 2010
- Posts
- 12,959
jb, how does descent from cruise work? You have mentioned ATC in the US can leave you with "High energy offsets". Is this all part of the same thing please?
Most of the time ATC will give you a descent from cruise either when you want, or earlier. Sometimes it will come after the desired point, in which case either increasing the speed, or using the speed brake will fix the profile fairly quickly.
Problems arise when ATC want either a higher speed at low altitudes, or when they don't clear you to descend as required inside 20 miles. The problem progressively gets worse the closer you get to the runway. A thousand feet of offset at 100 miles is trivia (3% or so), but that same thousand at 10 miles is 33%. Couple that with higher than normal speeds, and you'll eventually end up with an energy state that can't be reasonably fixed. Even requests to hold 160 knots until 4 miles (a very common one) cannot be complied with as it would often mean you'd exceed the 'stable approach' tolerances once you go below 1,000 feet.
This issue is most common in the USA, where they seem to have little concept of the problems involved in getting a very large aircraft down, but are used to compliance by locals in much smaller jets. It is easily the most common cause of company go arounds in Los Angeles. Whilst not the cause of the SFO 777 accident, it would seem to have been one of the links in the chain.