Between 1944 and 1945, a US Photo Reconnaissance Squadron or (or something alike) did a survey of the whole Chilean territory using the Trimetrogon camera system (228 rolls of film and about 49.000 photographies). This system consisted of three Fairchild K-17 cameras with 6" Metrogon lenses pointing left, right and down.

I'm a glaciologist doing research using those old images and I'm trying to learn more about the mission that carried out the survey. This is not only out of curiosity, as to do topographic reconstruction I need to resolve the lens distortion parameters and I need to know if all images were taken by a single system in a single plane, or if there were multiple systems involved.

The photographs have markings like this: enter image description here To interpret these markings a manual of the time gave the example "137PS-3M109-11-V-121" where the information was interpreted as follows (1) 137PS. Photograph made by 137th Photo Reconnaissance Squadron USAAF. (2) 3M109. The year is 1943, indicated by the fourth digit of the calendar year, that is, 3 for 1943. The letter M stands for mission, 109 for the mission number

Therefore, from the information in the picture above, the survey in Chile was done by a unit identified as 91PC but I've been unable to find a Photo or Recognnissance squadron/group/wing or whatever that fits this code (See UPDATE at the end below images). The number "5" stands for year 1945 and 4028 would be the mission number. But I haven't been able to find any information of such mission.

Good progress would be to identify the model of the plane(s) that carried out the survey, but the only clue I have are thousands of pictures porting the tip of the wing. But I don't know if that is possible for such a small clue. Can anyone identify this plane or has any further information about this surveying mission?

Below some examples of images of the wing:

enter image description here enter image description here enter image description here enter image description here enter image description here

UPDATE: On the Wikipedia page of the 311th Air Division it mentions the "91st Photographic Mapping Squadron (later 91st Photographic Charting Squadron, 91st Reconnaissance Squadron): 5 October 1944 – 26 August 1946". So, 91st Photographic Charting Squadron seems to fit the "91PC", and the dates match too. Anyway I haven't find much details about the missions performed by this squadron, planes used or any other detail.

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    $\begingroup$ Have you contacted both the Army and Air Force to ask them if they have information (long since declassified, I'd imagine) about these missions? I'm not sure which service would have the info now, but that seems like this would be the most accurate source of information for you. They may be able to not only give you a complete "decoder ring" for the text on each image, but give you information about the aircraft and cameras in use. $\endgroup$
    – FreeMan
    Apr 7, 2022 at 13:08

1 Answer 1


It looked too short to be a C-47 wing, which meant it was most likely a Beech 18. And sure enough, in the cropped image below (taken from a copyrighted image - uncropped pic at the link above), you can see the same aileron trim tab arrangement with an access panel just forward of it as the left wing in your images.

So there you go; it's a Beechcraft Model 18, widely used by the US military in WW2 as a utility light cargo, trainer, and short range executive transport airplane. It makes perfect sense that the photo recon missions you described would use these airplanes.

enter image description here

  • $\begingroup$ Thanks for the answer! However, I haven't been able to find any reference of the Trimetrogon system mounted on that plane. I found several references of it been mounted in a B-17, and to me, looking at pictures look more similar than this one. For example cnn.com/2019/10/02/us/b-17-flying-fortress-bombers-wwii/… $\endgroup$ Apr 7, 2022 at 4:14
  • $\begingroup$ I found a reference for Trimetrogon images on a Beech 18, and the images look very similar. I would still like to hear why you discarded the B-17 as an option. $\endgroup$ Apr 7, 2022 at 4:55
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    $\begingroup$ If the camera was mounted in the fuselage, the wing image of the B-17 would be way longer/more slender, and we might see an outboard engine in it. But mainly, the Beech aileron trim tab is near the centre of the aileron, the B17 one is at the inboard edge. So yeah, it does not look like a B17, it does like a Beech 18 $\endgroup$
    – Koyovis
    Apr 7, 2022 at 8:02
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    $\begingroup$ A little detail in support of this: the WP page for the 91st (in all its various guises) does briefly mention them operating F-2s, the photo-recon version of the Model 18 (in addition to both B-25/F-10 and B-17/F-9); source seems to be here $\endgroup$ Apr 7, 2022 at 11:38
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    $\begingroup$ Also as you can see here: en.wikipedia.org/wiki/Boeing_B-17_Flying_Fortress#/media/… the B-17's trim tab is at the inboard end, and it has a split flap so the wing skin inboard of the aileron is continuous to the trailing edge. $\endgroup$
    – John K
    Apr 7, 2022 at 11:43

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