Initiaitng descent

[md-100]

When do you initiate your descent? Is there any "good" rule of thumb?

[catchup]

ETA minus 30 min

regards

[md-100]

but in distance??

[Rainboe]

3 x Thousands of feet +10 then allow for wind (on a B737). For a B747- descend when the FMS says. So lose 25,000'- allow 85 miles (with slow down) or 75-80 miles keeping speed up. You would probably find this is a little generous- you could reduce this by 10 miles and increase speed if altitude too high.

It would be useful to say type of aircraft! Jet, turboprop, widebody?

[Old Smokey]

For a 'normal' jet, i.e. one that cruises 4000 to 5000 feet above the Mach/CAS changeover altitude, 3 times the Height plus 10 miles works quite well.

For high flyers, (Learjets et al) operating well above the Mach/CAS changeover altitude, 2.5 times the Height plus 10 miles would works better. (the Mach phase of the descent is an accelerating phase, and quite a bit steeper than the CAS phase).

The +10 miles is to allow for deceleration for approach, and if your aircraft has a high flight idle thrust, you might find that +20 works better.

Regards,

Old Smokey

[flying_elvis]

I use 3 x the altitude to lose PLUS 10 Plus 10 percent of the tailwind.

A good wag for rate of descent is 3 x your mach number. ie 3x.80mach = 2400 ft/min rate of descent. At crossover, still use your indicated mach for crosscheck.

[brain fade]

An Answer!

If you got , say 100 miles to go, you need to be at 300. ie your dist(in miles) x3

Only 25 to go? 25x3 ie 7500'

80 to go? 80x3 Should be at FL160

Toes? 5 each foot. 4 small + 1 big one.

[Intruder]

80x3 Should be at FL160

Your brain faded again... How 'bout FL240?

[popay]

For the A 330 we use factor of 4 until 10000 there after 3 + 10 %, but i generally always go with 4, cause that bird is absolutely a glider. Works fine with me and you are covered for the LW variations. By the way, would be interesting to know, do the guys usually know the finesse of the airplane?
Cheers.

[enicalyth]

seems to work

It’s a funny thing pprune. I was just having coffee with TMT when this very subject cropped up and the rationale behind rules of thumb.

Plain maths of sin, cos, tan suggests that if the flight path is at angle “k” to the horizon and true airspeed is “V” then RoD is Vsin k or as Jimmy Gilmour used to say “Vee Sink” is “Vee Sink”..

Of course if RoD is measured in feet per minute then so must airspeed.

I attach an example taken from Course Notes. Should you wish to check it out as I did then with an Excel spreadsheet it is useful to know that the time for descent can be calculated in simple stages. If H2 and H1 are two “heights” and F2 and F1 two “RoDs” then time = [{H2-H1)/(F2-F1)] x LN(F2/F1) where “LN” is the natural log function on your calculator. Easy to do in steps of 1000ft during smokoe.

The coursework example was an A330 and I just recreated a spreadsheet on the laptop out of curiosity obtaining a ground distance of 145nm from 39000ft. So 145/39 is 3.7 (indeed, why not use 4?) and another good and useful rule of thumb is tested and passes muster.

Amazin what you do while the missis is shopping.

PS image URL didn't seem to work!
PPS oh yes it did!!

[18-Wheeler]

In the 747 you allow for three times your altitude, about one mile per ten knots of head/tail wind, any altitude restrictions on the STAR, the weight of the aeroplane, the runway in use ...
... then go down at 110 miles.

[Farmer 1]

ETA - (No. of thousands of feet x 2)

[popay]

enicalyth, its also interesting to consider aerodynamic qualities of your airplane for a very rear occasion like dual flame out., in other words what is my finesse? Maintaining the same descent speed profile .82/300 due to technical reasons windmilling etc.... It takes about 100 NM (15 min) to come down from FL 400 or a factor of 4 or finesse of 25 with all engines stopped.
Cheers.

[enicalyth]

DS Solution

The Directing Staff solution to double flame-out, Mike, is to fly into the nearest thunderstorm and maximise the updraught effect. That's what Old Smokey would do. I wouldn't for two very good reasons, a) I have two engines remaining b) I don't want to lose those two by ingesting sparrows/geese/condors flying on oxygen.

[Old Smokey]

Jeez enicalyth, you could have quoted it in context.

And I thought you were a cyber-friend

Regards,

Old Smokey

[7FF]

When the Green T/D on the Magenta line hits the pointy tringle.

[411A]

There are those that calculate (and calculate...calculate...), and then there are those that just close the taps at 100 nautical miles and point the nose down...and accelerate as necessary.

I do the latter, and it has worked for me in over thirty years in heavy jet transport aircraft.

It ain't rocket science, now is it?

[flying_elvis]

Is it possible to fly 85 hours a month for 30 years and still loose your pilot skills and a good chunk of airmanship?...absolutely.

Yeah...I see lots of guys do Barber pole descents into turbulent air. Cloud reading skills have long attrophied.


340 kts moderate turbulence and what do they do? They pull the Speedbrakes full aft! Ooooo nicely done. 10000 hours in the plane and that's the best solution they come up with?

Yeah....still in the envelope. "U" in judgement. Always better to plan your work and work your plan.

No offense...I've ceased being impressed by high time pilots long ago.

[411A]

<<No offense...I've ceased being impressed by high time pilots long ago.>>

No offense taken, elvis, but oddly enough, lets look at an average situation.
Now, I don't fly the new fangled stuff, only the the 'ole Lockheed tri-motor.
100 DME seems to work for me, say from FL350.
Close the taps, descend at 320-330 (barber pole 350+), and adjust ROD to arrive at 30 DME at 250KIAS, is generally a breeze, winds aloft considered, no spoilers needed.

I once flew with a First Officer who, upon reaching cruise altitude, started 'calculating' his descent...even if it was six hours off.
Twisting his Breitling bezel 'til it smoked, it seemed, and yet...he got it wrong every darn time.

I have ceased being impressed by co-pilots...generally speaking.

[popay]

Mike Jenvey, well what appears a brand new set of rule for you is incorporated in our QRH under ALL ENGINE OUT FUEL REMAINIG and NO FUEL. I wasn't talking in particular about the appropriate procedure, like APU start and anti ice and changing the profile after APU BLEED available. However the first step is to determine the landing strategy if engine relight is unsuccessful. I was purely referring to the fineness of the airplane. Agree that is highly improbable but its in the QRH.
Cheers.