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I was watching some airport documentary and I noticed that the planes always had to park precisely. I honestly didn't think so much of it before, but in the documentary, an aircraft parked badly because the VDGS couldn't detect it correctly due to fog, so it had to be pushed back to park again with the help of a marshaller.

The only two reasons for this I could come up with are the following:

  • Jetways: I figured it's because of the jetways, but they can move about so as to adapt to where the plane is. Plus, when parking on stands, the accuracy is the same and there is no jetway.

  • Aesthetics: I also thought it could be about aesthetics, but the systems used to help the pilot park (a marshaller or VDGS) can be expensive so business-wise, it wouldn't make sense to be so strict on parking if it's just for the looks.

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This is to ensure that there is space for all equipment around the aircraft. It's not critical to the centimeter, but if it's off by a meter or more there may be problems. For example, the tail may stick out into the taxiway behind or the nose may stick into the service road in front of the aircraft. While most aircraft stop with the nose wheel on or very near the stop-mark, the main wheels can be off by a few feet, depending on how early/late the pilot turns into the stand.

Modern aerobridges have a wide range of movement to cater to various aircraft types, but nowadays there is a lot of equipment buried in the ground. You might see ground power and air conditioning hoses which plug into the ground next to the aircraft nosewheel area. If the aircraft is off by a few feet the hose/wires may cause obstruction to the ground crew. The hydrants for fuel also need to be near the fuelling ports - too far and the hoses might not reach.

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    $\begingroup$ “Its not critical to the mm/foot” is kind of a weird statement—1 ft is over 300 mm. American measures don’t really have anything comparable to millimeters, fractions of an inch are used but those aren’t units. And even an inch would be over 25 mm. Since millimeters were used in the question, it might just be easier in this case to stick with metric. $\endgroup$ – KRyan Aug 15 '18 at 19:13
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    $\begingroup$ @KRyan: Some industries use "mil" quite a bit (1mil is 0.1", i.e. 0.0254mm), and can use the phrases "dozen mils" or "hundred mils" as units of length measuring about 0.3mm and 2.54mm, respectively; such phrases accommodate constructs like "a few dozen mils" Metric, so far as I can tell, lacks anything between "millimeter" and "micron". $\endgroup$ – supercat Aug 15 '18 at 19:27
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    $\begingroup$ 1 mil is actually 0.001" (1 thousandth of an inch) $\endgroup$ – Glen Yates Aug 15 '18 at 20:18
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    $\begingroup$ The apparent accuracy the OP was talking about may be due to a known psychological effect. You can aim a gun more accurately if you aim for a point rather than a box. Likewise, it may be easier to get planes in their place every time, if they aim for an absurdly small target, rather than aiming for the actual box of acceptable positions. $\endgroup$ – Cort Ammon - Reinstate Monica Aug 15 '18 at 21:52
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    $\begingroup$ @Agent_L Micron is the proper term in much of biology. See also en.wikipedia.org/wiki/… $\endgroup$ – forest Aug 17 '18 at 9:19
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I worked for 3 years as a ramp agent and there are two reasons:

Jet bridges and safety.

Jet bridges have plenty of leeway, but not a lot. This gets worse in winter because those things hardly move at all with ice. Some jet bridges are poorly engineered, some are old. The painting on the ground tells the marshaller where to park the aircraft. By putting the forward landing gear on the right spot, you ensure that the jet bridge will reach the door and line up. Furthermore, it increases efficiency because the operator won't have to do as much adjusting to get the bridge into place, allowing for a smoother transfer of crew and passengers.

The bigger reason is safety. The painting on the ground determines where suction and jetblast are dangerous. By keeping the plane aligned with the markings, you keep your fellow ramp agents safe while they wait for the aircraft to park and turn the engines off.

Of course it isn't so bad as "a milimeter" off. You can miss the marker by 1 or 2 feet and still be fine in most cases (of course, your fellow ramp agents will make fun of you the rest of the day, but that's besides the point.) But the primary reasons are jet bridge limitations and safety.

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    $\begingroup$ Welcome to Av.se - nice answer. $\endgroup$ – Ralph J Aug 16 '18 at 2:44
  • $\begingroup$ The suction and jet blast areas don't make sense because the distance between the engines and the main door varies widely depending on model, consider a 737-600 and a A340-600. It may be something implemented for airlines that have few types, but it can't be done on a general basis. $\endgroup$ – user71659 Aug 16 '18 at 5:16
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    $\begingroup$ I would agree except most airports have stands of different sizes, which means that all aircraft that "fit" in one specific size will usually park at stands of that size, allowing those markings to be accurate most of the time. $\endgroup$ – Artefaxx Aug 16 '18 at 6:41
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    $\begingroup$ @user71659, in general that's true, however, each gate is usually marked for a select few aircraft that the gate is expected to regularly service, thus the markings are generalized for those aircraft types. For example, the JetBlue gate at PWM is only set up for the A320 and the E190, because those are the only two aircraft JetBlue operate (as of when I worked there anyways.) But yes, if a different type of aircraft that wasn't anticipated tried to park there, then the markings on the ground wouldn't be valid. At that point, you have to rely on your judgement. $\endgroup$ – Jonathan Kuhl Aug 16 '18 at 12:29
  • $\begingroup$ Bounty for real-world, real-facts answer, thanks $\endgroup$ – Fattie Aug 24 '18 at 5:07
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To add to Anilv's answer, here's a prime example of what happens when a jet isn't positioned correctly on a jetway (it's not parked, but the same principle applies)

Or this Royal Jordanian clipping a parked jet

The 'fender bender' happened when the Royal Jordanian Boeing aircraft's nose and wing hit the tail of the ChautauquaAirlines flight which was parked at a gate.

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    $\begingroup$ And a fender bender could easily cost an airline hundreds of thousands in lost revenue alone (downtime for repairs) even if the damage is small $\endgroup$ – Hanky Panky Aug 15 '18 at 16:47
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    $\begingroup$ It's a plane vs ground equipment issue too $\endgroup$ – user71659 Aug 15 '18 at 17:13
  • $\begingroup$ @user71659 RIP. $\endgroup$ – yo' Aug 17 '18 at 7:29
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    $\begingroup$ Regarding user71659's comment, if that is indeed an A332 as marked, a 'total loss' would be waaaaayyyy more than the $30M that posts mentions. More like 10x that figure. $\endgroup$ – reirab Aug 17 '18 at 19:19
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In addition to all of the reasons already mentioned (jet bridge alignment, alignment with safety markings on the ground, positioning relative to ground equipment, and staying out of taxi lanes,) another important reason for jets to be parked accurately (laterally) is keeping their wings in the proper bounds.

Jet airliners tend to have rather long wings. When you're parking a bunch of said jet airliners in a row, you need to make sure that each of their wings will stay within the proper bounds so that they don't clip either ground equipment or the wingtip of one of the adjacently-parked aircraft. Each parking stand is designed for aircraft up to a certain wingspan, so, as long as the aircraft are parked correctly, an aircraft within the size limits for its stand is guaranteed not to clip wings with the adjacent aircraft if it is parked correctly. If it is not parked correctly, all bets are off.

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    $\begingroup$ We certainly had that issue at PWM where I worked. I worked for United (gross) and United controlled Gate 2. We frequently parked their Embraer 170/175s there. Next to us at Gate 3 was Southwest, who only operates Boeing 737s. We'd often need to coordinate with the Southwest rampers and the fuel truckers to get equipment maneuvered and out of the way when we both had aircraft on the ground at the same time. Wingtips came very close, like maybe ten feet between aircraft, if that. It made push back operations rather intimidating. Another good reason why its important to park a plane well. $\endgroup$ – Jonathan Kuhl Aug 20 '18 at 1:19
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In addition to all the answers stated here, even in airfields where there are no fixed ground equipment, the reason the aircrafts must park accurately is to get the best use of the available space. If the airfield is crowded, each extra parking spot or free taxiway is a bonus. If everyone park their aircraft like some people park their cars however they want, the amount of aircrafts an airfield can receive will decrease.

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