Two quick questions about two flights

[Hokulea]

A couple of things on two recent flights occurred that I've never experienced before, and hoped someone might be kind enough to offer an explanation. This is simply to satisfy my curiosity.

1) Flying LAX to DFW on Monday, about 45 minutes from landing the aircraft suddenly slowed by about 20 knots. I thought we were actually starting an early descent, but checked the flight map on the IFE and saw we had slowed but not descending. About a minute or two later we sped up again. What might be the reason for this - sequencing?

2) Next day flew DFW to HNL. About two hours out we climbed to 39,500 feet (confirmed by the flight map and FlightAware). I can't recall ever being on a flight that flew with that increment of 500 feet above a "normal" flight level. What might be the reason for this?

My thanks in advance.

[DaveReidUK]

The IFE flight map usually shows speed over the ground. That will vary with the wind.

[Hokulea]

This one actually showed airspeed (as well as ground speed and "true air speed"), but the big clue we had slowed was the reduced noise from the engines and the sudden feeling of deacceleration followed a couple of minutes later by the reverse happening. I don't think it was wind.

ETA: I hope this link works:

https://flightaware.com/live/flight/AAL2284/history/20180403/0030Z/KLAX/KDFW/tracklog

At 10:31:25 pm EST you can see the speed drop and then recover shortly afterwards.

[DaveReidUK]

the big clue we had slowed was the reduced noise from the engines and the sudden feeling of deacceleration followed a couple of minutes later by the reverse happening

If you felt a deceleration (i.e. the effect of inertia), all that tells you is that your speed over the ground was reducing.

Your body has no way of telling whether or not that's due to an increased headwind component.

[Hokulea]

Then why did the engines sound like they had gone to idle? PS. I should also add that the map on the AA 321s also gives you the head or tail wind - that did not change.

[PAXboy]

The only folks who knew are the folks at the front end. I suspect that you will never know what they were testing and verifying at that point. Happily, all ended the way it was planned.

[Johnny F@rt Pants]

First I will start with a rhetorical question -

If you were puzzled, why didn’t you ask on disembarkation? Most pilots are a friendly bunch, and are quite happy to have a quick visit to the Flight Deck.

Onto your questions - (and any answers you get here will be pure speculation)

1 - possibly streaming, then decided against it. I have had numerous occasions where ATC have asked me to either speed up or slow down, then shortly after they have changed their minds and we have gone back to “normal” speed.

2 - was the Flight oceanic by this stage? If so, are there US offset procedures that includes flying at an out of the ordinary level? I’m clutching at straws........

[DaveReidUK]

Then why did the engines sound like they had gone to idle?

I have no idea.

The FR24 track, with plots about every minute, shows no sign of any loss of altitude so it seems unlikely that the engines could have been at flight idle.

[atr-drivr]

Possible block altitude request....I have done it many times with obvious approval from ATC...slowed down prior to TOD? Happens all the time.....been slowed 3 hrs away from JFK🙄

[Smokey Lomcevak]

The mind and body can play tricks. The deceleration to felt could well have been accurate - most likely at ATC request for en-route seperation or arrivals sequencing. Passenger aircraft are designed to be as efficient as possible, and that means minimising drag. In addition, the amount of kinetic energy involved in >50T of metal, fuel and flesh travelling at 500kts is significant.

What this means is that a deceleration of 20kts IAS or more could take a minute, or longer. During that period, one's body can adjust and perceive the deceleration as the new norm. When the target speed is achieved, more power will be required to maintain it (although typically not as much as the previous cruising speed), so the engines will spool up as required. This can feel like an acceleration - partly because one's body had created the new normal of a roughly constant deceleration rate, and because our minds associate an increase in thrust with acceleration. Furthermore, the new speed will require a higher pitch attitude than before, with more of one's weight acting through the back of the seat, rather than the bottom. It's a small change, but it serves to strengthen the illusion.

The most extreme examples of this are a descent with flap out at constant speed to level flight. If the level off coincides with a turn - and thus increased lift required, induced drag and,as such, power required; the resulting pitch change and increase in thrust can feel very much like the chaps/chapesses at the front have decided to "step on it".

[Hokulea]

Thanks, everyone, appreciate your answers. Smokey - I think you have the answer that satisfies me the most for the deacceleration - we were close to DFW so would not be surprised if we were slowed for separation. As for the altitude question - atr-driver, I assume a "block altitude request" means you can request a "block" of altitude space and then fly at any altitude within that block. Is that correct?

[Sultan Ismail]

Excuse my late arrival, the engines went to flight idle halfway across the Indian Ocean, well at least midway between Sumatra and Mauritius.
I have noticed this phenomenon on many round trips Kuala Lumpur to Johannesburg, more than 100 round trips, and agree it is in order achieve a spot on arrival time at destination.
It can be disconcerting when 3 hours from the nearest land mass.

[DaveReidUK]

Excuse my late arrival, the engines went to flight idle halfway across the Indian Ocean, well at least midway between Sumatra and Mauritius.
I have noticed this phenomenon on many round trips Kuala Lumpur to Johannesburg, more than 100 round trips, and agree it is in order achieve a spot on arrival time at destination.
It can be disconcerting when 3 hours from the nearest land mass.

If your engines went to flight idle in the cruise, we're lucky to have you still with us.

Still, it could have been worse - if they had stopped completely, you could have been up there all day ...

[Hokulea]

PS. This has been bothering me for a day or two:

"If you felt a deceleration (i.e. the effect of inertia), all that tells you is that your speed over the ground was reducing."

No, that is absurd. Anyone on an aircraft will feel a change in velocity in their reference frame, i.e., that of the aircraft, not the ground.

[DaveReidUK]

PS. This has been bothering me for a day or two:

"If you felt a deceleration (i.e. the effect of inertia), all that tells you is that your speed over the ground was reducing."

No, that is absurd. Anyone on an aircraft will feel a change in velocity in their reference frame, i.e., that of the aircraft, not the ground.

Oh I see, Newton got it wrong, then. :ugh:

[Hokulea]

Dave - I've been on flights where the ground speed varied from 450 to 650 knots. There was no feeling of acceleration on any of those flights other than take off, ascent, descent and landing. I suspect you have your physics wrong if you think anyone on an aircraft will feel a change in velocity because it changed wrt to the ground during the cruise. The frame of reference is the aircraft, not some point on the ground.

I do have a little bit of background in physics if you had taken the time to do a little research. It's a shame; I've asked for help on this site and also offered my advice on this site, but to come across someone who 1) didn't read my posts carefully and 2) doesn't understand Newtonion physics but insults me anyway, well, that's your choice.

For your info, I have used Newton's laws to find, track and take spectrosopic data of space debris among other things. I think you should sit back and reconsider your posts.

[DaveReidUK]

Dave - I've been on flights where the ground speed varied from 450 to 650 knots. There was no feeling of acceleration on any of those flights other than take off, ascent, descent and landing. I suspect you have your physics wrong if you think anyone on an aircraft will feel a change in velocity because it changed wrt to the ground during the cruise. The frame of reference is the aircraft, not some point on the ground.

I do have a little bit of background in physics if you had taken the time to do a little research. It's a shame; I've asked for help on this site and also offered my advice on this site, but to come across someone who 1) didn't read my posts carefully and 2) doesn't understand Newtonion physics but insults me anyway, well, that's your choice.

For the record:

1) I have read your posts carefully

2) I did not insult you, I simply pointed out that your denial ("that is absurd") of my statement that

"If you felt a deceleration (i.e. the effect of inertia), all that tells you is that your speed over the ground was reducing."is incompatible with Newton's Laws (with which I, too, have more than a passing aquaintance).

In fact you appear to have conceded that you will feel those forces on takeoff and landing, where again it is the change in speed of the aircraft over the ground that produces the sensation you feel. The only difference between that and acceleration/deceleration in the cruise is one of degree - the physics is exactly the same, only the numbers differ.

You go on to misquote me

"you have your physics wrong if you think anyone on an aircraft will feel a change in velocity because it changed wrt to the ground during the cruise"when I actually said that IF you feel a sensation THEN it's because the aircraft's speed is changing, not the other way round (I grant you that the sensation may be imperceptible).

I'll give you the benefit of the doubt and assume that wasn't an intentional straw man argument, but I stand by my comments.

[lomapaseo]

Can we get back to something I can understand, like conveyor belts :E

[Hokulea]

I stand by my comments, Dave, you have your physics wrong.

"when I actually said that IF you feel a sensation THEN it's because the aircraft's speed is changing, not the other way round (I grant you that the sensation may be imperceptible)."

You wrote:

"If you felt a deceleration (i.e. the effect of inertia), all that tells you is that your speed over the ground was reducing."

I'm not pursuing this further, it seems pointless. You've now agreed with my point that it's the speed of the aircraft that's changed, therefore those in that reference frame will feel the change in speed, but then back up your point with the argument that it's ground speed. Sorry, no more time for this nonsenseother than to say:
Thanks to those that gave helpful answers.

[DaveReidUK]

I'm not pursuing this further, it seems pointless.

We're agreed on that.

You've now agreed with my point that it's the speed of the aircraft that's changed, therefore those in that reference frame will feel the change in speed, but then back up your point with the argument that it's ground speed.I have never disputed that.

It's self-evident, thanks to Newton, that the only speed change a body can detect is acceleration in free space, which in the case of an aircraft implies a change in groundspeed. We learned that in 1728.

[GLIDER 90]

Was on a L-1011 Tristar coming back from Italy inbound to Gatwick some years ago on final approach about 15 minutes out from landing. The engines went from normal tone to silence the aircraft dropped this went on a few times, with one of the cabin crew looking alarmed and with some of the pax calling out. This was followed by a heavy landing.

[DaveReidUK]

Was on a L-1011 Tristar coming back from Italy inbound to Gatwick some years ago on final approach about 15 minutes out from landing. The engines went from normal tone to silence the aircraft dropped this went on a few times, with one of the cabin crew looking alarmed and with some of the pax calling out. This was followed by a heavy landing.

The two things you experienced (engines at flight idle and the heavy landing) aren't necessarily linked. :O

The term flight idle, and its cousin ground idle, are a bit of a misnomer because (particularly on big fan engines) a fair amount of thrust is still being produced at those settings (ask any widebody tug driver about pushbacks!).

But it's not enough thrust to maintain level flight - the only way you're going to go with flight idle selected is down, which happily is what you want to do as you approach your destination. :O

In fact the Holy Grail nowadays, for environmental reasons, is to complete a Continuous Descent Approach (CDA) using flight idle all the way from ToD to the threshold. Needless to say, that involves both careful planning and, crucially, the active cooperation of ATC, but it's certainly doable.

In the bad old pre-CDA days, descent and approach would typically include descent segments at flight idle interrupted by periods of level flight (for which the engines would have to spool up again). That's what your pilot would have been doing.

Here's a good intro to CDAs from Eurocontrol: A guide to implementing Continuous Descent (https://www.eurocontrol.int/sites/default/files/publication/files/2011-cd-brochure-web.pdf)

[GLIDER 90]

The two things you experienced (engines at flight idle and the heavy landing) aren't necessarily linked. :O

The term flight idle, and its cousin ground idle, are a bit of a misnomer because (particularly on big fan engines) a fair amount of thrust is still being produced at those settings (ask any widebody tug driver about pushbacks!).

But it's not enough thrust to maintain level flight - the only way you're going to go with flight idle selected is down, which happily is what you want to do as you approach your destination. :O

In fact the Holy Grail nowadays, for environmental reasons, is to complete a Continuous Descent Approach (CDA) using flight idle all the way from ToD to the threshold. Needless to say, that involves both careful planning and, crucially, the active cooperation of ATC, but it's certainly doable.

In the bad old pre-CDA days, descent and approach would typically include descent segments at flight idle interrupted by periods of level flight (for which the engines would have to spool up again). That's what your pilot would have been doing.

Here's a good intro to CDAs from Eurocontrol: A guide to implementing Continuous Descent (https://www.eurocontrol.int/sites/default/files/publication/files/2011-cd-brochure-web.pdf)

Much appreciated

[Andy1973]

1) Flying LAX to DFW on Monday, about 45 minutes from landing the aircraft suddenly slowed by about 20 knots. I thought we were actually starting an early descent, but checked the flight map on the IFE and saw we had slowed but not descending. About a minute or two later we sped up again. What might be the reason for this - sequencing?


The crew normally plan to fly an optimised descent profile, and the Flight Management Computer calculates a descent point based on that profile.

Sometimes, ATC restrictions mean that the crew cannot get a clearance to descend at the optimum point. The aircraft is then above the profile and has more energy than would ideally be required.

The crew could just use speed brakes when they get the descent clearance, but this isn't particularly smooth or fuel efficient. So what the crew could do is reduce the aircraft's airspeed - thereby reducing the kinetic energy of the aircraft. They would then accelerate once the descent was initiated.

So the most likely reason for the behaviour was that the crew could not get a descent clearance from ATC - rather than reducing thrust and going down, they reduced thrust and slowed down!

[WHBM]

We must be conscious that FlightAware and FlightRadar24 are both amateur products, done as well as they can, which use crowd-sourced data lifted from aircraft transmissions, plus an amount of interpretation. They do not have the accuracy of the professional systems. Five minutes looking at the track of aircraft on final approach to a runway, some aligned, others plotted coming in several miles offset, will tell you that.

Was on a L-1011 Tristar coming back from Italy inbound to Gatwick some years ago on final approach about 15 minutes out from landing. The engines went from normal tone to silence
The Tristar, designed in the late 1960s, had FOR IT'S TIME a surprisingly sophisticated, only partially computerised, Flight Management System, where you could put in a waypoint and an arrival time and it would fly the most efficient speed profile, to the second ! Literally. As the crew got various descent clearances it would be working all this out. Nothing unusual in 2018 but for a design from nearly 50 years ago, it was pretty nifty.

Some of the designers had come from Lockheed military projects, as Lockheed had not done a commercial airliner for a decade, but other team members were ex-Hawker Siddeley from Hatfield, where Trident development was running down and they had more good ideas than job opportunities. These personnel were part of the "brain drain" from Britain to the USA which politicians spoke about at the time - but whose stop-start procurement approach never ensured any career opportunities for them.

[Ancient Observer]

Blimey!
You 2 physicists are worse than 1 Economist.

[DaveReidUK]

Blimey!
You 2 physicists are worse than 1 Economist.

I seem to recall being told many years ago that a physicist was an engineer who had failed the practical. :O

[WHBM]

I seem to recall being told many years ago that a physicist was an engineer who had failed the practical.
... and an Economist is someone for whom the real world is a Special Case.

[DaveReidUK]

All economists should be amputated at the wrist, to stop them saying "On the one hand this might happen, on the other hand ..."

[WHBM]

All economists should be amputated at the wrist
I always say "should be put down at birth" ... but I think the sentiment is similar.