Descent Calculations

[pjc747]

For my Stratocruiser ops, I have elected to do my flights without ATC, since FSX's is annoying and I'm not at the seat for the long-haul flights. To calculate my descent, I have created diagrams like the one below, that gives me teh proper rate of descent, speed, and altitudes+distances for the flight's top-of-descent. I was wonder what you guys think of my handiwork, and any critique that could streamline it more. I think I refined it pretty well:



As perhaps you can tell, I'm flying Clipper 2, the old around-the-world flight of Pan Am. Currently, only Air New Zealand sell an around-the-world ticket.

[Norforce]

Its not hard 1000 ft a minute is a 1000 ft a minute 23000 ft want to desend to 3000 ft take the 3 from the 23 and look 20000 ft that 20 mins still good stuff Peter

[Nico081]

Descending alt (ft) / 1000 x 3 = Top of descent (gradient: 3°) in nm



Example:
If i want to descent from 29000ft to 5000ft, differencial altitude is 24000ft

24000ft / 1000 x 3 = 72nm

[seaniam81]

Peter, your math works. I double checked it with my electronic E5B. But only if your speed of 210 is ground speed and not airspeed.

[pjc747]

Peter, your math works. I double checked it with my electronic E5B. But only if your speed of 210 is ground speed and not airspeed.

Yeah, thats a reference to the FE's calculations.

[kilmer]

I read these posts about descending, and they were helpful, but I have a question about cabin pressure. In order to descend at 1,000 feet per minute at a ground speed of only 210 knots, you have to throttle back quite a bit. When I try to descend that quickly with the throttle, or more precisely the turbo boost, pulled back, the differential pressure drops to nothing and the cabin altitude starts climbing. I've had Heidi tell me that the passengers are uncomfortable because the cabin altitude has reached 10,000 feet. In order to maintain enough differential pressure to keep the cabin at any particular altitude, you have to keep those turbos throttled up, which in turn flattens out the descent rate to about 300 feet per minute in order to keep from overspeeding the aircraft. It literally requires hundreds of miles to come down from, say, 30,000 feet if I want to maintain the cabin altitude.

When descending, I reverse the procedure for climbing: that is, I have the throttles full forward and start reducing the turbo boost. When the aircraft is low enough that a boost from the turbos is no longer required, I zero the boost and start working back the throttles. Is that how you fellows descend?

Also, descending at more than 500 feet per minute makes the baby cry.

[CAPFlyer]

If you reference the B377 manual, you'll find that there is a minimum pressure required to be produced by the turbos to maintain pressure. As such, when you reach that point, you simply start using throttle to maintain MAP with the turbos set at their minimum pressure. At this point, also engage the FE to manage the turbos for you and he'll ensure they're properly set. That's what he's there for.

[bigjuicyspider]

Kilmer, like Capflyer says, use the VFE to manage the turbos on the way down.

You should be able to descend at around 1000 fpm if you want to and still maintain pressure. I've had the MP down to 28" on the descent, with the Turbo Lever obviously all the way forward in the early stages, and that will work for you.

********

CAPFlyer, do you know if the early pressurized propliners typically descended at a fixed rate of 500 feet per minute, or did they historically descend at the maximum rate that they were able to, as per their pressurization requirements?