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This image (as posted in another question) is a YouTube video's thumbnail:

https://i.stack.imgur.com/zXxHL.png

I noticed that the lead twin (737?) has gear down, the 747 behind does not (appear to) have gear down, but the third plane in line (another twin) does have gear down as well.

Why would the 747 not be gear down at this point?

FreeMan
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    FYIW the twin in the front is an Airbus (318-A321) – DeepSpace Mar 11 '19 at 15:28
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    There is a 75% chance @Terry sees this meaning there is a 98% chance he has the answer and a 0.001% chance it was him flying that 747... – Dave Mar 11 '19 at 15:45
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    @DeepSpace It looked like the engine nacel's were flattened on the bottom, hence my 737 guess. – FreeMan Mar 11 '19 at 17:54
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    Please tell me that photo was either staged, or taken with a really long telephoto lens. – jamesqf Mar 11 '19 at 20:06
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    @jamesqf Well, that fifth plane doesn't seem to have its landing lights on, even, so... – user Mar 11 '19 at 20:09
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    @jamesqf A telephoto lens coupled with the difference in size between #1 and #2 would them appear a lot closer than they really are. – JeffUK Mar 12 '19 at 08:49
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    Is it even given that the 747 is second in this sequence? Could it be that with the tele-lens-effect that the third plane from top (twin) is actually the 2nd in sequence, flying at a lower altitude? – Scrontch Mar 12 '19 at 13:23
  • @Scrontch given the separation required between planes, that third plane would have to be a very small regional to be closer to the lens, yet appear that much smaller than the 747. – FreeMan Mar 12 '19 at 13:29
  • @Scrontch I was thinking the EXACT same thing. Unfortunately the image is both too small (to see detail from the original image) and compressed (yay, artefacts) to make a proper deduction here. – Baldrickk Mar 12 '19 at 14:24
  • The image does not appear in the video. Do you have a link to the correct video at the specific approximate time? – Vladimir F Героям слава Mar 12 '19 at 15:43
  • @VladimirF - I pulled the image from another question (as was indicated, but was edited out). I've included the link to the original question. I've not seen the video, so I have no idea what the contents of the video are. – FreeMan Mar 12 '19 at 19:45
  • @FreeMan: The edit (which was after the comment) was because I've changed the photo in my question, see here: https://chat.stackexchange.com/transcript/message/49445592#49445592 –  Mar 12 '19 at 19:47
  • @ymb1 I see. OK, well... in that case, I guess Vladimir is out of luck. as I have no idea where the actual image came from... I'll leave it attributed to your question, though, as that was my source and what prompted the question. Since you kindly added the YouTube link, I'd think that'll work as "official" sourcing... (I hope) – FreeMan Mar 12 '19 at 19:51
  • @VladimirF: it's the thumbnail: https://goo.gl/images/oxcD6C –  Mar 12 '19 at 19:55
  • @Scrontch that small regional would surely not be that close behind the 747, the wake turbulence wouldn't allow it, so I think you're right, 747 must be #3 – JeffUK Mar 13 '19 at 10:30
  • The ye third 737 saw the camera, and dropped his gear early to make the 747 look bad ;) Seriously @ymb1 it is SOP for youtubers to fake their thumbnails. Youtube now allows larger posters to select the thumbnail and they often use a ringer, like a C-17 launching off a carrier. – Harper - Reinstate Monica Mar 13 '19 at 12:37
  • @JeffUK if you'll reread my comment, you'll see that I said it would have to be a very small regional to be in front of the 747 and still look that much smaller. A telephoto lens will compress distances making the planes look much closer to each other than they really are (search on [photography.se]). Also, see the math in Chris H's answer showing approximate distances between the aircraft – FreeMan Mar 13 '19 at 12:50

3 Answers3

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Why does the 747 not have its gear down yet?

Based on the image it is hard to say how far these aircraft are from the runway. The 747 could still be be quite far from the runway and thus does not need to have its gear down yet.

The 747-400 FCOM (NP.21.47 Normal Procedures) says:

At glideslope alive, call:

  • "GEAR DOWN"
  • "FLAPS 20"

It looks like the 747 in the picture has already intercepted the glideslope, but this is hard to say from this perspective. Also, operators may have different rules for when to lower the gear.

Then why does the third aircraft already have its gear down?

The most probable reason for this is drag. A flight crew may choose to lower the gear earlier than necessary to reduce its speed. ATC could have instructed this aircraft to fly at a lower speed for spacing and the crew then dropped the gear, which adds significant drag. This is pure speculation, of course.

Federico
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Bianfable
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    I second these assumptions based on knowing the approach procedures at the airfield in question (EGLL) and some familiarity with SOPs for at least one airline/type combination pictured here. – Cpt Reynolds Mar 11 '19 at 15:55
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Having spend a fair bit of time being paid (by NATS) to watch arrivals at Heathrow about 20 years ago that's a seriously long lens. I used 8-12x binoculars to get an earlier chance to ID types on arrival and you wouldn't get a view like that even with 12x. Without knowing the weather conditions on the day, but assuming they're typical, you can tell from the haze that the planes are far apart - look how much less sharp each one is than the one in front of it (the 5th is recognisable as an aircraft but only just). I suggest that the later twins are larger than the one at the front, further shortening the appearance, thus they're all further away than they look. This would also explain why the last doesn't have its landing lights on.

Adding up the wake vortex separation (relevant tables) as an estimate of the length of the shot and assuming we have a Medium (seperation not dictated by wake vortex so I've used a low figure of 2.5 NM) followed by 4 Heavies (4 NM required for a Heavy following a Heavy) we have 14.5 NM from the first to the last. With no ground in sight we can assume that the A320ish is still some way from the threshold, so it's likely to be just that it's further away than it looks.

CptReynolds reckons no. 3 is an A320 (like no. 1). If so, they're minimum* 7.5 NM apart and no. 3 is (by similar triangles) 2.5x as far away as no. 1. That makes no. 1 4.33 NM from the observer. We lack metadata and any direction or even time references but this map I found shows plane-spotting locations roughly 300-400 m (or 0.2 NM) E of the visible start of 27L and 27R (I assume the airport is on westerly operations because of the prevailing wind, and I won't consider displaced thresholds). If the observer was at one of those locations, that would mean we have, with rounding, something like:

A/C no. A/C type  Min distance from runway (NM)
1       A320      4.5
2       B747      7
3       A320      12
4       ?         14.5
5       ?         17

At this point, I admit to knowing much more about photography, maps etc. than aviation operations, and will let the experts weigh in on reasons (my suspicion is that no. 3 is limiting its speed)

* Of course there will be a margin on top of the minimum, but LHR runs pretty tight so I'll ignore the fact everything is probably a little further away.

The question really seems to be not "why doesn't the 2nd aircraft have its gear down" as "why does the 3rd", given that it's still some way out.

Chris H
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  • BTW similar calculations are made in the comments under the youtube video – Chris H Mar 12 '19 at 10:25
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    That's very interesting information and great insight, but I'm not sure how it answers the question - why does the 2nd plane, which is closer (as you confirmed) not have its gear down whilst the 3rd plane, which is obviously farther away, already have its gear down? – FreeMan Mar 12 '19 at 11:46
  • Chris, I think you got carried away with addressing one of the comments to the question, rather than the question itself. – David Richerby Mar 12 '19 at 12:04
  • @DavidRicherby possibly. This was going to be a comment but adding the links meant there was no way I could compress it. I won't change much, at least for now, but will add one important point – Chris H Mar 12 '19 at 12:11
  • I‘m pretty certain no. 3 is an A320 family aircraft, even though it might (for the pictured combination) not be the optimal sorting to stack‘em that way round, separation-wise. – Cpt Reynolds Mar 12 '19 at 12:26
  • @CptReynolds you could well be right. I might get the chance to look at the photo again on a big screen and draw some conclusions about distance in that case. It wouldn't change the overall length of the queue very much, as the separation in front of an A320 would increase, and behind would decrease. – Chris H Mar 12 '19 at 16:59
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    @David Richerby: But the answer to the original question is so simple. Because the crew didn't hit the "gear down" switch yet, no? – jamesqf Mar 12 '19 at 17:30
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    @jamesqf That would be a very poor answer, as it doesn't explain why didn't they hit the switch yet, given that the pilot four miles behind them already had. – David Richerby Mar 12 '19 at 17:33
  • @CptReynolds I've run the numbers (with one more calculation to follow I think - no, not enough data to work out the lens length etc.) – Chris H Mar 12 '19 at 21:33
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Larger plane more drag on the wheels perhaps? I'm only guessing, but it would make sense to me, the larger size of the second plane would mean there was already extra drag on it compared to the smaller planes, and lowering the gear too soon might have reduced their air speed by too much.

Andy
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    This seems very speculative. Sure, the larger gear of a larger plane will cause more drag in absolute terms than the smaller gear of a smaller plane, but you don't give any basis for suggesting that the gear of the larger plane would contribute a greater proportion of total drag than on the smaller plane. Sure, it might but it might not, too. We're looking for answers that are more authoritative than "I'm only guessing." – David Richerby Mar 12 '19 at 12:08
  • Well, it’s not only the drag but the total sum of force that counts. E.g it‘s also much higher weight pushing it down the glidepath, right? – Cpt Reynolds Mar 12 '19 at 12:27
  • I was merely putting forwards a possible factor for the planes gear still being up, but as you say David, "it MIGHT but it MIGHT not, too" (way to hedge your bets David) try sticking your hand out of the window of a car doing 60/70 mph. I'm guessing you 'Might or Might not' feel the drag that such a comparatively small thing can cause, now compare that to the wheel size of the plane, I would imagine the drag would be at least 'slightly' greater. I am not an aviator and do not claim to know anything about avionics, but I do know that the drag from the landing gear can slow air speed down. – Andy Mar 13 '19 at 13:52
  • The point of @DavidRicherby's comment is that at all the SE sites, we're not looking for "perhaps", "I'm only guessing."` or "I would imagine" answers. i.e. we're looking for facts as can best be determined, preferably with references to sources for those facts. If you have guesses, perhaps', or imaginations, the "comments" are the ideal place for those. You don't (yet) have enough rep to comment in general, but continue contributing and you soon will. – FreeMan Mar 15 '19 at 12:31
  • @Andy Read my comment more carefully. I acknowledged that lowering the gear causes drag, which is obviously true. But you claimed more than that: you claimed that lowering the gear affects big planes less than small planes. You gave no evidence at all for that claim and it's not at all obvious that it's true. In fact, I'd expect it to be false. – David Richerby Mar 15 '19 at 12:47
  • @ David I read your comment fine thanks Dave, but perhaps you should re-read mine instead, at no point did I claim that "lowering the gear affects big planes less". So perhaps instead of writing about what you think is there, you could actually read what was written. – Andy Mar 17 '19 at 02:42