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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?

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    $\begingroup$ FYIW the twin in the front is an Airbus (318-A321) $\endgroup$ – DeepSpace Mar 11 at 15:28
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    $\begingroup$ 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... $\endgroup$ – Dave Mar 11 at 15:45
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    $\begingroup$ @DeepSpace It looked like the engine nacel's were flattened on the bottom, hence my 737 guess. $\endgroup$ – FreeMan Mar 11 at 17:54
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    $\begingroup$ @jamesqf Well, that fifth plane doesn't seem to have its landing lights on, even, so... $\endgroup$ – a CVn Mar 11 at 20:09
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    $\begingroup$ 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? $\endgroup$ – Scrontch Mar 12 at 13:23
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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.

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    $\begingroup$ 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. $\endgroup$ – Cpt Reynolds Mar 11 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.

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  • $\begingroup$ BTW similar calculations are made in the comments under the youtube video $\endgroup$ – Chris H Mar 12 at 10:25
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    $\begingroup$ 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? $\endgroup$ – FreeMan Mar 12 at 11:46
  • $\begingroup$ Chris, I think you got carried away with addressing one of the comments to the question, rather than the question itself. $\endgroup$ – David Richerby Mar 12 at 12:04
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    $\begingroup$ @David Richerby: But the answer to the original question is so simple. Because the crew didn't hit the "gear down" switch yet, no? $\endgroup$ – jamesqf Mar 12 at 17:30
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    $\begingroup$ @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. $\endgroup$ – David Richerby Mar 12 at 17:33
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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.

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    $\begingroup$ 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." $\endgroup$ – David Richerby Mar 12 at 12:08
  • $\begingroup$ 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? $\endgroup$ – Cpt Reynolds Mar 12 at 12:27
  • $\begingroup$ 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. $\endgroup$ – Andy Mar 13 at 13:52
  • $\begingroup$ 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. $\endgroup$ – FreeMan Mar 15 at 12:31
  • $\begingroup$ @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. $\endgroup$ – David Richerby Mar 15 at 12:47

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