It may surprise you, but helicopters can routinely and safely land on the flight deck of smaller
ships in rough seas, while the ship is not only moving, but also rolling from side to side.
Even though there is very little room for error,
the landing is a bit easier than you might think, thanks to the beartrap.
That said, sometimes things can go wrong, and for that reason, there are two standard
emergency procedures for helicopters that cannot land on the deck of a ship.
But there is also a third option, a non-standard one I must say, and it is not what you think.
Wind speed, ship motion, and sea states all affect helicopter landings at sea.
Moreover the airwake from a ship's superstructure can also impact helicopter performance.
These and other factors, are combined into Ship Helicopter Operating Limits, or SHOL, or S-HOL.
SHOL is a polar plot of acceptable conditions in which performing flight operations is considered
safe. Stay within these operating limits, and a helicopter should be able to land safely.
But back in the day, landing a helicopter in rough seas
was considered to be outside of the operating limits and therefore was not a practice.
In the mid-1950s, navies were looking to introduce ship-based helicopters for specific missions,
but not being able to land those choppers in high seas
put a dent into those plans. That is until the late 1950s and early 1960’s,
when the Royal Canadian Navy started experimenting with a device that became known as a “beartrap”.
A beartrap is a rectangular device that allows helicopters to depart and land from smaller ships
such as frigates and destroyers in a wide range of conditions. The invention of beartrap permitted
the operation of helicopters on smaller vessels in rough seas up to sea states 6,
allowing them to handle 30 degrees of roll and 9 degrees of pitch, during day and night.
Following the success of Canadians with the beartrap, other navies,
including the Royal Navy and the US Navy adopted similar technologies.
A Canadian beartrap works as follows. A probe is attached to a cable which is lowered from the
helicopter to the flight deck. The flight deck crew, then using their “bare hands”,
attaches the cable through the center of the bear trap to a winch below the flight deck.
The cable is kept under constant tension which is controlled by an
automatic hydraulics system. This synchronizes the ship with the helicopter. When the pilot
and the Landing Signal Officer are comfortable that the motion of the helicopter and the ship
is relatively stable, the pilot decreases the power and the LSO increases the cable's tension.
As the pilot flies the helicopter down, the winch pulls the helicopter’s main probe into the center
of the beartrap which is then closed. Beartrap not only can catch and secure the helicopter in
place, but it can also move the aircraft in and out of the hangar using a rail system.
The original beartrap system, even though very helpful, had a disadvantage. Since it required
the flight deck crew to be on the deck to handle the cable, in stormy conditions,
there was a risk of man overboard. That’s why more advanced versions of beartrap were developed,
like Curtiss-Wright’s advanced handling technology known as ASIST. ASIST uses an optical positioning
system that continuously tracks and monitors the exact position of the helicopter relative
to the designated landing area. As the helicopter touches down, it is immediately secured using the
beartrap and then aligned and traversed into the hangar without requiring any personnel on deck.
Some navies refer to beartrap as Recovery Assist Secure and Traverse or RAST,
or MacTaggart Trigon system. While multiple versions of recovery assist exist, most rely on
a grappling device or a probe that is secured when the helicopter lands and the beartrap is “closed”.
An alternative to beartraps is a deck lock system which uses a steel grid in the middle
of the flight deck. The pilot hovers over the grid, waiting for the right moment to descend.
Once landed, a locking harpoon is deployed which engages the hydraulic actuator system
that pulls the helicopter onto the deck. The benefit of this system is that it
doesn't require any personnel to approach the helicopter, especially during rough seas.
A version of the deck lock system that utilizes a harpoon mechanism
was developed in the UK back in the 1970s, and was tested on this moving platform. A nice
feature of this system is that it can rotate the helicopter 360 degrees which simplifies handling.
For smaller surface combatants that rely on helicopters,
beartrap and other similar systems have strategic importance. In 2014, the Japanese government
expressed concern about a French company that sold their system to China. A few years later, in 2017,
Japan developed the first system that could automatically land a helicopter on a moving ship.
This system relies on GPS navigation to get close to the ship. Then the laser sensors are activated
on both vessels which calculate the relative position of the helicopter to the moving ship with
an error range of just a few inches. This allows the helicopter to land safely and automatically.
All this said, there are scenarios under which helicopters cannot land on the flight deck,
whether it’s a helicopter malfunction or an issue on the flight deck. Sadly,
there have been attempted landings that have resulted in fatalities. While emergencies
cannot always be prevented, emergency drills can increase the odds of survival.
The first alternative to landing on the ship, is to land somewhere else! Of course, this may not
always be possible, but since helicopters can be refueled from the ship while still airborne,
the refueling can increase the helicopter’s range to its maximum, which in some cases can provide
an alternative landing area somewhere else. Helicopter in-Flight Refueling or HIFR begins
when the helicopter lowers a bag attached to a cable onto the flight deck. The refueling crew
then attach the fuel hose to the cable, which is in turn hoisted back onto the heli and hooked up
to the fuel tank. Once fueling is done, the hose is lowered and the helicopter flies away. In case
you are wondering why there was a bag attached to the cable, that bag is used to pull up a fuel
sample that is visually inspected by the flight crew for fuel contaminants. The weight of the bag
also prevents the hoist cable from swinging around as it is being lowered onto the flight deck.
When landing on a ship is not possible, the second option is to land … on water.
This is what happened during a replenishment mission where a CH-46 Sea Knight experienced
an engine failure over the deck of USNS Spica. The pilot immediately reacted by releasing the load,
and clearing the deck. Next, by relying on autorotation, the pilot was able to
successfully land the helicopter on water. The Sea Knight capsized to the right,
as the crew escaped the fast-sinking vehicle. Everyone made it out alive, thanks to the
emergency egress training that the crew would have previously received. This was also one of
the first times that “HEEDS” bottles were used by the helicopter crew in a real life emergency.
Finally, the third emergency option, and this is quite a rare one,
but what do you do when multiple helicopters have to land on a ship, and there is only room for one?
This situation in fact came up when the United States was evacuating its troops from Vietnam.
As the evacuation fleet was sailing away, several South Vietnamee’s helicopters approached the ship,
and they were packed with refugees who were trying to escape. In a humanitarian effort,
the helicopters were allowed to land, one at a time! That’s right! After each landing,
the crew skidded the helicopter off the ship, to make room for the next one chopper to land.
In some cases, the pilot dropped off their passengers, then crash landed in water near
the ship to be rescued. But if you think this was impressive, which it was, I should remind you that
during these evacuations, a Cessna O-1 dropped a note on the deck of the American aircraft carrier,
USS Midway. The note read "Can you move these helicopter to the other side,
I can land on your runway, I can fly 1 hour more, we have enough time to move. Please rescue me.
Major Buang, Wife and 5 child.". That was a Vietnamese pilot and his family.
Midway's commanding officer ordered the clearing of the landing area,
where an estimated $10 million dollars worth of UH-1 Huey helicopters were pushed overboard.
Major Buang safely touched down, becoming the first South Vietnam Air Force fixed-wing pilot
to ever land on a carrier. Another Cessna O-1 was also recovered by USS Midway later that afternoon.