Question about Boeing's intent with DES NOW

[Will]

Pressing DES NOW within 50 nm of the TOD starts the aircraft down at 1250 fpm until the idle descent path is captured, at which point the engines go to idle and the rate of descent increases.

Question: Why doesn't DES NOW just calculate a new TOD at the present position, and descend at a constant non-idle rate?

I'm trying to think like Boeing. Maybe the advantage of the dog-leg (initial segment at 1250 fpm, final segment at idle) is fuel efficiency? You start down now, which is what you want when you press DES NOW, but presumably you use less fuel if you can fly a sizable portion of the descent at idle. So the aircraft starts down in accordance with pilot wishes, but then gets to save fuel later on by throttling back.

If not for that, then it would seem more intuitive that DES NOW activates a non-idle, constant-rate descent to the next FMC constraint. Is there any sort of other operational advantage conferred by the dog-leg?

[Hardy Heinlin]

I think it's more economic to stay near the optimum cruise altitude as long as possible. Imagine the worst case and you were to use a constant 500 fpm descent profile. In the later phase you would be flying at low altitudes for a very long time. So the steeper profile is probably the better one.

The NG FMC sets the descent thrust slightly out of idle. They found out that if it's not too much out of idle, there'll be no significant increase in fuel burn. In other words, if you go too much out of idle, it will be less economic.


|-|ardy

[Jeroen Hoppenbrouwers]

My guess is that no pilot ever wants to hit DES NOW anyway. You only hit that button when you are told by ATC to descend early, but are not given a specific altitude crossing constraint. 1250 fpm seems to be a number that ATC likes -- not 500 fpm.

I wonder how often this function is actually useful. Most ATC will want you down before a given point because of traffic.

Concerning the NG near-idle descent: the main reason of course is to have a bit of correction headroom in case the true idle descent does not cut it because of slightly more tail wind than expected/programmed. Apparently this extra correction headroom is worth the overall slightly increased fuel burn.

All these functions need to be seen in the context of a whole fleet burning fuel, not for any individual aircraft.

[Will]

But sometimes people want to lose altitude early... let's say you are flying a profile where you anticipate that energy management will be tricky, so you want to make sure you're actually down where you need to be for a crossing restriction. Or perhaps you anticipate flying through a level of decreasing headwinds, and you want to lose altitude early so the FMC doesn't have to deal with a larger error. Or maybe you just want your time to altitude calculation to be easy to calculate.

For these reasons, I would design the FMC with two options: ECON DES, which is the current DES NOW, and something new called PATH DES or something like that. The FMC already knows the fpm for a constant angle descent, so pressing >PATH DES would just create a new TOD at the present position and start a constant angle descent to the next altitude constraint.

But that's fantasy, of course.

[Jeroen Hoppenbrouwers]

Well we could turn PSX into a flight test airframe for new FMC functions  :-)

[Hardy Heinlin]

... let's say you are flying a profile where you anticipate that energy management will be tricky, so you want to make sure you're actually down where you need to be for a crossing restriction. Or perhaps you anticipate flying through a level of decreasing headwinds, and you want to lose altitude early ...

There are already options to manage this.

As we know, the FMC's initial DES profile calculator always plots backwards from a target waypoint which has the first altitude contraint. So if you want to have the TOD a bit earlier, just insert another along-route waypoint before that target waypoint, and set the first altitude constraint at that new waypoint. Now you have a level-off segment between the new and the old target waypoints that you can use as a headroom in case of unexpected tailwinds. When you're approaching that level-off segment and you see there are no increasing tailwinds, delete the first altitude constraint so that the second, original constraint becomes active: The FMC will recalculate the remaining profile and possibly switch to SPD | VNAV PTH with significant out-of-idle thrust for a direct, shallow path. Or it will go to THR > HOLD | VNAV SPD for a smaller descent rate to intercept that new, shifted idle path from below. (The VNAV SPD non-idle intercept descent rate will be 50 % of the VNAV PTH target idle descent rate; e.g. 800 fpm intercept for a predicted 1600 fpm target path. At high altitudes the idle descent rate is typically 2500 fpm; that's where the "1250" number comes from.)

Or enter some worst-case winds on the DES FORECAST page that are stronger than the official forecast winds. The TOD will shift automatically.

I think these options might be better than an additional option for a pilot defined path angle. They keep the system consistent.


|-|ardy

[Bluestar]

Or you could do what I did back in another lifetime, use the E6b.  🤣

Bode

[IefCooreman]

AFAIK the DES NOW forces a VNAV descent, disregarding the distance from TOD. ALT intervention is either a VNAV descent or a cruise descent, depending on the distance from TOD. Sometimes you don't want to have a cruise descent, and you use DES NOW.

The descent profile is indeed a fuel economy issue. However, when you discuss the use of the functionality, it really depends on pilot preference but the answer is usually related to shortcuts..

A situation where I almost always use DES NOW:
* nighttime small airports. TOD is a long way if you come in "opposite" to the runway in use. At night we can negotiate a straight-in with a little tailwind when we are alone in the skies. We will ask for an early descent ourselves to anticipate as the final answer might only arrive when we're in contact with approach.
* long snake-type transitions without altitude restrictions. Shortcuts can be expected and they are usually pretty huge in track miles.

When you discuss VNAV cruise vs VNAV path descents, a lessor known detail is the speed schedule. In cruise, a little increase in speed does not increase total fuel consumption to destination as much but it does change time to destination (also visible in the idea behind LRC vs MR). However fuel efficiency depends a lot on altitude, which is why descend speeds will be lower to allow the aircraft to stay as long as possible at higher altitudes (flying close to minimum drag is more important). This is very clear if you would descent to ie FL200-FL240 in VNAV, and then reselect a cruise altitude. The descent speed is the one the FMC uses to lose altitude as efficiently possible. The cruise speed is the speed to get to destination as efficiently possible. So if you use early descents in cruise, from certain altitudes you might end up too high for the descent profile, but also too fast. And any pilot knows that if you're too high, you need to be able to trade altitude for speed and create extra drag through speed. If you're still fast, you don't have that option anymore in case of shortcuts.