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Boeing aircraft decent technique?


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36 replies to this topic

#1 captainklm

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Posted 01 March 2013 - 07:56 AM

Hello all,
I have another tech-question for you all, particularly the real pilots ;) After watching a few of those "Just Planes" dvds, I`ve noticed that when the aircraft reaches its top of decent, it doesn`t use what I normally do, which is put the fmc altitude into the altitude window, and when the aircraft hits the T/D the VNAV does the decent.
In the dvd, after atc commands them to, they decent (wether its passed the T/D or not) using the vertical speed function. Is that commonly done? Is the VNAV decent at the T/D used?
As far as I know, there are three ways to initiate a decent, a) the VNAV, b )Vertical Speed bug and c) (Not sure if this is really used) the "Decent now) function in the fmc

So to sum it all up, after seeing various different methods, which is the most commonly used procedure for a decent from cruising level to any assigned altitude?

Thanks everyone!
Joe Barton
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#2 Fabo

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Posted 01 March 2013 - 08:09 AM

Whatever the pilot feels like. (apparently you will not wait until TOD hits when asked to descend)

BRGDS Peter

 

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#3 Los Pilotos

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Posted 01 March 2013 - 08:13 AM

I think the answer to your question is.. "it depends..." Most of the time I'll use VNAV but if only a shallow rate of descend is required I may use V/S or if taken off the programmed route FLCH may be more appropriate. So to sum up.. "it depends.."

René Pedersen

 

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#4 scandinavian13

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Posted 01 March 2013 - 09:01 AM

b )Vertical Speed bug

Used often by pilots to smooth the transition from level to the descent. It helps to avoid a more aggressive nose over, which the passengers may notice (the weird feeling in your stomach).

c) the "Decent now" function in the fmc

If it wasn't used it wouldn't be there. The function is there to initiate a ~1000FPM descent until re-capturing the calculated VNAV profile. Useful for the times ATC says "descend and maintain" some altitude before your T/D. Be careful though: "descend and maintain 15,000" and "cross DRUZZ at and maintain 15,000," may look similar, but the former says 'do it now,' and the latter says 'descend at your discretion as long as you meet the restriction.'

One reason a controller would use "descend and maintain" (while you're still in cruise) is if there's traffic that will conflict with your normal descent path, at a lower altitude. The controller will then instruct you to begin your descent a little earlier so that you're below that aircraft a little earlier than normal. Once you're below that altitude, the controller will most likely issue a crossing restriction (like the one above), which allows you to continue at your discretion (or, he or she may just give you further descent at pilot's discretion).

Kyle Rodgers

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#5 aceridgey

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Posted 01 March 2013 - 09:37 AM

It can also depend on which country you are flying in. In Europe there seems to be a attitude of cost saving where Air traffic states "[Callsign] When ready, descend to FLXXX. Pilots, certainly in the UK, are encouraged to descend on the aircraft profile as it gives an accurate T/D point

Alex Ridge [Operations - An Airline, UK]
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#6 Tabs

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Posted 02 March 2013 - 10:37 PM

The V/S "slow onset" method of starting the descent is very common in the real world. If you do it right the passengers will barely even be able to tell the descent has started. The VNAV onset tends to be a lot more noticeable.

Ryan Maziarz

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#7 Joduv

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Posted 03 March 2013 - 01:01 PM

Also if you start you descent miles after passing the TOD, for sure you'll be too high on the descent profile. An other technique is too stay on command VNAV and re-select a new higher speed into the MCP SPD window... VNAV is goind to pitch the aircraft down in order to reach the selected speed, and you'll regain you descent profile...

Same when ATC wants you too increase you rate of climb due to traffic separation or whatever, they could ask you to reduce your speed --> VNAV will get the aircraft pitch up and your ROC FPM will increase.

Joshua
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#8 777simmer

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Posted 05 March 2013 - 04:56 AM

I like this subject, so allow me to give a loooong answere :-)

There is no "normal" or "not normal" way to start a descend.
There is only optimal and less than optimal for the instructions given by ATC.
(and then there are company standard operating procedures that differ for each airline)

The whole idea of a VANV PATH descend is that the FMC supports the pilot by calculating a top of descend point that results in an idle (throttle) descend.
This in order to save as much fuel as possible and basically gliding to your destination.

The position of the T/D point depends on things like descend target speed (which is influenced by cost index), winds during the descend, etc)
At high cost index (descend speed for example 310kt) your T/D at FL330 could be something like 120nm from destination.
At low CI (descend speed for example 250kt) you T/D at FL330 could be something like 140nm from destination, this saving more fuel with a longer idle descend.

Now, since we dont live in a perfect world, the most optimal descend path can not allways be followed.
For instance intermediate descend restrictions can be higher than you would want to be at those points. You then have to intervene, diverting from optimal, or you will end up too high or too fast on final approach.
Or sometimes ATC just does not give you your descend at your calculated T/D.
Gone is the ideal descend path!

optimal would be if in the 250kt example:
You request descend from ATC at around 160nm.
ATC clears you for the descend (for example to FL200)
You set FL200 in the MCP
You press the Altitude selector
The aircraft now starts a shallow (early) descend (very comfortable for passengers) with the throttles as required (somewhere between 50% - 70%)
When the programmed VNAV PATH is intercepted the throttles reduce to idle and the aircraft follows VNAV PATH untill FL200.
In an ideal world ATC would clear you to succesively lower altitudes before you reach you cleared level.

If ATC instructs you to descend to FL200 but with a rate of 1500ft per minute, then VNAV is not the optimal mode to do that.
The V/S mode is!
So you set FL200, press the V/S switch and set 1500 fpm.
What about the FMC though?
It is still in cruise mode!
This is where the FMC descend now feature comes in.
With that you tell the FMC to change from cruise to descend mode (which in turn gives you the VNAV PATH deviation indicator on your ND so you can see how much too high or low you are compared to your planned, optimal path)

Then there are times ATC tell you to descend at a certain fixed indicated airspeed.
VNAV PATH will follow the planned path but it will not fly a fixed speed!
(VANV SPEED mode will, VNAV PATH will not)
In VNAV PATH the AP will vary pitch and thus speed to maintain the path.
FLCH is more appropriate for such a dscend.
If you initiate the dscend with FLCH you again need the FMC descend now feature to tell the FMC that the descend phase has started.

If ATC clears you after you have passed your T/D you can use VNAV to intercept you path but this is quite uncomfortable as the the aircraft will nose over untill it reaches 330kt or so to try to capture the programmed path.
FLCH with current speed is more comfortable for the pax in this case.
The you slowly increase the speed to increase descend rate.
You can arm VNAV so that once you are on the programmed path (and speed) again, it will automatically capture it and continue in VNAV PATH.

Sometimes you are being vectored around other aircraft, away from your planned route.
Again VNAV PATH is not ideal here, because if intermediate altitude restrictions are programmed in the FMC it will adhere to those restrictions.
FLCH or V/S are better in this case.
You dont want to level off at an intermediate altitude of like 12000ft (as could be indicated on the STAR and programmed in the FMC) when being radar vectored and cleared to descend to 10000ft!

Finally, not as important to simmers as in real life, the B777 cabin altitude controller and FMC work hand in hand to increase passenger comfort.
Cabin rate of descend is programmed by the controller depending on the programmed FMC descend path. Even when leveling off at programmed altitude restrictions, the cabin altitude will continue to decrease because the controller has taken this in account in its calculations!
In FLCH and V/S the cabin altitude controller will revert to an other (less optimal) mode.
So in that respect (and for fuel economy) one could argue that VNAV is the preferred mode for a descend.

Rob Robson

#9 cva1077

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Posted 05 March 2013 - 05:01 AM

If you do it right the passengers will barely even be able to tell the descent has started.


By this I assume you mean non Flight Simmers! Enignes spool down, here we go!

                                                               Best Regards, Ryan Cooper

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#10 captainklm

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Posted 05 March 2013 - 08:46 AM

Thanks everyone for all your detailed answers! I also had ATC, so I think the V/S control is more ideal ;)
Joe Barton
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#11 scandinavian13

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Posted 05 March 2013 - 09:27 AM

I also had ATC, so I think the V/S control is more ideal ;)


You're welcome, but why is that more ideal for ATC? From an ATC standpoint, I don't care what mode you're in. More specifically, my expectation is for you to follow the descent profile and cross at the restrictions I give. Even more specifically, if every other 738 pilot out there is using one method and you're using another, things can get awkward (then again, differences company policy for descent speeds can already mix things up).

If I say "cross HOGGS at FL180," I don't care how you do it, as long as you hit HOGGS at FL180.

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#12 777simmer

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Posted 05 March 2013 - 10:05 AM

If I say "cross HOGGS at FL180," I don't care how you do it, as long as you hit HOGGS at FL180.

Exactly!

And further more, it makes absolutely no sence to use V/S to start the descent per default.
Usually this method is prefered only by those who do not completely understand how VNAV works.
The B777 is designed to use Vnav for both descend and final approach.
V\S is a thing of the past (unless required as described above or if you enjoy flying like that, which is perfectly ok of course)
Rob Robson

#13 RWFeldman

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Posted 05 March 2013 - 10:46 AM

If I say "cross HOGGS at FL180," I don't care how you do it, as long as you hit HOGGS at FL180.

Exactly!

And further more, it makes absolutely no sence to use V/S to start the descent per default.
Usually this method is prefered only by those who do not completely understand how VNAV works.
The B777 is designed to use Vnav for both descend and final approach.
V\S is a thing of the past (unless required as described above or if you enjoy flying like that, which is perfectly ok of course)


Simply well put!

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#14 cva1077

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Posted 06 March 2013 - 03:16 AM

V\S is a thing of the past


Not really, in one of the ACA 777 vids(can't remember which) they used v/s(500fpm) for the step climbs because VNAV and FLCH are too aggressive for short maneuvers like an s/c.

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#15 777simmer

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Posted 06 March 2013 - 05:30 AM

We were talking about a normal descend and how to initiate that.
For inititiation of a normal descend from cruise level, (for example a clearance to descend from cruise FL380 to FL250) without any ATC restrictions, VNAV is the preferred mode.

Like I said, there are circumstances where other modes work better than VNAV.

Your example is another one of those exceptions where Vnav is not the smoothest method.
For short climbs at low altitude (where the aircraft has a lot of power available), for example climb from 6000ft to 7000ft, VNAV would climb with full CLIMB THRUST causing something like a 2500ft/min climb rate. This can set of a nuisance TCAS alert if another aircraft is above it in 8000ft.
The prefered mode for this kind of a climb would still not be V/S though in my opinion (although now it really becomes a matter of flavour:-) ) because if they would have used FLCH they would have had the same result with less buttons to push.
FLCH will try to reach the set altitude in one minute.
So, with 1000ft to climb, it will set thrust to maintain a 500ft/min climb.
With 2000ft to climb, it will set thrust to maintain 1000ft/min.
When the altitude to climb becomes much more than that, it will reach maximum CLIMB THRUST and climb with whatever v/s the engines can produce (which would be the same as a VNAV climb again).

Rob Robson