# Two planes at the "Same" Altitude, only one leaves a trail

Discussion in 'Skydentify - What is that Thing in the Sky?' started by Mick West, Aug 31, 2014.

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1. ### Mick WestAdministratorStaff Member

The first thing to notice here is that the planes are not actually the same type:

But even if they were, the visual difference in size between a plane at 36,000 feet, and another at 38,000 feet is just 5%. In this image, the visually smaller plane on the left is 57 pixels long, the large plane is 67. So if the plane on the right were at 36,000 feet, and the plane on the left were the same model of plane, then going by the lengths, the plane on the left would be at 36000*67/57 = about 42,000 feet, or 6000 feet higher.

And given that contrailing humidity can vary in as little as 1000 feet, that's plenty of room for there to be a difference.

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2. ### WeedWhackerSenior Member

Agreed.

There are a few common "keys" when viewing (or "planespotting"....an actual hobby of a surprising number of people. It is similar to "trainspotting" as another hobby focus).

In the OP photo, the airplane to the left (and therefore lower in altitude, as will become clear) is a narrow-body twin-jet....either a B737 or...I will go out on a limb here and identify it as in the Airbus A319/320 "family"...(NOT an A321...the fuselage is too short).

The other jet, physically "behind" but obviously at a different altitude (because of the parallax that distorts the apparent "proximity") is obviously a different airplane than the first. It might very well be another "narrow-body"....likely a B757....the apparent change (reduction) in fuselage diameter indicates that the second (right-hand of the image) airplane is higher...meaning, farther away from the camera when the photo was taken. (As is well known, I hope by now....the current "normal" vertical altitude difference is, worldwide. 1,000 feet. Since these two are on the same course and Jet Airway Route {at the moment of the snapshot}, in same direction, then normally...usually...they would be separated by 2,000 feet vertically. This is because of the typical "standard" of West-bound is 'even', and East-bound is "odd". Still for ATC purposes, the minimum of 1,000 feet can be implemented, when required, regardless of actual course direction).

To my trained eye, I would propose that this second (right side of image) airplane IS a B-757-200.

It is important to note that both the Airbus A-320 "family" and the Boeing B-757 "family" are approximately the same in fuselage diameter. Within 6 to 8 inches.

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3. ### TrailblazerModeratorStaff Member

I have also seen this picture passed around on Twitter. It astonishes me that anybody would claim that these planes were at the same altitude. Do they really think passenger aircraft fly one plane-length apart?

Could someone with the requisite aviation knowledge tell me what would be the minimum required horizontal separation if both aircraft were at the same altitude, as claimed?

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4. ### WeedWhackerSenior Member

Depends on the altitude, and thus the airspeeds. In a radar-environment AT typical cruise altitudes and speeds, it is 10 Nautical Miles ('NM'). (THIS is dependent on the distances OF the airplanes under radar-control from the antennas used to monitor them).

However, for the purposes of ATC ('ATC' = 'Air traffic Control') convenience (to reduce the workload on the controllers) they will typically use 15 to 20 NM at cruise altitude. This tends to be a "personal preference" per the Controller. This also allows for variations that occur in Actual Winds Aloft at altitude, as the airplanes move through the atmosphere. (IOW....a jet that is, say "leading the pack" can encounter sudden headwinds, and thus its groundspeed will be affected. The ones behind will then seem to be "gaining" on this 'leader'. This is an over-simplification, but used to illustrate the concept that in a DYNAMIC situation the Air Traffic Controller must manage).

In non-radar (such as over-water, mountainous areas, etc) the minimum that is strove for is 20 NM. BUT, in such circumstances, it is usually calculated in minutes, based on each airplane's ATC airspeed, as filed. (AND also, at Non-Radar Position Reporting Waypoints....the actual current Winds Aloft are provided, to ATC....included in the "Position Report". This helps to "update" what was previously forecast. A margin or "safety buffer" is usually added, to ensure sufficient spacing and altitude conflict problems (for airplanes that wish to change altitude).....these "buffers" will vary per ATC facility, worldwide.

The following text, below, veers away from the claims of "chem"trails at cruise altitude:

At altitudes where contrails do not form, such as takeoff/landing scenarios....the minimum is 3 NM....or, 5 NM when the preceding airplane is a "Heavy"(**). Not sure what the "In-Trail Spacing" is for a "Super-Heavy"....that would be only one airplane, the A-380 at present. (EDIT: Please see the PDF below....the spacing behind an A-380 varies from 6, 8 or 10 NM...depending on airplane type that is "in-trail").

(**) The classification (for ATC) as a "Heavy" has changed a bit over the years. 300,000 pounds (in the USA) was the typical number....but U.S. standards have tried to comply with ICAO (International) standards more and more. (Because of the "conversion" aspect between units: "pounds", "tonnes" {U.K., Canada and other places} and the Metric units).

...here is a Wiki ref. that is accurate (AFAIK):

http://en.wikipedia.org/wiki/Heavy_(aeronautics)

The B-757-200, although NOT capable of 300.000 pounds at takeoff...nevertheless was seen to produce significant wake turbulence, so it "falls" into the category, and is treated as "Large/Heavy" by ATC personnel when they sequence flights for takeoff and landing. (Even though the pilots of a B-757-200 do not need to add the "heavy" to their radio callsign)

This PDF may also be pertinent:
http://www.faa.gov/documentLibrary/media/Notice/N7110.567.pdf

Last edited: Aug 31, 2014
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6. ### WeedWhackerSenior Member

Yeah...kinda what I wrote above, except with a bit more detail. What's important to emphasize, in this context is....altitude (vertically).

1,000 feet. Yeah, to the non-pilot might not seem like much....(except it's just a bit less than the height of the TALLEST of the WTC Towers!!!)...BUT pilots understand that in terms of our "Vertical Velocity"...it can vary from....from as small as....say, 50 feet-per-minute to 4,000 feet-per-minute (or greater, when descending).

But, those ranges (although not absolute, it ALL depends upon airplane performance capabilities) are somewhat irrelevant, when it comes to nearby traffic, in airline operations. NO ONE wants to have a mid-air collision!!

This is just ONE (of many) reasons why TCAS is mandated.

TCAS has resulted in MANY, MANY safe resolutions since its inception....I dare-say there are online resources to back up this claim.

(ETA).....for MANY decades 1,000 feet vertically was the STANDARD for vertical separation for ALL airplanes that operated under Instrument Flight Rules....since the 1940s!!!

(ETA: AND, consider that airplanes (in the U.S.) that were NOT on IFR Flight Plans, but operating under VFR ('Visual Flight Rules") used altitudes only 500 feet off....IOW...a west-bound VFR airplane would choose 2,500, or 4,500, or 6,500 feet (ETC). East bound, used 'odd'-numbered thousands, but 500 feet off. SO....ALL of these VFR airplanes, which had NO REASON to contact ATC (except in certain airspace limitations) could operate, free and clear. THAT is the reality).

This 1,000-foot limit was good UNTIL the era of high-altitude flight, when there were some concerns about the accuracy of the pitot-static systems then....SO...
FL290 (or, "29,000 feet") became a "cut-off" point, back in the 1960s. THIS remained the "standard" until the late 1990s.....when RVSM standards were implemented, again ABOVE FL 290.

Last edited: Aug 31, 2014
7. ### WeedWhackerSenior Member

I am ADDING to my post, just above....for public consumption.

It is increasingly difficult to attempt to "teach" the public....MANY of whom are already well informed, BTW!!

But, there is an aspect of aviation that is SO specific....there is SO much to learn.....it seems (sometimes) nearly impossible to help those who are NOT a "professional pilot", much less an aviator of any type, to comprehend.

Hence, the "chem"trail myth. It propagates....and persists. (Pun). Because, CONtrails persist, and they (when conditions are suitable aloft) can propagate!!

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8. ### JuhaMember

IMHO first plane is higher and a small wide body. Perhaps A310 or 767-200? Well that's irrelevant anyway.

But they can not be at same level. 757s TCAS would be screaming F-words if they were.

9. ### cloudspotterSenior Member

Wasn't there another picture very similar to this where the two planes are actually overlapping? Pretty sure the one on the left is a Virgin Atlantic A320.

Last edited: Sep 1, 2014
10. ### JuhaMember

Slightly too much white at front of cowling for a Virgin?
Albeit there is lot's of similar liveries as Virgin.

11. ### cloudspotterSenior Member

I did of course mean to say the one on the left

So did I.

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14. ### JuhaMember

Ok, apology to cloudspotter. It defenetly looks in that picture a Virgin(or similar). And it's lower and narrow body. A320.

At first picture it looked like 1/3 from cowling edge is white. It's not.

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15. ### cloudspotterSenior Member

That's the one I'd seen. Couldn't recall if it had been posted on MB though.

16. ### Mick WestAdministratorStaff Member

Doh, there was a discussion Facebook about this a couple of weeks ago, but in a different context, someone thought one of the enhanced photos was fake:
There's some good skydentification in the comments there.
Although I think his original estimate of 30,000/38,000 is the more likely one, what with there being contrails.

The original full set of photos is here: