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I saw this picture in a local newspaper.

Planes with exhaust trails

My thoughts were, silly artists, that's the APU exhaust, it won't ever produce a trail like that during takeoff (or ever). Of course, the picture is clearly photoshopped, as can be seen by the planes' outlines. But somebody didn't do the research :)

Just to be sure, were my thoughts correct?

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  • $\begingroup$ Which paper? De Telegraaf? metro? AD? zucht... $\endgroup$
    – DeltaLima ♦
    Jun 8, 2015 at 12:29
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    $\begingroup$ those are obviously smoke trails and the aircrafts are performing an aerobatic display. </sarcarsm> $\endgroup$
    – Federico
    Jun 8, 2015 at 12:31
  • $\begingroup$ @DeltaLima Gelderlander $\endgroup$
    – Bart van Heukelom
    Jun 8, 2015 at 14:23
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    $\begingroup$ Not sure they meet minimum separation rules either... $\endgroup$
    – Flexo
    Jun 8, 2015 at 20:03
  • $\begingroup$ @Federico chemtrails, the evidence is in! $\endgroup$
    – jwenting
    Nov 2, 2016 at 8:59

3 Answers 3

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APU may be used in flight as second source of power or bleed air if one of the engine systems is inoperative. If it is so used, it might leave a contrail the same way engines will, though as DeltaLima notes, it's exhaust is hotter and the air behind the fuselage is more turbulent, so the range of conditions where it will leave a contrail is narrower than for the engines.

And it is an exceptional situation, so it is extremely rare. Most of the time the APU is switched off once the engines are running and only switched on again after landing, because when engines are running, they provide everything the aircraft needs.

In B737NG (the lower aircraft in the picture), the APU can be used as bleed air source up to 17,000 ft. This is too low for normal cruise, but if bleed system fails, pilots will descend to this altitude and start APU to provide air-conditioning for the remainder of flight to a suitable nearby airport. It can also be used up to 41,000 ft as power source, so the aircraft may cruise with one generator inoperative and running APU.

In A320 (the upper aircraft in the picture), the APU can be used together with one bleed air system up to 20,000 ft and for power generation up to 41,000 ft. So again, the aircraft may cruise with one generator inoperative and running APU.

However:

  • Normally there would never be contrail behind APU and none behind engines unless both engines failed and APU was started as backup power source (US1549 did start APU when the engines failed, though it is unlikely there was contrail at that altitude).
  • Contrail does not start right behind the engines. The water in the exhaust needs some time to cool down and freeze before the contrail becomes visible.
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You are right. Newspapers are rarely able to produce anything about aviation without letting their imagination fly.

First of all, the APU doesn't produce contrails. Second, contrails don't start at the exhaust but at a distance behind it.

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  • $\begingroup$ The APU could produce contrail if it is operating at cruise level. It just rarely is. $\endgroup$
    – Jan Hudec
    Jun 8, 2015 at 12:28
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    $\begingroup$ @JanHudec very unlikely as it isn't exactly a high by-pass system, so the exhaust air is relatively hot. This causes the condensation to happen quite far behind the aircraft. Since the exhaust is in the turbulent wake of the fuselage, the exhaust air will get mixed very quickly with the ambient air so there will be hardly anything visible. Now of course there are circumstances (humid air, just above dew-point temperature) that might cause an APU contrail but probability is very low. $\endgroup$
    – DeltaLima ♦
    Jun 8, 2015 at 12:33
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    $\begingroup$ Straight jets produce contrails too (older fighter jets do) $\endgroup$
    – Chris V
    Jun 8, 2015 at 15:28
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    $\begingroup$ A B-17 can produce contrails. aerospaceweb.org/question/atmosphere/contrails/… $\endgroup$
    – Sports Racer
    Jun 8, 2015 at 22:37
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    $\begingroup$ @DeltaLima ANY heat engine consuming hydrocarbons is going to produce contrails if the air the exhaust exits into is sufficiently cold enough or at a low enough pressure to be below the dewpoint. $\endgroup$
    – Romeo_4808N
    Nov 1, 2016 at 22:48
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Its photoshopped. If you look closely enough you will notice that the contrail from the A320 is a scaled down version of the one from the B737.

Something tells me the image came from a hysterical chemtrails conspiracy website.

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