I live about 20 miles from San Diego International Airport, pretty much under a major flight path.
About three or four months ago, I heard, for the first time, a jet’s engines throttle WAY down, then, a few seconds later, throttle up again. Now…I hear it all the time.
The first time, it scared me: I thought the engines had failed! But it seems to be the way things are done now. I don’t know if it is on aircraft coming in for landing, or climbing up to cruising altitude. But there is a very pronounced drop in the volume and frequency of the sound of the jets, a very distinct “throttle down” sound, followed very quickly by a throttle-back-up sound.
I’m pretty sure this is new! I’m also not the only person to have noticed this: I was in a parking lot when it happened, and several people turned to look up. The sound was apparently unfamiliar to them.
My guess is it has to do with noise abatement. Here’s a sample noise abatement map with procedures for San Diego. Are you in their green shaded area? Edit: More FAQs, this time dealing with Lindbergh Field. I’m sure you can google the applicable FARs, but that’s my guess for the throttling.
It would really help if you knew whether they were taking off or landing. When taking off there will generally be one power change at around 1000 feet or so, depending on the airport and company noise abatement procedure. That power change is usually a reduction in thrust but it can be a thrust increase, however it would never be a decrease followed immediately by an increase unless something abnormal had happened, like an engine failure.
On landing the thrust is adjusted as required to maintain the desired approach speed. Thrust will come off when the aircraft is slowing down and it will go up when it has slowed to the required speed. Thrust will also come up when drag is increased, for example when the gear and flaps are lowered and a speed reduction is not wanted. Another circumstance that causes thrust changes is turbulence. Has it been particularly hot and/or windy lately?
Something to consider is whether or not auto-throttle is being used. In my experience auto-throttle is quite a coarse tool and it will command large thrust changes to achieve a small change in speed. An auto-throttle can easily bring the thrust back to idle followed by a large increase in turbulent conditions. When the pilots are using the throttles themselves, they tend to be a bit smoother.
There has been no general change to the way aeroplanes are flown, so I think you’ve probably just noticed some large power changes in response to either turbulence or configuration/speed changes. Now that you’ve noticed it, you keep on noticing it. Or perhaps it is seasonal if it is due to turbulence.
One other possibility is that one of the noises you are hearing isn’t a change in thrust at all, but the sound of flaps being extended or retracted. Flaps can be very noisy and if the thrust is at idle, which it is for most of the descent, you might not hear the engines. So you could be hearing flaps extending and mistaking that for a thrust change, then you hear the thrust come up to maintain the approach speed with flaps out.
It’s won’t be thrust reversers as they are only used on the ground.
I’m just off the eastern edge of the map… The map confused me even more, as it doesn’t seem to match with the actual flight paths. It shows traffic landing at Lindbergh from the west…and, no, the planes take off to the west, but land from the east. Once they take off, they loop back around (well, the ones that aren’t going to Hawaii or Japan…) to the east. Worse, the ones that are coming from, say, San Francisco loop way out to the east before lining up to land. So the planes over my head could be going east…before going west to land, or simply going on east to Texas or wherever! No way to tell from listening…and no way for me (untrained) to tell by looking!
It’s definitely a decrease, followed almost immediately by an increase. The whole exchange is done in less than ten seconds.
Lately, very hot, but not much wind. However, this has been going on for months, since before the latest heat wave.
I haven’t a clue, but this makes sense. Might it be that this is just a correction to help the pilot get into proper spacing for landing? We often have long lines of planes in the air, coming in to land in sequence. If a guy was a little too far ahead, would a brief power drop be a good way to get into the proper spacing?
I do not know much about turbulence; San Diego has variable heat, but only infrequent winds. The “Santa Ana” winds are very noticeable on the ground, but they don’t seem to me to have anything to do with what I’m hearing.
And, yes, you’re very, very right that a kind of selective perception is involved. I almost mentioned that in my OP; I can’t be sure I had never heard it before, but I feel pretty firmly that I would have noticed it, given how very strongly I reacted to it the first time I did (consciously) hear it!
Entirely possible. I freely admit to very little knowledge. I haven’t flown as a passenger in years.
The last few times I have flown, I seem to remember the flaps being extended quite late in the descent. But I don’t have a good way of telling exactly where in the flight-path the sounds I’m hearing come from. The might be coming from well to my west, and I only think they’re overhead.
I’m pretty sure this isn’t it, because of the way the sound descends in pitch and then immediately ascends again. Also, the whole affair is in under ten seconds. I’m not able to rule it out, but it doesn’t have the usual auditory hallmarks of fire sirens, train whistles, screeching brakes, or other familiar Doppler-shifted noises. In those, the noise itself is fairly steady, with only the pitch varying. But with the airplane noises I’m hearing, there is also a strong drop-off (and increase again) of volume.
If you were alking about John Wayne Airport (SNA) I could confiirm that it is noise abatement so that would be my guess. IIRC the landing appoach to Lindburgh is right over a ridge and a freeway then you have to drop the plane onto the runway. Could that be the reason?
The map is for Montgomery field, not Lindbergh. It looks like noise abatement tracking procedures for the little guys. Noise abatement for big jets is covered by the standard arrival routes (STARs) for arrivals, and standard instrument departures (SIDs) along with noise abatement climb techniques. You tend not to see them laid out in the kind of map format shown for Montgomery because the pilots of airliners don’t get to choose how to approach the airport, they’re assigned a STAR and an approach and we, as pilots, assume that we are complying with noise abatement if we follow ATC instructions. For departures there is a noise abatement climb technique, but it only involves one thrust change, there shouldn’t be the decrease then increase that you are hearing. Therefore I don’t think you are hearing departures, I think it is arrivals.
Also, the direction the aircraft take-off and land in depends on the wind. They will accept a certain amount of downwind for noise abatement but if the wind is strong enough (more than 5 knots normally) then the take-offs and landings will be into wind so if you are under the approach path on one day you would be under the departure path on another day if they change runways.
Do you know how far in miles you are from the end of the runway at Lindbergh?
Any heat in the land will generate thermals and turbulence, it doesn’t have to be windy, just heat will do it. Anything above about 85ºF would be warm enough to cause turbulence over land.
Yes it could be. If you are told to slow down you need to pull the thrust off, if it is not already at idle, and once you get to the speed you want, you have to put the thrust back on. That would normally take longer than 10 seconds, but it could be an explanation.
Initial flap selection would normally be at about 3000 feet or about 10 nautical miles from the runway on final approach. Progressive flap and gear extension happens after that with the aircraft normally fully configured for landing somewhere from 3 to 5 miles out.
It’s hard to tell what might be happening because I don’t know the area. In ideal conditions the thrust is left at idle from cruising altitude down to the last 10 miles of the approach. That would suggest you are hearing changes associated with speed reduction on final approach or maintaining approach speed in turbulence. But it may be that the arrival sequence for Lindbergh requires periods with power on prior to the approach, that could mean you are hearing the result of speed reductions in response to ATC instructions.
Definitely could be… The landing path is over a ridge and several freeways, and zooms nice and low over the Hillcrest area… Pretty much right over downtown San Diego. I’ve heard it said that it is one of the worst approaches for landing in the U.S.
It’s a rare day indeed for planes to take off to the east, or land from the west, at Lindbergh. I’ve lived and worked in the area most of my life, and have never actually seen it.
In my OP I said twenty, but I was wrong; I’m ten miles almost exactly due east of the airport.
I think what I’m hearing is pretty far overhead, and not the sound of planes in the very final approach. But I’m far from certain!
Well, it’s been hotternell here lately, but when I first heard it, it was springtime and not so (blazin’!) hot.
And I probably had been hearing the same sound for years, and am only now conscious of it. Makes the most sense, anyway. Thank you (and everyone else who posted) for your information.
(One of the things I hope to do some day soon is hire a ride on a small plane and get a view of the region from the air from a lower altitude. I enjoy flying in jets, too; I remember studying the layout of Arizona’s Mogollon Rim from the sky, in a God’s Eye View about as pretty as anything I’ve ever seen.)
IANAP but I think it may very well be a possibility that because of the low approach they may be throttling down to descend then up to maintain the new altitude but if I had to bet I’d still go with noise abatement. Why not contact the FAA?
Neither of those really fit the scenario though. Noise abatement climb only involves one power change, from take-off thrust to climb thrust. Noise abatement descent procedures are kind of transparent to the crews, we have tracking to follow but we don’t know if that tracking is for noise abatement or separation from other traffic or to keep us in controlled airspace, and it doesn’t involve anything special with the thrust settings.
For the approach over the ridge and highway, that might look like it has an effect on the approach, but from a flying point of view you just fly a constant 3º slope down to the runway from about 10 miles out. The obstacles near the end of the runway aren’t allowed to infringe on the standard approach path. If they do, then the approach path might be steeper, say 3.3º, but it is still flown as a constant descent. In fact, ideally, the thrust is reduced to idle at 35000 feet or whatever the cruising altitude was and isn’t increased again until 1500 feet on a five mile final. That’s the ideal, but there can be lots of things that stop it from happening.
Trinopus, if you are at 10 miles from the runway then that is right about the time approaching aircraft would be starting to configure for the approach, that can definitely involve power changes both down and up. So that fits with what you are hearing, but as you say, you may be hearing other aircraft further away as well.
Here is a video of the phenomenon I think the OP is talking about. You can hear the pitch of the “whine” of the engines go up and down several times throughout the video.
I live near Atlantic City, NJ and hear something similar over my house. The jets are fairly high, I’m guessing 10,000’+, and after they pass I hear the engines spooling up. I always attributed to the Doppler effect and that the spooling up sound was just the approaching jet but that I don’t hear it until the jet has passed. I’m hearing it in one place an seeing it another. The only thing that makes me think that this may not be the case is that it happens pretty quickly. The sound goes from nothing to max pitch in less than 10 seconds and I never hear the descending tone you’d expect.
I’m not doubting you but the noise abatement procedure coming out of SNA has always felt like two throttle changes. The lowering of the trottle causing the “drop” (I know the plane doesn’t drop) and then a few second later a gradual throttle up and climb. Now I’m curious. Does SNA use a different noise abatement procedure? Is it even noise abatement or is it something else? Is it one throttle change that feels like two to the lay-passenger?
The sound on these videos is close to what I hear. But, in the vids, the sound drops and rises again by, what, maybe 15% (judging solely by ear.) The really dramatic ones I hear in the sky drop and rise by maybe 60%. (And…my ear is not professionally trained; I’m guessing!) It’s the really pronounced drop and rise again that caught my attention.
So… Does the same phenomenon ever happen, but, like, four times more strongly?
Really fun videos! I gotta fly again some day soon! A quick visit to SF or LV…
I think you are correct for SNA. I haven’t found anything official yet, but a few anecdotal cites suggest that the thrust is reduced below climb thrust for a quiet overflight of the most noise sensitive area then increased to climb thrust. This isn’t the standard noise abatement procedure but each airport is different and there are some that stipulate different procedures from the standard.
Yes. In your video the conditions are very stable and smooth. Low cloud/fog like that with clear skies above are a sign of smooth conditions. On a turbulent day the turbulence causes variations in airspeed. If the airspeed gets a few knots too fast the autothrottle can come all the way back to idle to try and fix it, then once the speed has come back to where it should be, the autothrottles will power up very quickly to maintain the speed. If the speed gets slow the opposite happens, and if the speed goes fast to begin with then too slow, the autothrottles can make very big power changes each way to try and keep up with what is happening. The pilot manually operating the thrust levers can do the same thing, but they tend to be smoother because they have the advantage of knowing what the aeroplane is likely to do ahead of time while the autothrottles can only react, they can’t think ahead. Jets need a lot of thrust to correct a slow speed while props don’t, so you get much bigger thrust variations in a jet, the variations are also more obvious because of the whining noise the engines make.
Super cool! Thank you! There are just so darn many things in this world, far too many for any one person to know. But now I know a little more than yesterday!
Reading the FAA noise abatement procedures here, it looks like a thrust reduction to below climb thrust followed by an increase to climb thrust could be quite normal in the US in general. It’s a bit different to the ICAO procedure that just stipulates a reduction to climb thrust. In particular where the FAA says:
That thrust setting could easily be less than the normal climb thrust.
Looking further in to this, it looks like the ICAO noise abatement could involve the same power down followed by power up. I’d been thinking that the noise abatement procedure for the aircraft I fly, because it complied with ICAO, was typical of other aircraft types as well, but if anything it might be a special case because it is already a very quiet aeroplane. ICAO stipulates a thrust reduction to the noise abatement thrust setting specified in the aircraft flight manual. For us that is normal climb thrust, but for other types it is probably less. We are all learning something today :).