Good question and I don't know the answer. Most of my experience has been as a ground pounder, so we don't often interact with signals designed to attract attention from the air. I have occasionally ridden as an observer and as cargo enroute to a remote dropoff point. In that context, we would investigate any kind of signal that might be connected to our operation.

That doesn't say anything about the signal that might be in view of non-SAR aircraft however.

I'll bet AKSAR has a more definitive answer....

I am reminded of the Gary Larson cartoon showing a helo in flight and HELF spelled out on the ground, with the caption along the lines of "Sorry, I thought it said HELP - my mistake"

one of his very best...

I suspect that very few people these days have heard of Morse code, or cn recognize any specific letter. But it doesn't hurt in the least to try. Most ETSers will know....

In the Continental USA, I'm told the Civil Air Patrol (CAP) is the lead agency for SAR air search. Their training material is on the CAP site: http://nesa.cap.gov and Ground to Air and Air to Ground signals are covered in the curriculum for several roles.

The Ground Crew curriculum is at:
http://nesa.cap.gov/gsar-curriculum/
In Task Guide: Ground & Urban Team Task Guide
See "Task O-0703 EMPLOY GROUND TO AIR SIGNALS" on pages 148-150 of "Ground & Urban Direction Finding Team Tasks 24 May 2004"[1][2]

The Mission Aircrew School Curriculum is at:
http://nesa.cap.gov/mas-curriculum-2
In Mission Scanner Task Guide - Dec 14
see "MS O-2021 INTREPRET EMERGENCY SIGNALS AND DEMONSTRATE AIR/GROUND TEAM COORDINATION"
{ at a quick glance, these seem to be a superset of the Ground Crew signals. Also, the "Mission Scanner" is the guy in the plane who is looking for the survivor. Looking at the pilot training - they are taught to signal to the ground, but not to interpret ground signals}

[1] http://nesa.cap.gov/gsar-curriculum/ also has
two other documents that seem pertinent:
Task Guide: GTL & GTM Reference Guide
(see Chapter 10, pp. 77-83)
Power Point: Air to Ground Coordination
[2] My personal fetish, signal mirrors, are covered in
Task O-702, "USE A SIGNAL MIRROR" of the Ground & Urban Team Task Guide, and in Chapter 10, p81 of the GTL & GTM Reference Guide, though there are some errors in the instructions. I wonder how to submit errata? Some US government manuals have an explicit procedure for that ...

AKSAR, that is exactly what I was looking for, thanks. I will go through these and the other resources and see if there is a conflict or just a problem with incomplete listings of signals.

Thank rafowell, not me!

In my experience, spotting a person from the air (as opposed to the wreckage of an aircraft) is extremely difficult, at least in our local terrain. I would suggest that the single most important thing, rather than some specific signal, is to wear or have available some bright, contrasting clothing. An orange jacket or vest stands out from miles away.

Years ago, a series of experiments were conducted in Arizona to attempt to quatify the POD (Probability Of Detection) of a subject from a helicopter. I believe hikermor may have participated in those experiments? The studies are available on the saraz.org Documents page. They did a Mountain Searches experiment, and a Desert Searches experiment. In my experience here in Alaska, their PODs were quite optimistic. That's probably due to the fact that we tend to have a lot more brush and ground cover.

My bottom line advice is if you want to get found from the air, wear some bright orange clothing!

I love that cartoon!

Spoiler alert ... Houston, I think we have a problem.

The CAP training for ground-air distress signals for the CAP people on the ground lists the 5 internationally standard (line-style) distress signals[1], the first (and arguably most important of which) is:

"V" for "require assistance"

The CAP training for the line-style) ground-air distress signals for the airman tasked with looking for them (Mission Scanner)[2] ...

Doesn't list "V" at all!

What we have here is a (rather appalling) failure to communicate. The remaining four standard signals are included in the 25(!) signals taught to mission scanners, but the scanner is given the incorrect definition for a second one. The standard definition of "X" is "Need Medical Assistance" and the scanner is taught the archaic definition[3] that "X" means "Unable to Proceed", which could be a fatal miscommunication. (I sent a note to the CAP academy pointing this out - hopefully they have an errata sheet or something.)

[1] The 5 line-style distress signals taught the "ground people" are precisely those listed in these references:

(a) "Joint Pub 3-50: National Search and Rescue Manual Volume I: National Search and Rescue System", 1 February 1991, Appendix C: Emergency Signals, page C-2, where they are listed as "IMO/ICAO" signals (IMO == International Maritime Organization; ICAO = International Civil Aviation Organization) in the figure: "Figure C-1. Surface-Air Visual Signal for Use by Survivors" This document is at http://www.dtic.mil/docs/citations/ADA357500
and a searchable version at www.public.navy.mil/surfor/Documents/3-50-1_Vol1.pdf

(b)the Feb 2018 version (hopefully current :/) of the Australian National Search and Rescue Manual, page 326, in Table D-1:2 Ground-Air Visual Signal Code for Use by Survivors, which is preceded by the words: "The following visual signals are internationally recognised. They are authorised for use in the Australian SRR."That document can be downloaded from https://natsar.amsa.gov.au/natsar-manual.asp

(c) The 15 July 1985 version of United States Air Force AF Regulation 64-4, Volume 1, Search and Rescue
SURVIVAL TRAINING, Figure 24-19, p. 471 (see p. 473 for the postural signals). This document is at http://www.dtic.mil/docs/citations/ADA325861
[The fact that this document is freely downloadable is not widely advertised. Also - great section on signal mirrors!]

[2] MS O-2021 INTREPRET EMERGENCY SIGNALS AND DEMONSTRATE AIR/GROUND TEAM COORDINATION

[3] Back in 1956, "V" was not on the list, and "X" was listed as "Unable to Proceed", but both were fixed by 1985. Here's the 1956 table: https://archive.org/stream/DTIC_ADA367029#page/n17/mode/2up/search/signals

Hearty agreement, and it applies not just for aerial searches. I have spent way too much time pounding on the ground looking for pastel tinted victims who blended into the landscape effortlessly. Bright colors rule, and personally, while shy and mild mannered (just ask Mrs. Hikekrmor), I always have something bright handy. The best ever was a reversible down jacket, loden green on one side and international orange on the other.

I remember the desert search experiments quite well, being on occasion one of the eagle-eyed participants in the helo. I had moved to Channel Islands National Park by the time of the mountain work.

When I recall those experiments, I inevitably think of John Bownds, principal author of the work, an extremely active and effective SARA member, who contracted Valley Fever, most likely from inhaling road dust while conducting these trials. The disease forced him to leave Tucson within a few years and took him from us before he attained 50 years of age -a truly fine and capable person.

When flying low in commercial planes (mostly just before landing) I have been surprised by how visible blue objects are - tarps on roofs, pools, ...

That has led me to wonder if bright blue would be a better color for detection - especially during the fall when many trees in the midwest lean toward the red and orange colors.

Any experience on that?

Good point about fall colors. The important factor is contrast, in order to stand out from the background. In addition to color, motion is very important, if not even more critical.

i am a fan of signal mirrors in daylight and a fire at night (properly maintained!! - epic wildfires have been generated by those who didn't. The combination of light and smoke will attract attention, while warming you.