Here it is again !
https://airfactsjournal.com/2020/09/stop-calling-it-the-impossible-turn/
Obviously, the advice has to be taken with pitch of salt as the author flies PC12 from US federal 5km runways, at least my instructor who told me it’s doable flies one in 600m (of course any PC12 pilot will tell you it’s doable, for AOC flying they have to show CAA/FAA they can make a runway on departure/en-route no matter what happens 
)
The maths are easy, in PC12 you get +17% climb gradient and -6% decent glide and you only need 400m to stop
Guess what? PA18 does +20% climb and -10% glide and you only need 200m to stop
Guess what? C172 does +9% climb and -11% glide and you need 300m to stop
So, in reality you will never make a “strict 180 back” in a heavy loaded C172, especially with pilots struggling to clear the 50ft obstacle in the first place, let alone do 180 back 
To compare types one has to account for winds (improves climb gradient & improves glide) but stopping distance with tailwind increases, also loss of height in 180turn or pushing nose down and assume VY, VTH, VBG are the same….
Ibra wrote:
for AOC flying they have to show CAA/FAA they can make a runway on departure/en-route no matter what happens
The ‘power-off 180deg turn to a precision landing’ is part of the FAA CPL syllabus and I had to do this earlier this year. The maneuver itself isn’t that hard, it’s the precision aspect (- 0 / + 200ft of a defined point on the rwy) that makes it a bit tricky.
What was the REAL eye-opener however was when we did the same maneuver with a tailwind during my training. It took me three attempts to get it halfway right. On the first try I prob90 wouldn’t even have made the 3500ft runway. This was with about a 3-4kt tailwind! While I did account for it, I totally underestimated the effect. Brilliant lesson, very glad we did that.
172driver wrote:
The ‘power-off 180deg turn to a precision landing’ is part of the FAA CPL syllabus and I had to do this earlier this year. The maneuver itself isn’t that hard, it’s the precision aspect (- 0 / + 200ft of a defined point on the rwy) that makes it a bit tricky.
I had the impression during FAA CPL examinations is from downwind to final on active runway no? or is it back to opposite runway after takeoff?
Ibra wrote:
I had the impression during FAA CPL examinations is from downwind to final on active runway no?
Yes, that’s correct. However, in a real EFATO you would prob90 land with a tailwind, that’s why I found the tailwind experience so enlightening.
Got it, yes 5kts tailwind landing is enlightening, +10kts tailwind landings the physics are hard to explain I am sure an EFATO is not the right moment for someone to learn about precision landing, reverse wind gradient and tail gusts…
Same as flying & navigation at 0ft agl (ahem 500agl), worth testing the skill & the route in sunny days and see if it’s your style or personality fit and not pick that when cloud-base is OVC005
The FAA now recognises that the “impossible turn” is not only not impossible but after 2018 instructors are required to demonstrate when it can be done safely.
Page A-12:
A.11.4 Return to Field/Engine Failure on Takeoff. Flight instructors should demonstrate and
teach trainees when and how to make a safe 180-degree turnback to the field after an
engine failure. Instructors should also train pilots of single-engine airplanes not to make
an emergency 180-degree turnback to the field after a failure unless altitude, best glide
requirements, and pilot skill allow for a safe return. This emergency procedure training
should occur at a safe altitude and should only be taught as a simulated engine-out
exercise. A critical part of conducting this training is for the flight instructor to be fully
aware of the need for diligence, the need to perform this maneuver properly, and the need
to avoid any potential for an accelerated stall in the turn. The flight instructor should
demonstrate the proper use of pitch and bank control to reduce load factor and lower the
stall speed during the turn. After completing this demonstration, the flight instructor
should allow the trainee to practice this procedure under the flight instructor’s
supervision. Flight instructors should also teach the typical altitude loss for the given
make and model flown during a 180-degree turn, while also teaching the pilot how to
make a safe, coordinated turn with a sufficient bank. These elements should give the pilot
the ability to determine quickly whether a turnback will have a successful outcome.
During the before-takeoff check, the expected loss of altitude in a turnback, plus a
sufficient safety factor, should be briefed and related to the altitude at which this
maneuver can be conducted safely. In addition, the effect of existing winds on the
preferred direction and the viability of a turnback should be considered as part of
the briefing.
I have actually never heard the term “the impossible turn” before reading it in some English speaking magazine, long after I got the PPL. The way I learned, and the way we still do it here is: below X alt – land straight ahead (on the runway preferably), above X but below Y or no more runway left – find a “suitable” field in the general direction of flight, above Y – turn back. The idea is simply to turn back when you have enough alt. Doing it below an altitude that allows a 180 + degree turn is of course physically impossible. So what this “impossible turn” is supposed to describe, still eludes me. That phrase is not a part of the vocabulary here, it’s all a matter of of high you need to be to be able to turn 180 degrees.
My home field extends straight out into the sea at 27. This means not turning towards land is a very stupid thing to do. It is more a matter of how far towards land are you able to get, and turning 180+ degrees is always preferable – if possible. The sensible pilot will therefore start at the far end of the runway, not at some intermediate point. This works well when taking off, but does not work all that well when training touch and go for instance.
