Can an autopilot slow the rate of descent?

[Nicholas49]

Hello

Hope it's OK to ask this question here.

If you are descending at 3,000 ft per minute (i.e. a 'quick' descent) in vertical speed mode, as you approach the altitude selected on the autopilot, can a commercial aircraft (e.g. B737 or A320) slow the rate of descent automatically to ensure a smooth level off and prevent it busting through the altitude in the (extremely unlikely, I know) event that the pilots failed to reduce the vertical speed manually? Obviously this applies to rate of climb too.

Hope that makes sense!

Thanks,
Nick

[Lord Spandex Masher]

Nicholas, yes they can and do.

It's called various things but it usually occurs at about 15-20% of the descent, or climb, rate. For example, at 3000fpm the altitude 'flare' will begin at about 600 feet before the selected altitude to achieve the desired smooth level off. This will happen regardless of using VS, FLCH or FPA.

So we don't need to reduce our rate of descent as the AP will cope as it was designed to do. However, to reduce your vertical speed within a couple of thousand feet is desirable as it helps to prevent TCAS pinging somebody.

[SNS3Guppy]

I prefer to reduce vertical speed manually to 1,000 fpm within a thousand feet of the target altitude.

[Northbeach]

It depends on whether or not the autopilot has been able to successfully enter the “capture mode” (my name as I am not sure of the technical software label associated with this function). If you are in rapid vertical speed manually flown descent and only 3 meters/10 feet above the altitude you were assigned and at that small distance/time above your desired altitude engage the autopilot. Under those circumstances the autopilot will not successfully capture the altitude.

On the other hand if you are in the same vertical descent and engage the autopilot with enough time/altitude for the automation to function within its design limitations above or below the selected altitude then the automation will capture and hold the altitude. In capturing the altitude the autopilot will vary the rate of descent/ascent.

The exact time and/or distance above/below the target altitude where the automation will or will not capture the assigned altitude will vary with the software version, mechanical interface between the autopilot and flight controls, age and condition of all the related equipment, weight of the aircraft and energy involved, rate of descent and time between autopilot selection sequence complete and time/distance to target altitude.

[Denti]

As mentioned above autopilots can and do it, no need to manually reduce the rate of descent/climb. However it is a recommended practice to reduce the vertical rate during the last 1000ft prior to level off. Using high rates however the automatic mode (ALT ACQ for altitude acquire in the 737) can activate pretty early. Our SOP therefore is to manually override the AP with a control column input if altitude acquire activates too early and holds too high a rate during the last 1000ft. The 737 AP will enter CWS pitch mode in that case and re-enter ALT ACQ if possible.

[Nicholas49]

Thanks for the replies guys.

Can I ask a secondary question? It links to what Northbeach said about 'capturing' the altitude and what Lord Spandex Masher said about the 15-20% margin. Again, I recognise fully this is a purely theoretical scenario which is not, hopefully, going to materialise on a real flight.

Imagine you are at 30,000 ft and you change your target altitude to 29,000 ft. However, you select a 3,000 ft per minute ROD. Now, in this situation, when you hit "GO", would the autopilot say "no, I don't think you want to do that", and apply automatically a slower rate of descent?

In other words, does the autopilot function that slows the rate for a smooth level-off also take into account the magnitude of the altitude change, adjusting the ROD accordingly?

I appreciate the answer may differ for different aircraft / flight modes.

PS - Denti, on re-reading your post, I think you are saying that for the 737, the answer is "not necessarily"?

[kharmael]

The autopilot wouldn't necessarily crank on 3000fpm worth of descent immediately, it's more of a gradual thing and the altitude capture would kick in before you even attain that rate of descent.

I can't speak for other types but if you use the pitch sync button to select your rate of descent it's quite feasible to whack the nose down (this would never happen on an Airline!) and get a stonking rate of descent which the AP might not be able to cope with in 1000ft. :8

[BOAC]

N49 - the answer is no, the A/P that I am familiar with (737) does not cater for the actual altitude change but if it were only 1000' as kharmael says there would not be time to generate a high rate. The A/P would, however, probably 'overdo' the actual effort in pitch and power needed, hence the 'human' intervenion.

Denti is NOT saying that at all. He is saying that we just don't do it that way. Both passenger comfort and attention to other traffic dictate a different style (hopefully!)

We had the same problem in BA on the 737 at one time - the 'glitterati' insisted that VNAV should always be used for altitude changes and after several (highly predictable) TCAS 'events' on departure high rate climbs to 1000' below incoming traffic we managed to get them to see sense.

The 'problem' with V/S selections is that some a/c do not have any speed protection in that mode, and there have been some extreme speed events because of inattention.

I have no idea what a 'pitch sync button' button is!!

[kharmael]

Pitch Sync button on the yoke allows you to set the attitude you want the aircraft to adopt on climbing or descending. So you can select a 3 degree descent by holding the sync button on the yoke, pushing the nose down until you're 3 degrees down and releasing the button. The aircraft will then fly at 3 degrees down until it captures the pre set altitude. Similarly you can pitch the nose down until you capture a rate of descent or indeed a speed. Naturally this also works in the opposite direction!

[BOAC]

Why not tell us which aircraft you are talking about?

[SNS3Guppy]

Nicholas,

Different autopilot systems accomplish the functions about which you're asking, differrently.

In the Classic 747, we have several methods of performing a descent or climb, depending on which options are installed on the autopilot panel.

We must first select an altitude in the altitude select window then have an altitude switch which gives us altitude hold, off, and altitude select functions. If we place the switch in the altitude hold position, the autopilot will hold whatever altitude exists at the time we move the switch. If we leave the switch off, we can manually direct the airplane up or down using a "pitch wheel." If we move the switch to Altitude Select, the airplane will capture the selected altitude using whatever method we choose.

We must then choose a climb or descent mode. We can select from airspeed, vertical rate, PMS, or Mach modes. In airspeed or mach mode, the airplane will pitch using the autopilot to maintain a selected speed. It will hold that speed until we're close to the selected altitude, then begin to level off. The autopilot uses 10% of the vertical rate to decide when to begin leveling off: a 2,000 fpm descent rate will lead the level-off by about 200' above or below the selected altitude.

If we select Vertical Speed mode, then we use another wheel on the autopilot panel to set the desired rate of descent, and the airplane will hold that rate until it approaches the selected altitude. While the airplane is supposed to lead level-off (altitude capture) by 10% of the vertical descent rate, we recognize that the autopilots are getting older, and we treat them accordingly. Within a thousand feet of our target altitude, we reduce the vertical speed to a thousand feet per minute, making leveling off much easier on the autopilot. At high rates of descent, the autopilot may not level in time, and it shouldn't be expected to capture at descent/ascent rates of 5,000 fpm or greater.

We also use a PMS (performance management system) mode in some airplanes, which lets a performance management computer determine the best speed and descent for the airplane. PMS can manage aircraft control and autothrottles. All we need to do is put in certain information and tell it what altitude it is to which we want to climb or descend, then let the computer calculate the best way to do it. There are various options. In that scenario, we leave the altitude select switch off, and the performance management system does the rest. It will determine when to level off and does a good job. It isn't used for reduced vertical separation minimums airspace (mostly higher altitude airspace) because it varies altitude slightly up and down to hold a constant airspeed and simplify it's job, so we shut it off and use altitude hold in cruise.

As you can see, there are various techniques or methods for performing climbs or descents, and for leveling off from them, with this one autopilot system. Change airplanes and systems and options on the system, and the answers will be different. Additionally, if we need to slow down during the descent, such as when passing through 10,000' (we slow to 250 knots), we need to lead the altitude a little more. Typically I'll begin reducing the vertical descent rate around 12,000' and have the power to idle, as I'm shooting for a speed reduction from 300 knots to 250 knots by 10,000'. I usually do this by pitching up to slow, if I'm hand flying, or by using a reduced vertical speed and idle power. If it's timed properly, I should be arrive at 250 knots as I arrive at 10,000', and then I'll continue down from there using the airspeed hold feature, most of the time. If air traffic control wants a different speed, then we'll do that, instead.

[Nicholas49]

SNS3Guppy - thank you for that.

Thanks BOAC too for the explanation. You now have me wondering whether 'glitterati' is a euphemism for 'chief pilots'. ;)

As it is relevant to this thread, there is also a somewhat less technical question about rates of descent that's bugged me on my last few commercial flights.

When the aircraft commences its descent, often following the "update on arrival and thanks for flying with us today" PA from the flight deck, I've felt a 'sinking' feeling, a little like a tame roller coaster. It is noticeable but it is not uncomfortable. Is it the result of:

a) the ROD being increased by the pilots or the autopilot;
b) the descent being initiated;
c) either of these?

I have sometimes felt it more than once during the descent, so I am guessing it occurs each time you leave a new altitude? Out of interest, do you guys feel it up front? And do you know what ROD creates that sensation on your aircraft type?

Thank you again for taking the time to reply.
Nick

[Northbeach]

Imagine you are at 30,000 ft and you change your target altitude to 29,000 ft. However, you select a 3,000 ft per minute ROD. Now, in this situation, when you hit "GO", would the autopilot say "no, I don't think you want to do that", and apply automatically a slower rate of descent?


The short answer is "yes" you are correct.

I fly a Boeing 737. In this equipment with the autopilot engaged at 30,000* then selecting 29,000 and beginning a 3,000 ft per minute ROD the automation will accept what you imputed. However almost immediately the autopilot will come out of the 3,000 ft per minute ROD to whatever rate it calculates to "capture" the altitude. So in non-technical speak the autopilot "says" (figurative language only) the pilot wants me to descend a thousand feet. And the pilot wants me (the autopilot) to descend at 3,000' per minute. However, there is no constraint on how long I must maintain the 3,000' rate of descent. So I (the autopilot) can begin the descent at 3,000' per minute to comply with the second variable (the rate of descent) once complied with (momentary only-perhaps less than a second) the autopilot is no longer bound by the 3,000' per minute ROD. In fact the airplane will probably never actually achieve the 3,000’ ROD.

A note of caution; different airplanes have very different flight automation and logic. For example the last jet I flew; if during the automation's capture mode the pilot touched any of the variables (vertical speed or indicated airspeed) then the automation, by design, would ignore the previously assigned altitude and fly right through that altitude at whatever the last input assigned.

A few terms you may or may not be familiar with. When speaking about altitude changes the "armed" mode is when the pilot selects a different altitude and the autopilot is aware of the new assignment and is set up to acquire that altitude. On my Flight Mode Annunciator (FMA) the "new" altitude is displayed in white colored font.

When the autopilot has changed the flight path to acquire the newly assigned mode it has entered the "capture" mode. When the altitude is captured then the color of the displayed altitude changes to green and the numerical representation of that altitude changes to words.

*When speaking about altitudes there is separation in terminology at 18,000' above sea level. At and above 18,000' those altitudes are referred to as "Flight Levels". For example 30,000' above sea leves is called "Flight Level three zero zero". We do not consider the 100' increments; that is why the last two digits are "dropped".

Again 30,000' above sea level would be "Flight Level three zero zero". 30,000' avove sea level is never referred to as "Flight Level three zero zero zero zero" or "Flight level three hundred"; both of those are incorrect.

“Flight level” is abbreviated by the letters FL. Therefore FL300 would be 30,000' above sea level; FL290 would be 29,000' above sea level and so on.

Altitudes below 18,000' would be referred to in thousands; 17,000’ above sea level would be written as 170 and spoken as "one seven thousand".

Remember these like most things have a "textbook perfect" answer; however different regions use different textbooks. And then there is practical application that will vary greatly from strict adherence to local slang (sometimes completely unintelligible).

And a final word; in aviation there is often several different ways to accomplish the same thing. Please do take anything I, or anybody else says on such matters, as “the answer”. For example in this discussion I have completely omitted the VNAV (Vertical navigation (path) descents. I spend the vast amount of my time flying in VNAV; I don’t actually manually stipulate rates of descent. Cranking in a 3,000’ ROD to change 1,000’ would give the passengers a lousy ride, and the flight attendants would not put up with it.

Respectfully,

Northbeach

[safetypee]

In many autopilots the transition from vertical speed mode (VS controlled by elevator) is triggered by a combination of VS and the altitude remaining to the target (altitude error), e.g. a simple rule like alt error + vertical speed = constant.

At the point of transition the autopilot changes mode to altitude capture and the elevator controls altitude (autothrust if used, changes to speed hold). The flight path is flared, reducing the VS, by demanding elevator control in proportion to the altitude error.

In many systems this is a submode of altitude hold mode. Attempting to change VS during the transition would be ineffective, or alternatively changing the selected altitude could cause the auto system to recompute the flare path with variable results, or revert to a basic pitch mode – attitude hold, thus it is important to look at the mode indicator before changing parameters.

Re descent:
The situation as described is commencing a descent. However there are alternative methods and modes of operation for achieving this. Autopilot modes (control laws) are usually compromises to balance the desired aircraft performance / positioning and a comfortable ride for passengers. An experienced crew will learn which mode / method to use for each situation; this can be modified by weather conditions, ATC constraints, and other operational variables.

[kharmael]

Sorry BOAC. That is on King Air B200s and C130-J! :8

[Denti]

@Northbeach re your explanation of flightlevels vs altitude, remember that nicholas gives his location as london, in europe we use much lower transition levels than in the us, so 18000 would be a FL over here, usually the switch is done somewhere between 4000 and 6000, but depends on local regulation, sometimes even per airport and not only per country.

[Nicholas49]

Northbeach - thanks for the detailed explanation.

As I wrote 30,000 ft, I knew I would be corrected! May I respectfully suggest that to encourage correct use of terminology, you say in your PAs "Ladies and gentlemen, we are currently cruising at Flight Level Three Eight Zero"? ;) Seriously though, I do understand that 'confusing terminology' is not encouraged, so I suppose us passengers who understand the difference will just have to sit there and smile quietly to ourselves!

Denti, that's interesting as I hadn't realised the cross-over between thousand feet and Flight Level is not standardised throughout the world. Surely that gives rise to the potential for some confusion?

[Denti]

Nicholas, it is just one of many things to be aware of. For the way down it is the ATCOs responsibility, on the way up we have to remember it ourselves, thus it is part of the pre-departure briefing. Additionally it is coded into the FMC and the aircraft reminds us if we forget (actually, it reminds us both ways).

[SNS3Guppy]

I'm not sure what you mean by "on the way down it's the ATCO's responsibility." Climb planning and descent planning is always a flight crew responsibility, subject to ATC or route limitations.

In most places, while ATC will at some point (usually) direct a descent), the crew should plan for, and request their own descent. Specific level-offs or descents, or prescribed arrival procedures will dictate changes at the air traffic controllers discretion, but initial descent planning should be done by the crew.

[flydive1]

Guppy, I believe Denti was talking about changing from QNH to QNE and back

[A37575]

I've felt a 'sinking' feeling, a little like a tame roller coaster. It is noticeable but it is not uncomfortable.

Probably the captain chancing his arm at hand flying and consequently ham-fisted on the controls. Many airline pilots are so reliant on the automatic pilot to do everything, they forget it is just another aeroplane with standard controls like elevators and rudders and things which can be flown perfectly smoothly by an all-around competent pilot. Since the advent of sophisticated automatic pilots a long time ago, there are not too many around, nowadays. (competent pilots, that is - not autopilots):ok:

[Nicholas49]

Thanks to all, again, for the very helpful answers.

Nick