This is an excerpt from John Dullighans wonderful "Cinderella Bird"
Most of the time, the H-46 was flown just like any other helicopter, especially if it was in formation with others. But if you were assigned a medevac, especially if the LZ might be ‘hot’, a maneuver called the ‘Buttonhook’ was particularly useful. H-46 pilots claimed that by flying a ‘Buttonhook approach, the ‘Phrog’ could get into a LZ quicker than any single rotor helicopter. The less time the aircraft was exposed to hostile fire the better so it was a good idea to get into the landing zone, land, pick up the casualties and get out as quickly as possible. Doing a long straight in approach from altitude, gradually slowing into a hover would get you shot down very quickly. The Buttonhook was started directly from cruise speed and low level by setting up a high drag turn which resulted in rapidly decreasing speed and decreasing radius of turn which resulted in an increasing rate of turn. The airplane was never going in the direction it was pointing and it was rapidly decelerating.
H-46 pilots used the characteristics of the twin rotors to develop the 'Buttonhook', so called because from above, the maneuver traced a path over the ground that looked like an old fashioned buttonhook. It was most effective when the desired landing direction was 180 degrees from the initial heading, but the heading difference could be as low as 75 degrees. The aircraft appeared to start into a normal turn but as it slowed rapidly it rotated sharply, seeming to hang in space at the end before swiveling into position.
A ‘Buttonhook’ is begun at an airspeed above 120 knots at an altitude no higher than one hundred feet. The start position is close abeam the intended point of landing/hover, one hundred meters but no more than two hundred. First reduce collective pitch to a minimum (trying not to disengage the sprague clutches and dropping the engines off line) then raise the nose of the aircraft from five to twenty degrees, thereby maintaining or reducing altitude slightly. Then roll the aircraft to bank angle not exceeding forty-five degrees while applying sufficient yaw input to maintain balanced flight. At the same time smoothly come back with the stick (full aft cyclic) and increase collective pitch, which will cause the aircraft to "hook" around in the direction desired with no gain in altitude. The desired result is a controlled loss of altitude accompanied by a substantial loss of airspeed; usually dropping from 120 knots to below 40 knots in five seconds. The increase in collective pitch must be substantial to ensure engines are on line and spooling upwards, in preparation for landing power. Then as the aircraft rotates to the desired heading, roll out level and increase collective to full power if necessary. The aircraft is now almost stopped in a position to transition to a hover or a landing whichever is desired. This maneuver required practice to get right. Common mistakes were:
· Not keeping power on during the entire maneuver, so that on roll out on the desired heading, the engines are at idle and cannot spool up quickly enough to prevent a hard landing. A hard landing could result merely loosening the fillings in your teeth to requiring the airplane to be carried back home slung from a Sky Crane.
· Maintaining too low a collective pitch setting. This extends the maneuver by not slowing the aircraft rapidly enough thus overshooting the landing/hover area and having to 'drive' to the zone after aligning the aircraft on the new heading. This left the aircraft low and slow and out of position, extremely dangerous if there was any opposition.
· Not reducing the collective sufficiently, resulting in a climb in the sharp turn. This increase in altitude unnecessarily exposed the aircraft and extended the time it took to achieve landing or hover.
· Allowing the nose to drop during the sharp turn to new heading. This sets up a dangerous, low-powered rate of descent at very low altitude.
It wasn't taught in flight school but any pilot who got shot at, quickly learned it from the experienced pilots. The 'Phrog' had so much excess power most of the time, it was rarely at maximum weight, and would accelerate much quicker than a Huey. You could jink nicely with it since it had a very high crosswind component, thirty knots at least, so you could point somewhere other than the track you were making, at the expense of higher drag.
To learn a ‘Buttonhook’ was not easy and there were many hard landings. But the typical pilot in Vietnam was flying most days, well over one hundred hours in the average month, sometimes more. When you fly that much, you get good. The aircraft seems to do what you want without any conscious thought. Yes, it was dangerous, but it saved lives. My friends from that era, all now retired, conclude that there is nobody left in the Marine Corps who can fly a buttonhook. I described the maneuver recently to a young Marine H-46 pilot, and I could tell that he found it hard to believe that people did such things. He certainly wasn’t prepared to try one. And given the amount of time he flew, he was wise. But if he was being shot at, he might change his mind.
The entire text can be viewed @ http://www.airsceneuk.org.uk/oldstuff/h46/h46.htm