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Descent AP mode: FLC vs VS

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Descent AP mode: FLC vs VS

Old 12th Oct 2019, 08:08
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Descent AP mode: FLC vs VS

The aircraft I'm about to fly is a slow aircraft, with a VMO of 250KIAS/.63M, there is no auto-throttle, and it will be in single pilot operation

In some busy TMA's (such as Paris) the ATC often expects you to fly at 250kt (or even more) during your descent ;
The operator's Operating Manual says that climb should be in FLC mode (seems safe to me) and descent in VS mode.
I think that a descent in FLC mode would unable the pilot to keep a speed of say 240kt, without the risk of busting the VMO, and hence accommodate ATC, during a high workload part of the flight.
In VS mode, you must constantly adjust thrust to keep close to 240kt.
In FLC mode the descent rate would be tuned with the Thrust Lever. A poor monitoring would have little consequence, it would just require a higher Thrust variation to get back to the desired descent path. In case of very high workload, you would set the thrust on the low side, and then reach you target altitude a little to soon, without any safety consequence.
What do you think?
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Old 12th Oct 2019, 08:42
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Tp or jet?
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Old 12th Oct 2019, 09:07
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FLCH gives speed protection so would seem safer. Does the manual really say V/S for descent? Approach, certainly.
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Old 12th Oct 2019, 09:37
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Light aircraft tend to produce quite strong fluctuations of vertical speed when operated in FLCH in descent. Climb is always okay, but beware in descent. I used to fly Citation XLS and we practically never used FLCH in descent due to this - it could be really uncomfortable. Rather use VS and watch out for your IAS. And if your plane is limited to 250 KIAS then rather give it some margin and fly max 240 KIAS - that will avoid headaches. If ATC requests 250 KIAS, just tell them that your unable and that your max speed is 240.

My current aircraft does not even have a FLCH-mode for descent. For climb there is a CLB-mode that is nothing else but FLCH, but can only be used to climb. For descent we have to use PATH or VS and it does work very well.
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Old 12th Oct 2019, 10:10
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Originally Posted by EatMyShorts! View Post
Light aircraft tend to produce quite strong fluctuations of vertical speed when operated in FLCH in descent. Climb is always okay, but beware in descent. I used to fly Citation XLS and we practically never used FLCH in descent due to this - it could be really uncomfortable. .
On "my" aircraft (C680) its exactly the other way round, unless you donŽt yank the throttles around like mad, FLC in the descent is very gentle, yet in climb its almost unusable. (and there the throttles are usually just in the MCT detent) Go through a temperature change and youŽll have someone going sick (in the climb), where as VS is rock stable (I know the drawbacks of VS in the climb)

Maybe ours is just the oddball, but then I have NEVER flown anything with a IAS or FLC mode that really worked that well. (Never flew an autothrottle aircraft)
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Old 12th Oct 2019, 10:16
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Originally Posted by 172510 View Post
In FLC mode the descent rate would be tuned with the Thrust Lever. A poor monitoring would have little consequence, it would just require a higher Thrust variation to get back to the desired descent path. In case of very high workload, you would set the thrust on the low side, and then reach you target altitude a little to soon, without any safety consequence.
What do you think?
I think you got that right. BUT: in these airspaces you usually also have altitude contraints to meet, agreed busting them is usually of less consequence then blasting through Vne, but still it comes down to:

stay alert and prepared. SP ops is sometimes really high workload (I still fly a KingAir B200 single hand, with Vne/Mne being not far from yours), so the key here is to make sure to have the setups etc done ASAP, free as much capacity as early as possible.
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Old 12th Oct 2019, 11:29
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If it's a CE500, 550 or 560 incl XL and XLS - VS all the way down.
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Old 12th Oct 2019, 12:25
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Originally Posted by His dudeness View Post
On "my" aircraft (C680) its exactly the other way round, unless you donŽt yank the throttles around like mad, FLC in the descent is very gentle, yet in climb its almost unusable. (and there the throttles are usually just in the MCT detent) Go through a temperature change and youŽll have someone going sick (in the climb), where as VS is rock stable (I know the drawbacks of VS in the climb)

Maybe ours is just the oddball, but then I have NEVER flown anything with a IAS or FLC mode that really worked that well. (Never flew an autothrottle aircraft)
Possibly I do not remember my times on the XLS well enough, I changed fleet to the Falcon in 2007... I definitely remember that we would hardly use FLCH for descent, but rather during climb, maybe not always. When you only have to climb a few thousand feet it will be VS anyway and not CLB/FLCH to avoid large rates of climb and upsetting other crews.
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Old 12th Oct 2019, 15:01
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Or just say unable and give them a speed you fee comfortable within the limits / mode.

Does the AFM require or suggest VS descent?
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Old 12th Oct 2019, 17:13
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Originally Posted by Capt Pit Bull View Post
Tp or jet?
Jet C510 Cessna Mustang
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Old 12th Oct 2019, 17:15
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Originally Posted by Oh gaim View Post
Does the AFM require or suggest VS descent?
The Operator's Operating Manual does, not the AFM

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Old 12th Oct 2019, 18:54
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Then why not ask the DFO for his reasoning - it doesn’t hurt to ask?
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Old 13th Oct 2019, 08:21
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Originally Posted by Oh gaim View Post
Then why not ask the DFO for his reasoning - it doesn’t hurt to ask?
What if you donŽt have one ? (Hint: single pilot on a jet in EASA land means private ops) Why not ask here ? (IŽm am a DFO, well was, now IŽm a NPFO in EASA speak...)
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Old 13th Oct 2019, 10:31
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Originally Posted by 172510 View Post
The Operator's Operating Manual does, not the AFM
Fait enough - but this read to me like an OMA or NCC equivalent which would then be the operator of the aircraft. Perhaps this is the nomenclature for the manufacturers POH?
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Old 13th Oct 2019, 17:53
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172510

The 250kts is a maximum speed. You should not fly what you think air traffic control expect you to fly. You fly what you are comfortable with. 250kts will be very uncomfortable for your pax in a small jet and will massively increase your workload, try flying at 220kts as it will be much more comfortable and this is the expected Eurocontrol downwind speed then 180kts on base leg and 160kts when on an intercept heading. Many small jets will fly through the localiser at anything more than 160kts. Make sure you tell ATC what speed you are going to fly and let them manage the separation. You are flying the aircraft, not ATC. There are many published articles about things going pea shaped when pilots let ATC take control of the aircraft.

MM
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Old 13th Oct 2019, 19:13
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Many DFOŽs are hired for other purposes than making sense of flight operational manuals and has in many cases spend their recent years behind a desk doing less operational things - and some have forgotten what it means to be out in the real world on a daily basis. Probably the operators OM-B is just a piece of junk - written to satisfiy the authories... Say it isnŽt so..

The OP, 172510, is clearly on the ball here with his question and challenging the operators (NCC I assume) manuals, with good reason. In a single pilot jet operation, flying into eg. Le Bourget etc. and you have the choice of using a speed (NOT V/S) control mode (such as FLCH) - that would be a wise choice from a flight safety perspective - any time the workload is high. With experience on type - heŽll find the thrust setting values and master this somewhat comfortably. Smart guy.
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Old 14th Oct 2019, 19:36
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In practice the Mustang works well in VNAV to hit all those STAR levels into Paris, VS works just as well.
Aim for 240kts max and the power changes required are easily managed SP.
If the A/p is engaged you cant easily overspeed it as the Garmin will start to pitch up as you approach VMO.
160kts max on an ILS intercept or it will go through, and back, and through, and.........
Great fun to fly, but low on power at higher levels and if you are used to something heavier very sensitive to power changes on approach, not helped by the delay in spool up on those little engines...
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