https://www.youtube.com/watch?v=onNxigIjkYw
In this video, you can hear the engine constantly changing tone. at :07, you can hear the engine drastically slow down, almost to idle speed. Then, at :36, you hear it pick speed back up. at :58 you can hear it very gradually picking up just a bit more speed. Then at 1:15, you can hear it pick up some more. Then, at 1:27, one last time it picks up just a TAD bit more speed, before gradually slowing back down for the landing.
Why do they have to slow way down like that, then pick back up? I used to notice this ALL the time, but I also noticed it more so back in the day with the older aircraft where the engines had the distinct whine that the JT-8's and RB211's had.
In today's newer aircraft, I don't really notice it much. Is that because there is newer technology where it's not as necessary to have to keep speeding up and slowing down, or do I just not notice it because the sound of today's engines are completely different, where all you really hear is the air flowing through them, so changes in speed aren't as noticeable?
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- Starlionblue
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Re: Why do aircraft continually speed up, then slow back down within a short time?
The thrust response of turbofans is highly non-linear. At cruise, a turbofan is at 80-85% N1 for almost max thrust. At idle, it is at 15-20% N1. On approach fully configured, it is at 45-55% N1. Most of the thrust is in the top 50% of the N1 range.
(As more than one instructor told me: "You must know your pitch and power settings!")
In clean configuration, when descending for approach, the engines are easily at flight idle, which might be what happened at :07. then at :36 there may have been a level segment. However the aircraft may also have to decelerate in the level segment, but hit the target speed at :58. Jets tend to be very "slippery". On the glideslope, an A330 will accelerate at idle unless you're at Flaps 2 or more.
As the flaps come out with the aircraft on the glideslope, thrust needs to increase with each flap level plus gear extension. The aircraft slows but the glideslope is constant.
Turborfans also don't give instant thrust response like props. On approach, you're holding a relatively high thrust level against flaps so you can accelerate the engines quickly to respond to speed changes, and in case of a go-around.
Today's engines aren't that different really. They accelerate a bit faster, but mostly you're not sitting right next to them.
(As more than one instructor told me: "You must know your pitch and power settings!")
In clean configuration, when descending for approach, the engines are easily at flight idle, which might be what happened at :07. then at :36 there may have been a level segment. However the aircraft may also have to decelerate in the level segment, but hit the target speed at :58. Jets tend to be very "slippery". On the glideslope, an A330 will accelerate at idle unless you're at Flaps 2 or more.
As the flaps come out with the aircraft on the glideslope, thrust needs to increase with each flap level plus gear extension. The aircraft slows but the glideslope is constant.
Turborfans also don't give instant thrust response like props. On approach, you're holding a relatively high thrust level against flaps so you can accelerate the engines quickly to respond to speed changes, and in case of a go-around.
Today's engines aren't that different really. They accelerate a bit faster, but mostly you're not sitting right next to them.
Re: Why do aircraft continually speed up, then slow back down within a short time?
Starlionblue wrote:but mostly you're not sitting right next to them.
Methinks that is the key.
Ever sat in the last few rows on a DC-9 or MD-80? It's LOUD, and it seems like you can clearly hear every little change in thrust. They probably sound much more extreme than they are. You don't get anywhere near than feeling with wing-mounted engines. Or at least, I don't.
Since the video shows the last 2 minutes, or maybe 1800 feet of descent, these thrust changes are probably to capture and/or hold the approach speed, and maybe to adjust for the last flap setting.
Re: Why do aircraft continually speed up, then slow back down within a short time?
Flying 101: For a stable approach, across most types: Keep the attitude, descent rate, etc. constant, stable. Altitude especially (which can potentially deviate with wind changes and a multitude of other factors): It's taught early on to correct situations with power, and not control surfaces (in routine situations). And yes, those rear-mounted JT8D-200s definitely clue you into the power setting much more than other engines. On one flight as a passenger on a DC-9 years ago, I actually heard the engines briefly spool up strongly, and felt acceleration maybe ~100' over the displaced threshold.
Perhaps the pilots' indicated airspeed was a little too slow, for a greaser? He/she certainly accomplished one ~ 1,000' beyond the aiming point (which is legal, and not a poor practice. No need to set it down on the "aiming point" just within the "touchdown zone."
By happenstance (living in ATL at the time), I thought I had no business, no certifications to allow "loggable" hours in an MD-80 sim, but I did get about 15 hours in one in the 90's (apparently, it counts!), and yes, it's a very slippery airplane.
Perhaps the pilots' indicated airspeed was a little too slow, for a greaser? He/she certainly accomplished one ~ 1,000' beyond the aiming point (which is legal, and not a poor practice. No need to set it down on the "aiming point" just within the "touchdown zone."
By happenstance (living in ATL at the time), I thought I had no business, no certifications to allow "loggable" hours in an MD-80 sim, but I did get about 15 hours in one in the 90's (apparently, it counts!), and yes, it's a very slippery airplane.
Re: Why do aircraft continually speed up, then slow back down within a short time?
As said height / ILS track is managed by thrust, keeping IAS constant.
Re: Why do aircraft continually speed up, then slow back down within a short time?
keesje wrote:As said height / ILS track is managed by thrust, keeping IAS constant.
Not on a jet. Thrust controls speed, and pitch controls glidepath.
Re: Why do aircraft continually speed up, then slow back down within a short time?
Tartarus wrote:keesje wrote:As said height / ILS track is managed by thrust, keeping IAS constant.
Not on a jet. Thrust controls speed, and pitch controls glidepath.
Ah I was indeeed referencing to a prop flight.
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- GalaxyFlyer
- Posts: 12400
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Re: Why do aircraft continually speed up, then slow back down within a short time?
Tartarus wrote:keesje wrote:As said height / ILS track is managed by thrust, keeping IAS constant.
Not on a jet. Thrust controls speed, and pitch controls glidepath.
Don’t tell that to a carrier pilot!
GF
Re: Why do aircraft continually speed up, then slow back down within a short time?
You might also experience thrust changes (up and down) at cruise altitude during areas of moderate turbulence (throttled down during the chop, throttled back up when the chop subsides).
Re: Why do aircraft continually speed up, then slow back down within a short time?
TriL1011Star wrote:https://www.youtube.com/watch?v=onNxigIjkYw
In this video, you can hear the engine constantly changing tone. at :07, you can hear the engine drastically slow down, almost to idle speed. Then, at :36, you hear it pick speed back up. at :58 you can hear it very gradually picking up just a bit more speed. Then at 1:15, you can hear it pick up some more. Then, at 1:27, one last time it picks up just a TAD bit more speed, before gradually slowing back down for the landing.
Why do they have to slow way down like that, then pick back up? I used to notice this ALL the time, but I also noticed it more so back in the day with the older aircraft where the engines had the distinct whine that the JT-8's and RB211's had.
In today's newer aircraft, I don't really notice it much. Is that because there is newer technology where it's not as necessary to have to keep speeding up and slowing down, or do I just not notice it because the sound of today's engines are completely different, where all you really hear is the air flowing through them, so changes in speed aren't as noticeable?
The title of your question suggests that the aircraft is speeding up and slowing down; this is not the case in the video you show. What you're hearing is the engine increasing and decreasing. Clearly the airplane is landing, which. means getting configured, which means lowering flaps and landing gear. When approaching to land, a power reduction is made for descents and to slow, and when a configuration change has been made (flap setting, for example) power is adjusted to hold whatever airspeed is desired at that configuration. It may be a power reduction to slow to the flaps speed, then an increase in drag as the flaps are extended, and a power increase to hold the speed.
The airplane will slow down in stages as it approaches the airport terminal area. It may have descended at 300 knots, but slows to 250 at 10,000' and as it gets closer to the airport, air traffic control may prescribe a particular speed (eg, maintain 180 knots until 5 miles, etc); this necessitates power changes, configuration changes, etc. As the airplane approaches to land, it continues to slow until it's at its final approach speed for the desired flap setting. Power may increase or decrease to maintain that speed.
In cruise, aircraft tend to fly.a particular speed and hold it.
For takeoff, speed is up to 250 knots, or higher if heavy and needed, until 10,000, then typically accelerating to a higher climb speed in the range of about 320 knots until level, then maintaining a speed in the range of about .76 to .84 Mach.
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