Originally Posted by
PLovett
I have tried a quick search through the forums but without success so here goes.
What vertical rates of descent are used in jet transports from TOD to approach?
Some years ago I was fortunate enough to visit the cockpit of a 717 for a brief time which included the top of descent. I recall the FO setting a vertical rate of 3,500' in the autopilot controller and then watching the glass VSI going to that rate (with the autothrottles retarding). Now that rate would be positively dangerous at lower altitudes so I presume a lesser rate is dialed in at some stage.
I know the TOD point is roughly 3 x altitude (in thousands) plus a fudge factor to get the beasty slowed but it is the rate that has me baffled. Any information greatly appreciated.
The old rule is that you want to hit certain 'gates' with a specific amount of energy. For example, if you are going straight in to the airport, the first gates are 30NM at 10,000ft at 250kts or 40NM at 10,000ft and 350kts. You can expect to slow 100kts in about 10NM in level flight, idle.
The next gate is 10nm at 200-210kts (unless DC-9/MD-80 where you can be considerable faster or in a Jurassic 727.. all of these vehicles have great abilities to slow rapidly if that makes sense).
The final gate is 3nm and 1000ft and near Vref for a stabilized approach and 3deg slope in wx.
As noted earlier.. you can use Mach number to figure a lot of stuff. At 0.80m you are travelling about 8 miles per minute (no wind). You're at FL350. Your clearance is to cross X at 10,000 and 250kts. You have 25,000ft to lose. Choose your rate of descent...
At a modest rate of 2400fpm, you will lower the nose 3 degrees from your level flight attitude, use about 11 minutes (round to the high side) so you begin 88nm before your crossing fix.
Of course, if you choose rate you have to accept speed as whatever the descent rate produces.
And then, on most stuff, you just begin descent when the dot tells you to..
Doesn't always work but at least you have something to blame if it fails.