Descent Profile

[Golf Alpha Whisky]

Can one of you guys flying the heavy metal please advise what a typical descent profile would be for a modern jet airliner from cruising altitude? Say you are cruising at 35000 ft mach 0.8 and are at top of descent ..... what would a typical vertical speed be when reducing altitude and do you reduce the air speed back or maintain air speed during the descent.

At what point do you start to slow the thing down?

Thanks.

[Capt Pit Bull]

Well, I only flew medium jets rather than 'heavy metal' but I can give you an outline.

Initially, a descent will be conducted at a fixed mach number. As the aircraft descends into gradually warmer air this equates to an increase in TAS so the aircraft needs to accelerate and thus the rate of descent will be relatively high.

This is all highly variable depending on type and load, but an typical ROD might be around 3000 fpm with a typical load.

At some point IAS becomes more limiting than Mach number and the profile will change as a result. Typically an automatic change over from holding Mach to holding IAS occurs. Now you are looking at a situation where a fixed IAS, since you are descending into denser air, must equate to a decrease in TAS, i.e. the aircraft is now decelerating. As a result, the ROD will decrease.

The differences between these two stages can be quite marked - perhaps 1,000 fpm difference.

Another factor that may significantly affect the descent rate will be Icing conditions. To supply enough bleed air for the ice protection systems it may be necessary for the flight idle rpm to be raised (either automatically or by flight crew input). Hence residual thrust is increased and Rate of Descend decreases as a result.

Once you get under ATC speed control, reduction in IAS will result in very modest descent rates - perhaps a few hundred feet per minute. As to when this will happen - depends hugely on the airspace.

Is that any use?

pb

[Olendirk]

Take the milega to the airport and add a little bit for approach, e.g. 10nm, so multiply by three then you should have the altitude you should be(ok, be aware to add some elevation).
The TD symbol is quit nice for descend, sometimes theres a SLP or an alt constraint in the fmc you nearly forgot. well nice fmc. thanks.
initiatially i take 2000ft a min and watch out for the GS x5.

:ok:

[airseb]

it depends on your type, but typically, for an A320 at standard load and standard weather (STD temp 1013hPa, ...)you'll get:

4° angle of descent at 340 kts/Mach 0.80
3° angle of descent at 250 kts/Mach 0.76

you have to factor in the wind, about 10% correction for 50kt component, and eng anti ice. finally most places and companies now require 250kts under FL100.

so for you're example, no wind, it's about 75 miles from FL370 down to FL100 (270/4 + 5 miles reduction from 340 to 250 kts)

then about 35 miles under FL100, not counting the deceleration.

typically, level flight will give you 5 miles from 250kts to 180 kts

seb

[Golf Alpha Whisky]

Thanks guys - I'm curious as to what you actually manage and what you "set" during the descent from cruise.

For example say you are cruising at 35000 ft mach 0.8 and your initial descent is to 20000 ft what actions would you take?

Do you set the Rate of Descent (say 3000 ft / min) and manage the target speed with engine power or set the target speed and manage the rate of descent to maintain this? .... or does the AP / FMC do it all for you?

[Capt Pit Bull]

If there are no airspace / ATC issues, the optimum descent is achieved with thrust lever closed. As I said earlier, the AP will be used to maintain a Mach number or IAS and the ROD will do its own thing as I described.

An FMC can be used to calucate all that lot, taking into account airspace constraitns and forecast winds, and the Top Of Descent commenced accordingly. Various types of managment can be used to account for variation in real life from the plan.

e.g. if the winds are not as forceast, something has to give. Either the aircraft will not descend on the planned profile (but can maintain the specified speed) or the speed is maintained (in which case the profile is lost).

pb

[Golf Alpha Whisky]

Thanks pb.

Is it the case then that the AP is commanding a pitch down attitude to achieve the desired airspeed and is not controlling engine power?

[tom775257]

It depends on the mode you are in.

On the airbus in Open Descent (OP DES), and I believe on the Boeing in Flight level change (FLCH), the autopilot will pitch to maintain the airspeed with thrust idle in the descent.

In other modes this will not happen.

Tom.

[411A]

Descending with a large heavy jet simply does not take the planning associated with an Apollo moon mission, yet I have watched First Officers over the years start deciding when/how to descend when they were at TOC.:ugh:

The usual 3xaltitude+10 works nearly every time.
So, at FL350 it would be 35x3+10....descent 115 miles from destination.
Headwind?
Eliminate the +10.
Tailwind?
Add another +10.

This worked very well in the heavy jets I've flown over the years...even the B707, long ago.

In the end of course, it's ATC that will do the 'deciding'.:E

[Old Smokey]

Boeing gave us a super whiz bang VNAV function on the FMC to provide optimum TOD and Descent profile, all at great customer expense.

Strange how it works out to be 3 X Ht + 20 to within a mile or two.......:}

Regards,

Old Smokey

[Castle Don]

For the MD11 the FMS will calculate a TOD point but it will usually end up putting the aircraft high and fast, unless we enter some data to fool the computer (eg. an extra 50KT tailwind.). So a lot of us tend to ignore the "magenta" and do it manually. We use the same old 3X rule that seems to work for most aircraft, then adjust it for HW/TW, ENG ANTI-ICE, and gross weight.

Assuming no ATC or arrival procedure restrictions, we plan to arrive at 10,000ft/250KIAS at 35NM from the runway. This we do as a flight idle descent from TOD and avoid use of spoilers if at all possible. Prior to TOD we program a descent speed into the FMS..generally 320KIAS/M0.82. This gives us some room to adjust speed on the way down if we start to deviate from the desired profile...eg: if the aircraft starts to get high of profile, increase to say 340 to get back on it...more speed/more drag/higher VS.

(As a technical aside...this line of thinking doesn't work when we are at low fuel loads. The MD11 max. IAS is significantly reduced once we burn below approx. 30,000lbs due to wing bending limitations. In that situation we can shallow the descent but really can't steepen it. Well...unless we use those spoilers. Which, incidentally, don't produce dramatic results anyway.)

As for slowing down...passing 12,000ft reduce VS to about -700ft/min and bleed the speed off gradually to arrive at the 250KIAS/10,000ft point. From there on in it's just a matter of judging when to start configuring.
(Actually, at high weights, 250KIAS is below our min clean maneuvering speed. Slats extend would get us down to 205 or so.)

[flite idol]

The black art of FMC VNAV! It works well if you thoroughly understand it and follow the basic rule of garbage in-garbage out but leads to more "whats it doing now`s" that I`ve had hot you know whats. It will only perform based on the information and constraints in the database and manually entered by the weakest link, me that is! Always back it up with situational awareness and mental maths.

[411A]

The L1011 was the first widebody civil jet transport to be equipped with an FMS, which provided complete RNAV/LNAV and thrust management modes of operation.
It was manufactured by Hamilton Sunstrand and it provides very accurate TOD information for a variety of descent speeds, M.86
350KIAS/250KIAS or M.82/320KIAS/250KIAS or M.80/300KIAS/250KIAS, or...indeed any combination in between, all pilot selectable.
Want to be 20NM from the airport at 230KIAS?
Simply choose that point as the EOD point, and sit back and watch PFM at work.
It is superbly accurate, even to this day.
And, why not....it's on a TriStar.

Ahhhh....Lockheed.:E

[TheGorrilla]

411A, for me that would almost work. However, I Find I have to do: 3 x (ALT/1000) + 20. The beastie is a little slippy below 260 kts or so. Failing that I use Vnav and fistfulls of speedbrake.

Another method I was taught is 3 x pilots age + inside leg measurement of the most junior hostie. One can get into serious trouble before the descent checks are finished though.

[Capn Bloggs]

Ahhhh....Lockheed.
Ahh, also McDonnell Douglas oops Boeing 717...
Tell it where you want to end up, with any ifs or buts in between (and the descent winds) and as long as the Altitude Selector has been wound down below you, the jet descends automatically at it's FMS descent point, prioritising the vertical path, using thrust to control the speed ie close to idle. Decel at 10,000 to 250 occurs automatically (or nay other target you put in to the FMS plan), as does final decel for landing. Works spot on (almost) every time. Makes me feel guilty taking home my pay.

If it can't stay on the path with idle, it'll tell you to "add drag" ie speedbrake.

If the wheels fall off, 16k@50track miles, 12k@40tm, 9k@30tm, 5k@20tm, 3k@10tm, using Vertical Speed (speed with thrust) or Level Change (speed with Pitch/idle power).

Nice stiff wings too so no wing bending problems! :ok:

[World of Tweed]

Don't know if this is true but during my training we were told the VNAV kit developed for the 757/767 was derived from the Space Shuttle Orbiter's descent nav. system. Interesting seeing as the Shuttle uses a M1.0+ 21Degrees slope.

Although I have to say on the basic 757 "it" invariably panics just after TOD and demands speedbrake. However if you take no heed it generally sorts itself out without having to resort to the "rumble Handle" after a few mintutes.

Very clever pieces of kit but because I'M the one who programs it I'll never fully trust the "Pink line".

[TheGorrilla]

Well, it's all a bit gash anyway. Even the best fmc model can only work to the forecast winds inserted. Assuming someone remembered to insert some! Anti-ice, inversions, turbulence speeds, weight differences, rough/smooth handling, bank angles, performance index of a particular aircraft, luck, cunning, skill, daring, finese, grit, determination, sheer rugged good looks (hostie defined) etc... can all effect ones profile.

[bubbers44]

Always do that 3Xaltitude plus 10 with wind corrections even when in VNAV and reference the airport not your miles remaining in FMC. Places like Bogota and Mexico City will clear you for this certain arrival then dump you on to the approach with you high and dry if you don't. I always put the airport in fix mode in my B757 so they can't get me by a last minute short cut. The Guayaquil approach includes a holding pattern which you won't ever do so leaves you high if you just VNAV the approach. It is always better to be down too soon wasting gas than way too high wondering if you can land.

[A37575]

Flew the 737 Classic for a German operator long time ago. The F/O was female with 500 hours who relied totally on FMC to descend. One crystal clear night over Europe the view was just magnificent and the track from 100 miles out was almost in line with duty runway. Well before top of descent I asked her (the F/O) would she like to try a profile descent of the sort described by 411A in his excellent post. Three times the height plus 10 DME.

She baulked and was quick to tell me this was non-standard procedure as the standard was to rely on the FMC. I admitted this was true but that on the other hand to be able to judge the profile using DME versus height was a useful skill. She had never heard of this technique.

To cut a long story short I promised her faithfully not to tell a soul in the company that she had actually hand flown a descent profile using basic skills.
So she hand flew the 3Xht+10 and the result was a beautifully flown approach straight in to Hamburg from 115 miles out culminating in a perfect flare and touch-down. After the pax had departed she turned to me with a lovely smile and thanked me for what she described as a taste of REAL flying - but then urged me never to tell a soul that she had flown a non-standard procedure...