[ncc1701e]

Hi, I was wondering is there a formula one can use to get a proper descent. I mean for example when you're in an airliner flying at high alt. how and when do you start descending to make sure you end up at or near traffic pattern altitude when you get to within about 10nm of destination airport. I know its not as simple as an equation but just wanted to get an idea or tip. Im sure initial alt. IAS and STAR as well as rate of descent are all involved in the answer but Im just looking for that guiding light to put it all together into a nice little formula that'll work everytime.

thanks

[six7]

I've seen some information posted here and XPFW forums concerning specific aircraft, descent rates, speed & flap settings etc. HERE'S ONE... a Google search will probably uncover others.

THIS might also be of some help.

Also, just in general - determining how far out to begin your descent can be as simple as an equation... Pythagorean Theorem:



If you know any 2 of the variable you can easily find the 3rd.

[AndyGoldstein]

The rule of thumb in the airlinepilotforums reference is about right - 3 NM out per 1000 ft cruise altitude. On my many flights east into KBOS, they always seems to start descent right about when they cross the Hudson River.

This assumes an initial descent rate of about 2500 fpm. Most heavies will do this with the engines just slightly above flight idle. Remember that you will be decelerating all through the descent, so don't get panicked by the absurdly short ETA the GPS is giving you. You'll be reading 10 minutes to destination for quite a long time.

- Andy

[-TheoGregory]

Also thats what your speed brakes are used for, If you hit in your vert speed to -2500, engage speed brakes and decelerate to 240 knots, you should be at about horizon level while losing 2500 feet a minute...

If your anywhere around 30,000 ft, ATC generally will tell you to descend 100 nm away from your destination airport though if your decending into somewhere like LOWI they give you vectors through the mountains and can tell you to decend in your last 60 nm to avoid high mountains

[AndyGoldstein]

Also thats what your speed brakes are used for...

Well, yes and no. I've seen it both ways. Most of the time they like to do most of the descent clean, using speedbrakes only as necessary to decelerate into final. I can recall only one time where the pilot rode the brakes all the way down. Brings to mind one of the traditional ATC jokes:

ATC: Delta 432 heavy, please descend to 4000 ft over the next 10 miles.

Delta 432: Unable to do so.

ATC: What's the matter, don't you have speedbrakes?

Delta 432: Yes, but they're for my mistakes, not yours.

- Andy

[dpny]

Question about descending without speedbrakes: when descending in a heavy I sometimes find it necessary to use speedbrakes most of the way down to keep within the specified descent speeds. It occurs to me, however, that as the air becomes more dense on the way down, the plane may begin to slow without the speedbrakes due to increased air resistance. Is that true, or just wishful thinking on my part?

[chksix]

If you descend with LVL CHG you increase the rate of descent with speedbrakes since that mode is holding speed. This mode is the most fuel efficient.

[dpny]

If you descend with LVL CHG you increase the rate of descent with speedbrakes since that mode is holding speed. This mode is the most fuel efficient.

I never use XP's LVL CHG functions. It seems to only go straight up or straight down, with nothing in between.

[chksix]

I've tried it in a plane under development and it works if you set the throttles to a flight idle setting (can often be around 35 to 40% N1). If you set zero thrust you get steep descents

The x737 LVL CHG mode is better though

[-TheoGregory]

Andy has a point though i still maintain what i said.

I got the chance about a year ago to hop into the Air New Zealand 737 simulator at Auckland and that was cool (A family friend is a pilot for ANZ) but as we decended into Wellington we used the speedbrakes from about 30k - all the way to about 10k.

Though i do think its just up to what the pilot wants to do, his aircraft, his atc airspace limitaion and his altitude