Random acceleration when descending

Hello IFC,

I was flying overnight from KJFK to RJTT and was coming up to the approach on 16L at RJTT via STAR. My autopilot was set on 240kts as I was at 8000ft with VNAV armed to descend to 6000ft. However the speed kept on creeping up when descending until around 260kts, even at a very small vs speed (-1100 approximate), which caused me to get a level 1 violation. There was not much tail wind (8kts) either so I really don’t understand how this happened. Despite not having deployed flight spoilers, the acceleration still seems really odd as the speed was pretty standard for an approach.

Anyone got any ideas what could’ve happened here? (Just curious why this could’ve occurred, though I will have little chance of appealing the violation) Thanks for any response in advance.

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Hello!

Depending on aircraft and configuration, -1100fpm can be too much and cause acceleration. It’s unfortunately not that uncommon. Aircrafts are great gliders :)

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What aircraft were you flying?

If you were on the heavier side of things, you’d have a little trouble slowing down too.

The acceleration isn’t random. It’s just the aircraft wanting to do what it does best…fly!

Also, 240 KIAS is a little above standard. It’s close though. Airliners usually fly the last portion of the STARs at 220 knots or below.

Whenever I start my descent, depending on the aircraft, as a rule of thumb, I set my speed to M0.65 - M0.67after crossing my TOD, and keep it around there, until I go below FL180, then slow down further to 220. Works well most of the time and keeps me far away from overspeed violations.

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There’s a possible compounding effect being a bit fast on a STAR decent with VNAV. The faster your forward speed, the higher VNAV needs to make your vertical speed to make the next waypoint altitude. Such an increased decent gradient (angle of decent), increases gravity’s effect on airspeed. So the faster your starting speed into the STAR the more that speed risks diverging more quickly to violation speed than if you were a bit slower. (This is a bit speculative as I have yet to test it for myself)

You’re at M0.67 at 35000 feet?

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Regarding the gravity accelerating you bit, IF’s VNAV tries to maintain a 3 degree slope so it’s probably not true

1-2 seconds before I descend from 35,000, yes, I set my autopilot speed to M0.67.

So I’m not at M0.67 at FL350. Rather, I’m slowing down just before, and during my descent from FL350.

The idea is to bleed off your airspeed as you descend.

That’s why it’s so important to be at your device about 10NM before your top of descent when VNAV is armed.

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(edit: I believe the glide ratio below is correct, but my stated consequences of descending at a steeper angle than that are wrong. You get acceleration to an equilibrium speed unique to whatever decent angle you choose. The glide ratio angle gives you the best distance. But you may well exceed 250kts in a glide at or less than the best glide angle. But for an aircraft with superior glide ratio, the more you have to monitor speed and be ready to add drag. Intuitive after all…)

A 1,100fpm decent rate is 10.9 knots down.

For decent angle to be 3 degrees requires airspeed to be 204 knots (with 1,100fpm).

If airspeed is 250 knots the decent angle is 2.5 degrees.

But my thinking is still flawed. If you’re at a given point and altitude and you need to get to a distant lower point, there is one and only one decent angle between those two points. It’s that angle and not the speed you fly that angle that determines what percent of the aircraft’s weight acts to accelerate you down the “hill.” It’s this angle that works against you in trying to keep the speed down on approach. (Obviously a 45 degree angle would be a massive acceleration.)

Anyway I see airliner glide ratios seem to be around 3.5 degrees. So, excluding idle engine thrust, you get “downhill” acceleration beyond that kind of angle.

So any kind of steep decent would seem to require getting speed controlled very early (Aspen?).

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What aircraft?
And BTW if VNAV can’t maintain speed you need to put out spoiler or flaps.

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I echo this, especially the 757 - very slippery!!

Also, for the most direct indicator of how much gravity is affecting your forward speed on decent (other than the airspeed indicator itself), keep on eye on the velocity vector (small circle on the HUD), specifically, how far it is below the horizon. That has a direct relation to how much your speed will increase during idle decent.

My mistake. VNAV targets a 2 degree descent gradient, unless you descend late and then the limit is -3000 fpm. Sure, it is intuitive that a steeper descent will require less thrust/more drag to maintain the same airspeed.

But wouldn’t the decent angles vary for different standardized approaches? Waypoint altitudes in mountainous regions sometimes need to descend more rapidly to clear terrain?

The “3° descent angle” that you’re talking about makes a difference only on final approach. That’s when you sometimes have approaches (e.g.: ILS Z 16R at KVNY) which have more than a 3° glideslope, in mountainous regions.

For STARs (which is probably where the OP was when they had an overspeed on descent), you’ll usually have a range of altitudes to be within, at waypoints. Infinite Flight doesn’t exactly depict altitude “ranges,” so sometimes you need to be wise and pick your own altitude within that range. For example, if one waypoint has a 10,000 foot altitude denoted on the flight plan, and the next one has a 3,000 foot altitude, you might want to consult the charts and see if those are hard altitudes or altitude ranges (“at or above,” or “at or below,” or “between”).

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I’m not sure I understand exactly. Are you saying some of the STAR waypoint altitudes have been manually altered to give fixed number altitudes for waypoints that may have a range of values irl? And that some such transcribed transitions may be too steep?

That’s interesting. Where did you find the reference for 2 degrees or 3000fpm? (maybe I came across it previously and forgot)

Scroll down a bit, you’ll see it under the Pro Tips

i do believe this is an actual bug, having experienced it before.

i don’t know how to reproduce it, and haven’t encountered it in recent flights

most notable occurrence was during a flight from NZAA to KLAX

was approaching KLAX via the RYDRR2 STAR, descending through 9000ft over santa monica

A/P was full on (spd, LNAV, V/S, and ALT)

i had turned off VNAV after establishing a good rate of descent

aircraft was at 240 and slowing to 220 when the throttle suddenly jumped from idle to ~100% N1 (can’t remember the exact number) aircraft speed shot up to over 260, and i got the overspeed warning

fortunately for me, i was monitoring my device and i immediately disconnected the autopilot, extended flight spoilers, set throttle back to idle, and pitched up to 5 degrees

i did avoid a violation but i have to say i was getting ready to end my flight as a last resort

Click the Autopilot off and fly it…problem solved!

But in all seriousness, IF only gives speed violations after 10 seconds of continuous overspeeding, with an audible, and visual alarm (with a 10 knot leeway up to 260 indicated) it shouldn’t be to difficult to disengage VNAV and level off with speed brakes to bring your airspeed below 260

Yup. Not exactly altered, but, suppose a waypoint in the chart has this written next to it:

———
10000
5000
———

That means “between 10,000 and 5,000 feet.”

But in Infinite Flight’s flight plan it’ll probably show up as 10,000 (the upper limit) because flight plans don’t reflect ranges, only hard altitudes.