Can any RW pilots perhaps help me (a 'seasoned' Microsoft/X-Plane pilot with over 10,000 hours in the a319/320/321! ) with this?

I'm wondering about what really happens on descent through FL100 (and reduction to <250kts).. do pilots use DES mode, and if so, does the speed in the FMC show a decrease as the waypoints get closer to FL100, slowing the plane? In other words, maybe a waypoint in the FMC at FL180 will indicate a managed speed of 320kts. Will subsequent waypoints above FL100 all also show 320kts, or will the flight computer have 'graduated' them (i.e. @FL140, 280kts in the FMC; @ FL110, 260kts..). If 320kts is indicated all the way down to 10,100', the plane would (surely) have to level off pretty sharpish to lose all that airspeed in so short a space of time! (Not 'elegant' IMHO, but what is happening in the X-Plane simulator).

Do RW pilots descend through FL100 while in DES mode, in fact, or do they prefer OP DES, or even use the V/S dial, along with the SPD selector? (I tend to switch between all three, depending on the current need).

I hope my question makes sense at least! Many thanks for any feedback.

Martin

It's been about six years since I flew the 'bus but if I recall correctly, there is a deceleration point that changes the FMS speed target from the planned descent speed to the 250/10000' restriction. You may need to help it out with speed brakes. The danger of going to an open descent while on a vertical profile descent (managed descent) is that it cancels out any altitude and speed restrictions and will go to flight idle at the designated speed to the altitude in the altitude preselect window.

In real world applications, there is no right way to manage the descent. You do what is needed in that moment. In a perfect world, fully managed for as long as possible is preferred for efficiency and to meet any constraints but it's not always appropriate given the realities present in that moment of the flight.

ILUV767 wrote:

It's been about six years since I flew the 'bus but if I recall correctly, there is a deceleration point that changes the FMS speed target from the planned descent speed to the 250/10000' restriction. You may need to help it out with speed brakes. The danger of going to an open descent while on a vertical profile descent (managed descent) is that it cancels out any altitude and speed restrictions and will go to flight idle at the designated speed to the altitude in the altitude preselect window.

In real world applications, there is no right way to manage the descent. You do what is needed in that moment. In a perfect world, fully managed for as long as possible is preferred for efficiency and to meet any constraints but it's not always appropriate given the realities present in that moment of the flight.

Been a while since I've flown the 'bus as well, but that day is coming back soon.

100% on the first part, it hits a decel point, that in my experience is 1000% too late to actually slow down, especially if your CI had you at 320 in the descent. The airplane doesn't like to slow down, so if you have a decent tailwind, you're probably putting out some speed brakes, or floating along between 10-11k while the airplane gradually slows down. It's smart enough in managed speed not to go over 250 under 10k.
Open Descent was helpful if you just need to get lower and do it fairly quickly, at least it behaves like a reasonably well-thought-out airplane in that mode, and it doesn't pin the forward FA against the FD door and make everyone think they're about to die, unlike the first time you chop power in the CRJ and descend in SPD HOLD.

Profile descents always seemed to be fairly reasonable, but where and when I flew the thing, the procedures were not nearly as complex with OPD STARs into every airport, runway specific transitions, more aggressive descent profiles. Most of our flying was fix-fix-fix-cross fix at 10k-fix-fix-vectors. I have no idea what it's like to ride it down through window altitudes now, probably more parking your hand on the speed brake lever to stop it from crossing everything 50 knots too fast and 1000 feet too high.

Thank you for the replies! I guess, from what I have read, that I am doing things pretty much OK. Just had a great flight KJFK to CYHZ (Halifax) in a Toliss a321 (an amazing training-level reproduction of the aircraft, certified by Airbus I believe). My ATC programme usually calls for me to reduce speed if I am much over 260kts by about FL130, but I usually don't wait for them to tell me.. I hit the speed dial (in DES mode) and dial in a slower speed myself. Once under FL100 and at 250kts, I can press the speed dial (back to one dot) and return to fully managed mode: the Toliss aircraft (do you guys know of it?) behaves superbly (maybe even better than your real-world ones, from your comments, LOL!), and I Ieave the plane in fully managed mode thereafter, adding flaps as required, until I am on the G/S, when disengage the AP and hand fly down to the runway.

I noticed yet again though that on the FMC, the managed speed was set high - 296kts at 10500'. If I leave the a/c on fully managed mode, it pretty much levels off at FL100 to get down to under 250kts: that is what I was questioning really: I don't see any deceleration point in the sim model (like you mention), it keeps going until it almost reaches FL100 and then tried to slow to <250kts.

As I say, sounds like I am doing more or less what is needed (what kind of CI do you use BTW? I tend to use around 25??) - as I commented in my original question, I use a combination of DES, OP DES and (albeit rarely) V/S mode, with spoilers as needed. Amazing to me that anyone can programme a PC to create this kind of realism - just as amazing as the real-world planes in its own way! Still wish I could get my hands on a real a320 though!!

Thanks again for the replies.

Jeez, whats your cost index to be descending at 320? I sometimes have trouble staying on speed and profile with the boards out and 280 managed descend in the fenix 320

The managed speed that you mentioned seems to be rather on the high side. Usually the managed descend speed is around 270kts, unless flying at a very high CI. Normal cost indexes nowadays are quite low (depending on the variant and airline around 10-20, if the aircraft is in metric system, CI for aircraft in pound would be slightly higher), most of the time even below LRC. The behavior you mentioned is correct: the FMGC calculates an idle descend profile (if there are no constraints) with a deceleration point that assumes a deceleration at a reduced vertical speed (usually around 500fpm), so most of the time the deceleration point is quite close to FL100 at around FL105/110, depending on the speed.
In real world there are several techniques that depend on the ATC but also on the pilots preference. Personally, I almost never use managed descend (except in a busy airspace with a lot of constraints where it really helps) but initially (from Top of Descend) V/S - 2500 until the changeover from Mach to Speed (the reason is that the aircraft will reach quite high rates of descend, sometimes up to around 4000/4500fpm and a quite steep pitch down when descending in Mach, therefore I feel using V/S mode for the initial part of the descend is better for passenger comfort and also to avoid overspeeds in case of wind shifts - but that's a personal technique, OP DES/DES works as well but in my opinion not as smoothly), then OP DES. I try to keep the speed managed if possible, but I will select the speed as appropriate for the profile (getting high on profile at high level=>increase the speed). The reason is prefer OP DES is that most of our fleet is not equipped with DPO (Descend Profile Optimization), therefore the engines will not be at idle in a managed descend (as the profile is calculated kind of conservative). Therefore OP DES should theoretically save some fuel (if done correctly...which is not as straightforward as it sounds).

"It's complicated."

As mentioned above, there's a deceleration point automatically programmed in the flight plan at 10000ft. It shows as an amber ball on the green track line on the ND.

Speed will be managed or selected. If it is managed, the deceleration point will be respected. If selected, speed will just stay where you plonked the speed target.

The plane doesn't level off abruptly to slow down. It will slow the descent to reach 250kn by 10000 feet. By 11000 feet or so, depending on your current descent rate, you'll see it level off slightly.

We will be in DES or OPEN DES going through 10000 feet, and rarely V/S. It really depends on what we need to do. If we're in a hurry down it will be OPEN DES, possibly with speedbrake. That would happen if we get big track shortening, for instance. As soon as you get a big shortcut, the best thing is to pull for OPEN DES and see what that gets you, obviously being mindful of descent constraints.

DES is the "safe" option since it respects the altitude constraints. If in doubt, use DES. It can save you from embarassing yourself. But many places will cut you in on a base depending on how low you are. If you're still too high for it, you'll just get more track miles. OPEN DES can help you get down faster, and thus get cut in faster.


If appropriate, you can always ask for high speed below ten thousand if you need to get down faster. Many places will give it to you. The higher your speed in OPEN DES, the higher your descent rate.

Once on the approach, V/S is often used to finess the glideslope intercept. DES likes to "dive and drive", meaning it will get you down to the lowest current constraints (or the altitude target), then fly level. That's safe, but not very elegant. Finessing with V/S is more elegant, and more efficient, though tbh it's not like you're saving tonnes of fuel.

Thank you very much for taking the time to write detailed replies. I need to read them again when I am 'in' the a321 (the most sophisticated, sim-wise, of the Airbuses I fly - a319/a320/a321/a330 & a350!). I think, as I say, by using DES, OP DES, or V/S as I feel the situation warrants, I manage things more or less logically and effectively. The 'complication', as I did point out, is that my ATC programme will call me to reduce speed, probably to 260kts, at around 13000', so I have to intervene to comply with that - usually I switch from DES to OP DES and dial in the required SPD, adding some speedbrake if needed.

(My CI by the way I set between 15 and 50: seems like I should keep it under 20 or so, though I am not sure how accurately the CI is modelled: yes, 320kts is too fast for descent - if that is what shows in the FMC I make some changes; I am comfortable descending at 290kts, max., which I reduce further below around 20,000').

Thanks again!

You don't need to go from DES to OP DES to slow down. Just go to selected speed. OP DES will descend you at idle, so use that to increase descent rate. The higher the speed, the higher the descent rate. And there's always the speedbrake.

In the real world, the one thing you always have to watch for with OP DES is that it won't respect altitude constraints. So we tend to try to use DES if we can, and not just leave it in OP DES or V/S. Distractions are a thing.

Martinlest wrote:

Thank you very much for taking the time to write detailed replies. I need to read them again when I am 'in' the a321 (the most sophisticated, sim-wise, of the Airbuses I fly - a319/a320/a321/a330 & a350!). I think, as I say, by using DES, OP DES, or V/S as I feel the situation warrants, I manage things more or less logically and effectively. The 'complication', as I did point out, is that my ATC programme will call me to reduce speed, probably to 260kts, at around 13000', so I have to intervene to comply with that - usually I switch from DES to OP DES and dial in the required SPD, adding some speedbrake if needed.

(My CI by the way I set between 15 and 50: seems like I should keep it under 20 or so, though I am not sure how accurately the CI is modelled: yes, 320kts is too fast for descent - if that is what shows in the FMC I make some changes; I am comfortable descending at 290kts, max., which I reduce further below around 20,000').

Thanks again!

Normally there is three types of descent speed, first is the transition speed on descent, next a terminal speed such as 250 below 10,000, the other a deceleration for approach. ATC may say "transition speed 260 kts" on the descent page you would program the descent something like 0.78/260, next is the terminal speed, this is done on 6R and is normally automatically programmed as 250/10000, if ATC says reduce to 250 at 12,000 you can simply put in 250/12000 on the page after you press 6R. The next is the decel point on the approach. the magenta D in a circle. This changes the phase from DES to APP, and the target managed speed clean becomes green dot. As you select flap the managed speed reduces to F/S/Vapp.

Open descent will basically ignore the vertical path programmed and use elevator to fly the managed or selected speed (at idle thrust), DES will prioritize the computed vertical path, the managed speed will be a window.

"You don't need to go from DES to OP DES to slow down". Yes, sorry, my mistake, not sure why I put that. I don't: I usually stay in DES and select the speed!
"So we tend to try to use DES if we can, and not just leave it in OP DES or V/S. Distractions are a thing." Same in X-Plane - though the consequences of forgetting you've switched out of DES are less dire!

Thanks Zeke. As I say, I think I have a reasonable feel for whether DES or OP DES (or even VS) is more appropriate at any given moment.. I do try to descend in DES mode as far as possible, to match the altitude restraints. (I realise though as I write this that I am not at all sure in fact when or why a pilot would choose OP DES instead of DES + selected speed?).

One problem in X-Plane that you won't have in a RW Airbus (I assume!) is that sometimes tweaking these speeds leads to an excessive descent rate - maybe -5000ft/min , and then the plane accelerates like mad. That's a programming issue no doubt (of course, at the end of the day, I have to remember that these X-Plane aircraft, amazing as some of them are, are just simulations and not 100% guaranteed to 'behave' exactly as a real world plane would 100% of the time!). I usually correct this 'plunge from the skies' with a short spell using V/S to bring the descent and speed back to a reasonable rate.. Good job I only have 'virtual' passengers (though in X-Plane, there is some really blood-curdling screaming comes up if you do anything really terrible!).

I am never quite sure whether to reset speed restraints 'on the fly' via the FMC LSKs, or by using the SPD dial (when in DES mode).

Thank you guys.

Martinlest, at my operator, in the Training Department we teach that a smooth, well planned descent begins with an accurate Top of Descent point and maintaining good situational awareness throughout the descent and approach. It helps make the entire descent process very straightforward.

If you have a STAR or other descent procedure in the flight plan, the TOD is generally set based upon the constraints embedded in the procedure. If you are flying to a destination without an arrival procedure, we recommend crews insert an "artificial" waypoint between the last enroute fix and the destination airport at a distance 30 n.m. from the airport (or final approach fix, as appropriate) and insert constraints 250 KIAS and 10,000 feet (U.S.) at that point. This helps them maintain situational awareness and prevents them from getting behind on the descent. If ATC has not issued descent clearance at the TOD point in either of these two cases, we query ATC and request descent.

Our company policy is to use managed procedures to the maximum extent possible, so for the descent, we generally use DES profile and insert .78/300 in the Descent Perf page. Using CI is acceptable, but in lieu of specified speeds on an arrival procedure, we teach that 300 KIAS is generally acceptable to ATC for maintaining traffic separation, and also provides a comfortable profile.

The point at which the aircraft will begin to decelerate is noted as a magenta dot on the ND. The aircraft does not level off for speed reduction; rather, the rate of descent is reduced so as to achieve 250 KIAS at 10,000 feet.

Below 10,000 feet, almost every pilot I have observed will go ahead and pull for Open Descent (provided no additional altitude constraints) and select 250 KIAS. This allows them to go ahead and get down to the assigned altitude without delay and also puts them in position to reduce to green dot/S/F/Vapp as necessary. Most do not routinely use V/S, although of course, as noted above, it is an option, as well as the use of speedbrakes if needed.

The important points are planning the descent, monitoring the profile, and maintaining SA.

With regard to your comment:

Martinlest wrote:

Still wish I could get my hands on a real a320 though!!

Why not just go ahead and apply to an airline that operates a fleet of Airbus aircraft? I think you would enjoy it.

e38

That's great, many thanks. It is good to see that I already do many of the things you recommend - I often add an 'extra', dummy waypoint or two into my plans, partly for the reasons you say, and partly because my ATC programme uses waypoints to instruct me to descend. If there is a long gap between waypoints in my plan, I add a few (usually not more than 2). It helps. (Yes, I most often fly ILS/RNAV with SID/STAR, as well as transitions when appropriate. These high-end XP aircraft mostly handle them very well).

I just flew a route you RW pilots will doubtless never do - the (in)famous Kathmandu to Paro, Bhutan, and back. Spectacular. All went really well, with a nice safe touchdown after a long period of hand flying around the peaks. With satellite imagery showing the real terrain all the way, and using a VR headset too, it doesn't get much better, in a sim at least! I do tend to use DES though below FL100, not OP DES, for as long as possible. Sometimes it works brilliantly all the way to gear down... The key, of course!, is to monitor closely what is happening at all times.

As I said earlier, I will have to read all the comments folks have kindly posted at a time I am in the sim and an Airbus (I do fly Boeings too, BTW! As well as Embraer and a host of others - one of the advantages of a simulator over real world: variety!). Like many who fly some of the training level aircraft in X-Plane 11 (and now, XP12), I try to keep up with what is realistic procedure real world. One never knows when that moment will come when someone goofed: both pilots ate the fish, and the cabin crew are going around quietly asking the passengers if any of them knows how to fly the plane!! My 15 minutes of fame ("Passenger Flies A321 into Hillside"!! )

Thank you all.

Martinlest wrote:

. I do try to descend in DES mode as far as possible, to match the altitude restraints. (I realise though as I write this that I am not at all sure in fact when or why a pilot would choose OP DES instead of DES + selected speed?).
...

I am never quite sure whether to reset speed restraints 'on the fly' via the FMC LSKs, or by using the SPD dial (when in DES mode).

You'd pull for OPEN DES if you're in a hurry. As soon as you hear "expedite", or if you think it's getting a bit "tight", OPEN DES is your friend. You'll be at idle so you're descending as fast as you can. You can then finess it with speed and configuration. If you're in DES the plane will try to keep flying the programmed profile, which might not be appropriate, for example if you expect to be cut in (track shortening).

Another reason to have OPEN DES is when DES is not available, e.g. radar vectors.


Whether to use the MCDU or not is dependent on the situation. Do you really want to go "heads down" at low altitude in busy airspace? If you're at 20000 feet and it's not too busy, go for it. If you're at 6000 feet, better to stay heads up and select instead of fiddling with the MCDU.

Martinlest wrote:

That's great, many thanks. It is good to see that I already do many of the things you recommend - I often add an 'extra', dummy waypoint or two into my plans, partly for the reasons you say, and partly because my ATC programme uses waypoints to instruct me to descend. If there is a long gap between waypoints in my plan, I add a few.

You should not need dummy waypoints. The aircraft will fly the profile either way. However, there is sometimes a tendency to "dive and drive" since a descent rate of 500fpm or more must be maintained. Having a level segment can be useful for deceleration, or you can choose to use V/S.

If the aircraft is not doing what you want in DES mode, it is the pilot's job to use a selected mode (OPEN DES or V/S) to control the descent profile.

Just descend at 290, level off at 10000 and hit the brakes (he’ll fly right by).

Martinlest wrote:

I am never quite sure whether to reset speed restraints 'on the fly' via the FMC LSKs, or by using the SPD dial (when in DES mode).

We basically never adjust speed constraints via MCDU during descent. It is usually faster and you have more precise control when you do it through FCU speed selection. One exception is the SPD LIM point which is 250/10000 by default, in case ATC wants us to go fast we might change it to 250/5000 or similar to get proper profile calculations if we continue 250+ knots below 10000 feet.

For altitude constraints it is different. In case ATC gives as altitude constraints for some waypoint we insert it to flight plan usually. This helps with monitoring the descent and many times we are able to fly the with DES mode.

We usually are trying to do a continuous descent approach which means that we don't level off during the transition from descent to final approach. This is often done with V/S selection during the last 1000-3000 ft before capturing final descent (ILS GP for example).

In addition to make things more complicated in our fleet there are planes in three different configurations regarding profile computations. We have the original version with quite a long deceleration level-off and we have the descent profile optimization (DPO) version with reduced speed reduction and thrust margins. Then we also have some aircraft with H3 FMS modification that doesn't have level segment at all and also has some modifications to DES mode functions and the way aircraft wants to follow profile during managed descent.

But as others have also said, there is no absolute right way to do things and handle profile. Every approach is different and requires different actions from the pilot. Safety is the highest priority, so the most important thing is to obey constraints in the procedure or given by the ATC (and of course limitations of the aircraft). Then comes the efficiency and passenger comfort. Efficiency might mean a lot of things, sometimes its flying idle from T/D until final approach and minimizing the fuel burn, sometimes its minimizing the flying time and so on. And as a pilot I feel that I've done a good job when I have managed to accomplish all these things in one approach (safety, efficiency and comfort).

You'd pull for OPEN DES if you're in a hurry.

Yes, that makes sense of course. I get worried though when, having gone over to OP DES from DES, the XP a/c can get to -5000ft/min, or even more. Then the speed starts climbing of course - adding drag is no help then. How does that compare to real-world, is it a 'fault' in the sim, or can engaging OP DES cause such a high descent rate in the real plane; if so, what do you do to reduce it? Go to V/S??

Do you really want to go "heads down" at low altitude in busy airspace?

We basically never adjust speed constraints via MCDU during descent. It is usually faster and you have more precise control when you do it through FCU speed selection. One exception is the SPD LIM point which is 250/10000 by default, in case ATC wants us to go fast we might change it to 250/5000 or similar to get proper profile calculations if we continue 250+ knots below 10000 feet.

Absolutely: that is just as true in the sim, and the main reason I tend not to adjust parameters via the FMC, unless it is at cruise altitude, say, and there's not much happening.

As for dummy waypoints, maybe they are more helpful in the sim than real-world, but they certainly make the ATC instructions more timely (and I usually do get enough level segments to make any necessary adjustments to my airspeed). But that's just a PC programming thing of course.

Just descend at 290, level off at 10000 and hit the brakes (he’ll fly right by).

I am sure I've done that before.. I'll give it another try LOL. The Toliss A319/321 at least should do more or less what the real plane would do. (My a320 is great but not quite at that level of realism).

But as others have also said, there is no absolute right way to do things and handle profile. Every approach is different and requires different actions from the pilot.

Exactly, and as I said above, I am fairly happy that I do that in my simulated flights.

Great input again, thank you once more. My original question really was about FMC speed restraints being so high just a few hundred feet above FL100/250kts, but more stuff has come up. Most interesting for someone like me who has a passion for flying these planes but is restricted to a home simulator! Luckily, some of the best payware mimics the real aircraft very closely.

(I remember, back in the days when such things happened, British Airways recruitment staff visiting my school, asking boys (it was an all boys school) if anyone was interested in training with BA to become a pilot. I put my name down, but ended up sticking to 'Plan A' (university) in the end! So I guess I could have been flying these aircraft for real had I made different choices. No regrets though!).

Martinlest wrote:

You'd pull for OPEN DES if you're in a hurry.

Yes, that makes sense of course. I get worried though when, having gone over to OP DES from DES, the XP a/c can get to -5000ft/min, or even more. Then the speed starts climbing of course - adding drag is no help then. How does that compare to real-world, is it a 'fault' in the sim, or can engaging OP DES cause such a high descent rate in the real plane; if so, what do you do to reduce it? Go to V/S??

Do you really want to go "heads down" at low altitude in busy airspace?

We basically never adjust speed constraints via MCDU during descent. It is usually faster and you have more precise control when you do it through FCU speed selection. One exception is the SPD LIM point which is 250/10000 by default, in case ATC wants us to go fast we might change it to 250/5000 or similar to get proper profile calculations if we continue 250+ knots below 10000 feet.

Absolutely: that is just as true in the sim, and the main reason I tend not to adjust parameters via the FMC, unless it is at cruise altitude, say, and there's not much happening.

As for dummy waypoints, maybe they are more helpful in the sim than real-world, but they certainly make the ATC instructions more timely (and I usually do get enough level segments to make any necessary adjustments to my airspeed). But that's just a PC programming thing of course.

Just descend at 290, level off at 10000 and hit the brakes (he’ll fly right by).

I am sure I've done that before.. I'll give it another try LOL. The Toliss A319/321 at least should do more or less what the real plane would do. (My a320 is great but not quite at that level of realism).

But as others have also said, there is no absolute right way to do things and handle profile. Every approach is different and requires different actions from the pilot.

Exactly, and as I said above, I am fairly happy that I do that in my simulated flights.

Great input again, thank you once more. My original question really was about FMC speed restraints being so high just a few hundred feet above FL100/250kts, but more stuff has come up. Most interesting for someone like me who has a passion for flying these planes but is restricted to a home simulator! Luckily, some of the best payware mimics the real aircraft very closely.

(I remember, back in the days when such things happened, British Airways recruitment staff visiting my school, asking boys (it was an all boys school) if anyone was interested in training with BA to become a pilot. I put my name down, but ended up sticking to 'Plan A' (university) in the end! So I guess I could have been flying these aircraft for real had I made different choices. No regrets though!).

Why does your speed start climbing in OP DES? The elevator should be adjusting to maintain the target speed. You do get transient speed increases but OP DES is a "speed on elevator" mode so it will simply adjust. The aircraft maintains a speed with idle thrust and you get whatever descent rate results depending on configuration and weight. Seems like a simulator fault.

You do get high descent rates in operations. If you're really in a hurry to get down, OP DES, high speed, and full speedbrake will get you a descent rate in excess of 6000 fpm. The obvious application is emergency descent with no suspected damage, but I've seen in excess of 5000fpm in normal operations, for example, if you get a massive shortcut on descent.

If you want to reduce the descent rate in OP DES, you retract the speed brake and/or decrease speed. Or change mode to DES or V/S. Situation dependent.

Regarding MCDU use on descent in the real world, the PM can go heads down while the PF remains heads up, so it is simpler than in your case where you are simming alone. Again though, we probably would not touch the MCDU more than necessary below MSA and/or around 5000 feet.

Of note is that if you want to set a constraint, you don't have to first line-select the waypoint with the R keys in the F-PLN page to open the page for that specific waypoint. You can save yourself some time and keystrokes by entering XXX (for speed) or /XXX (for altitude), then pressing the R key corresponding to the applicable waypoint. This will limit the time heads down.

Another time-saving trick is to use R6 on the F-PLN page if you need to quickly change the QNH, instead of going to the PERF page and clicking your way to APPR PERF.

Why does your speed start climbing in OP DES? The elevator should be adjusting to maintain the target speed.

No, I think that is my error. I must have meant DES, not OP DES. Sorry. If (probably due to ATC instructions) I am a bit 'behind' in my descent, the a/c will try to catch up in DES mode, and that is when it can reach more than -5000ft/min. At that rate it is hard to control the speed - full spoilers don't always help, not enough at least. As I say, in those situations, I do go to V/S mode for a while, until things are more under control.

Martinlest wrote:

Why does your speed start climbing in OP DES? The elevator should be adjusting to maintain the target speed.

No, I think that is my error. I must have meant DES, not OP DES. Sorry. If (probably due to ATC instructions) I am a bit 'behind' in my descent, the a/c will try to catch up in DES mode, and that is when it can reach more than -5000ft/min. At that rate it is hard to control the speed - full spoilers don't always help, not enough at least. As I say, in those situations, I do go to V/S mode for a while, until things are more under control.

If you're "behind" in your descent, OP DES and high speed are your friends.

In DES the speed target is allowed to fluctuate within a range (the two magenta lines on the speed tape). This means the aircraft can adapt the descent speed based on conditions, e.g. actual wind different from predicted. If it "maxes out" the speed and hits the top of the range, you may need speedbrake to stay on profile.

You are doing the right thing intervening. If the aircraft isn't doing what you want it to do, take action!

Starlionblue wrote:

Martinlest wrote:

. I do try to descend in DES mode as far as possible, to match the altitude restraints. (I realise though as I write this that I am not at all sure in fact when or why a pilot would choose OP DES instead of DES + selected speed?).
...

I am never quite sure whether to reset speed restraints 'on the fly' via the FMC LSKs, or by using the SPD dial (when in DES mode).

You'd pull for OPEN DES if you're in a hurry.

if you are really in a hurry. Select V/S first to initiate the descent, the. Go OP Des to manage it. Otherwise you are waiting for the engines to get to idle before you start to descend.

EssentialBusDC wrote:

Starlionblue wrote:

Martinlest wrote:

. I do try to descend in DES mode as far as possible, to match the altitude restraints. (I realise though as I write this that I am not at all sure in fact when or why a pilot would choose OP DES instead of DES + selected speed?).
...

I am never quite sure whether to reset speed restraints 'on the fly' via the FMC LSKs, or by using the SPD dial (when in DES mode).

You'd pull for OPEN DES if you're in a hurry.

if you are really in a hurry. Select V/S first to initiate the descent, the. Go OP Des to manage it. Otherwise you are waiting for the engines to get to idle before you start to descend.

Would that really gain you any time? The engines need to spool down either way, though perhaps not as much with V/S.

I don't actually know. Just wondering.

It bears mention that the emergency descent procedure, which emphasizes getting down fast, does not involve V/S. I suppose that could be for simplicity, though.

Starlionblue wrote:

EssentialBusDC wrote:

Starlionblue wrote:

You'd pull for OPEN DES if you're in a hurry.

if you are really in a hurry. Select V/S first to initiate the descent, the. Go OP Des to manage it. Otherwise you are waiting for the engines to get to idle before you start to descend.

Would that really gain you any time? The engines need to spool down either way, though perhaps not as much with V/S.

I don't actually know. Just wondering.

It bears mention that the emergency descent procedure, which emphasizes getting down fast, does not involve V/S. I suppose that could be for simplicity, though.

In the grand scheme of things it doesn't gain much, but it is enough to notice. V/S is instantaneous, as soon as you select a negative rate, the aircraft will start down. It will disregard the selected or managed speed, while the thrust adjusts, to maintain the V/S. There can be a lag with Open Des before it actually reaches an idle thrust descent.

Thank you for continuing to post comments. Yes, I do use OP DES and V/S modes on descent, in addition to DES.. I probably do what you do (I like to think!) and switch modes when one isn't really doing what I need at any given moment. If I use V/S mode, I try to match my altitude and speeds to what the flight computers are expecting at that (way)point (i.e. what shows in the FMC): if you fall behind, you can find yourself high at approach time of course (happens to me more often than I'd like, when DES mode seems to lag behind rather and I haven't intervened for some reason!)..

Currently moved away from Airbus for a week or two - concentrating on the MD-11 cargo (!) (superb rendition from 'Rotate'. Not sure if any real-world pilots here can't get enough of their 'daytime' jobs and fly X-Plane too! ).

Martin

Martinlest wrote:

Thank you for continuing to post comments. Yes, I do use OP DES and V/S modes on descent, in addition to DES.. I probably do what you do (I like to think!) and switch modes when one isn't really doing what I need at any given moment. If I use V/S mode, I try to match my altitude and speeds to what the flight computers are expecting at that (way)point (i.e. what shows in the FMC): if you fall behind, you can find yourself high at approach time of course (happens to me more often than I'd like, when DES mode seems to lag behind rather and I haven't intervened for some reason!)..

Currently moved away from Airbus for a week or two - concentrating on the MD-11 cargo (!) (superb rendition from 'Rotate'. Not sure if any real-world pilots here can't get enough of their 'daytime' jobs and fly X-Plane too! ).

Martin

As an alternative to V/S, try switching to TRK/FPA and setting a descent angle which puts the hockey stick on the waypoint.

We don't use that often but it's perfectly doable. One advantage is that unlike V/S, FPA compensates for wind. Just be careful that your vertical speed doesn't get too low, or ATC may become annoyed.

Yes, I don't think I have ever used that function, in fact. It always makes me think, for some reason, of the real-world accident caused by a pilot setting a descent rate of 3300 ft/min, instead of 3.3 degs down (Air Accident Investigation programme!). I will try it out next time I fly an Airbus (still immersed in the MD11 at the moment. What a plane!!)

Martinlest wrote:

Yes, I don't think I have ever used that function, in fact. It always makes me think, for some reason, of the real-world accident caused by a pilot setting a descent rate of 3300 ft/min, instead of 3.3 degs down (Air Accident Investigation programme!). I will try it out next time I fly an Airbus (still immersed in the MD11 at the moment. What a plane!!)

The display symbology was changed after that accident to be more clear.

Also, ALWAYS set based on what you see on the FMA/PFD, not the FCU/MCP/ACP. The numbers on the glareshield are just what you set. The numbers and modes on the FMA/PFD are what the plane is actually doing.