Precisely operating a helicopter to and from a small piece of land on a calm, sunny day is challenging enough. Imagine flying that same helicopter to and from a small spot on the deck of a small ship — in a storm and at night. This is the task naval helicopter pilots worldwide must do.
Compared to flying over land, low-level flight over the sea is more difficult. It is much harder to judge altitude and “groundspeed” by looking at moving waves instead of at stationary trees and house. At night, of course, the situation is worse, and the pilot must rely on instruments until he is close enough to the ship to use its lights as a visual-reference system.
It has been standard practice to make approaches on a 30- or 40-deg angle to the ship, to stay out of the ship’s turbulence as long as possible. For this type of approach, the pilot must solve some tricky navigation problems in his head to keep up with the ship and to account for winds.
If, for instance, the ship is raveling at 15 kt into a 20-kt wind, the helicopter must maintain a 35-kt component of airspeed along the ship’s path just to keep up. Considering the closure rate need to catch up, the pilot must hold an airspeed of about 45 to 50 kt during the final phases of the approach.
At this low airspeed, the helicopter is flying on the “backside of the power-curve.” Here, power management is difficult because it takes more power and collective pitch to fly slower instead of faster as in normal forward flight.
The objective is to solve all these navigational and piloting problem so the helicopter can be brought to a hover just over the landing spot on the stern. More than once, pilots thinking they were doing a good job have found themselves hovering 100 feet behind the ship. This situation is especially likely when visibility is poor, so electronic devices such as TACAN and radar are routinely used to provide range and closure rates.
If the pilot is having trouble with all of this, he may require more than one time-consuming attempt to get the helicopter safely onboard. For this reason, simpler straight-in, over-the-stern approaches have been recently recommended to speed up the operation, despite the complication of spending more time in the turbulence generated by the ship’s superstructure.
A Soviet pilot writing advice to new helicopter pilots points out that paying too much initial attention to the deck may lead one into trouble. There have been instances in the Soviet navy in which the pilot was continually chasing the moving deck, i.e. he was using the deck as a fixed reference as if it were a spot on land. If, for instance, he tried to maintain a descent angle of 6 deg with respect to the deck and the ship pitched up 7 deg, the pilot’s intended flight path would take him 1 deg under the water’s surface!
The ship prepares
There is also action aboard the ship prior to the arrival of the helicopter. About 15 minutes before, the ship will set “Flight Quarters”. All antennas, guns and railings that could interfere with the helicopter are stowed or lowered, and a crash and fire team is set up. As the helicopter approaches, the captain will — if possible — turn the ship into the wind to provide wind over the deck. He will also try to pick a course that provides the smoothest ship motion through the waves.
Landing is the greatest challenge. There are two big problems: the effect of the ship’s superstructure on the wind patterns at the helideck, and the timing of the touchdown (considering the motion of the deck).
The helicopter platform’s usual location is at the rear of the ship, on the fantail. There, wind flow is most affected by how it has negotiated its way around the forward part of the ship. If the ship is going fast or heading into the wind, the structure produces a flowing wake of relatively low-speed air, just as does a truck on the highway. Flying the helicopter into this wake will result in “drafting,” sucking the helicopter forward toward the hangar. This changes the helicopter’s relative airspeed and the control positions for steady flight.
As you can imagine, landing safely on a pitching and rolling deck takes great piloting skill. After getting the signal that it is safe to land, the pilot will attempt to line up with a stripe painted on the deck, positioning the helicopter’s nose over another mark. He tries to anticipate an instance when the motion of the ship is relatively quiet. When that moment comes, he plants the landing gear firmly on the deck with full down-collective pitch, while the deck crew installs chocks and tiedown chains.
RAST and SAMAHE
At some “sea state” (a measure of the wave and wind conditions), the ship’s motion and the gustiness will be so extreme that no amount of pilot skill is sufficient to make this type of landing, nor to ensure that the helicopter will not slide off the deck before the ship’s crew can secure it.
Not to worry! To help the pilot in such situations are systems that actually attach the helicopter to the ship while it is still in the air. The one used by the U.S. Navy is known as RAST (recovery assist, secure and traverse), which was first developed in Canada. Its basic function is to attach a cable to the hovering helicopter and then firmly — but gently — winch it down.
As the helicopter hovers at an altitude of about 15 feet, it lowers a light cable down to two crewmen on the deck. They attach this “messenger” cable to a much heavier “haul-down” cable. The heavier cable is then drawn up and locked to the RAST probe, which is permanently attached to a strongpoint under the helicopter’s center of gravity. Figure 13-2 shows a Canadian Forces Sikorsky Sea King attached to the RAST system.
AT this point, the landing safety officer (LSO) takes command from his vantage point in a bunker, with his eyes at deck level. For a helicopter the size of the Sikorsky SH-60 Seahawk, he applies approximately 2,000 pounds (900 kg) of tension to the cable to stabilize the aircraft. By using a constant-tension winch, the system maintains this force as the helicopter lowers to about four feet above the deck, in preparation for touchdown.
During this operation, not only is the required rotor thrust increased by 2,000 pounds, but the cable also tends to put some rolling and pitching moments on the helicopter. These are somewhat different from what the pilot is used to in free flight.
The LSO waits for the pilot to get ready and for the ship to be more-or-less steady. At that moment, he applies 4,000 pounds (1,800 kg) to the cable to pull the helicopter down.
AT all times, the pilot can release the haul-down cable and abort the landing, or by pulling more than 4,000 pounds, he can pull all 200 feet (61 m) of cable out of the winch and fly away with it (something frowned upon by the ship’s crew).
Provided it is a good landing and the helicopter is pulled firmly down to the deck, the jaws of the “bear-trap” (the rectangular device properly called a Rapid Securing Device) closes on the RAST probe, thus locking the helicopter to the deck.
The British and French navies use a device whose purpose is similar to RAST but which differs in operation. The SAMAHE (system de manutention pour Helicopteres) consists of a grid into which the hovering helicopter shoots a harpoon attached to a cable from a winch in the helicopter. The winch is then used to pull the aircraft down to the deck.
Stopping and folding rotors
Once the helicopter is down and secured, the next step is to stop the rotor. Much of the resistance to flapping on a rotating rotor comes from the centrifugal forces acting on the blades. Slowing the rotor down eliminates this stiffness and allows the blades to respond to gusty wind conditions.
For this reason, devices known as “droop stops” are incorporated in the rotor hub. These are spring-loaded devices, which, at normal rotor speeds, are kept out of play by centrifugal forces.
As the rotor slows down and the centrifugal forces become less than the spring forces, the droop stops engage and limit the amount of down-flapping that the blades can do. This prevents them from striking the tail boom or the top of the cockpit, and gives clearance for persons working under the rotor while it is turning slowly. For instance, the droop stops on the Seahawk are designed to not let the blade tips come lower than seven feet about the deck. On some helicopters, the amount of up-flapping is also restricted.
As the main rotor slows down, a vulnerable time comes just before the droop stops are actuated. A sudden down-gust coming over the hangar can cause the tip-path plane to come dangerously close to the helicopter structure or to anyone standing under it. A similar situation can exist even after the stops have come into place, since they can only restrain the flexible blades at their roots.
Once the rotor is stopped, the blades must be folded to get the helicopter into the hangar. This can be done manually on small helicopters, but large helicopters use electric and/or hydraulic actuators that extract one pin from the blade-hub joint and then pivot each blade about the other one.
Once the blades are folded and fastened down, the helicopter is moved into the hangar, either by hand or by the hydraulically powered RAST or the cable-actuated SAMAHE trolley.
Understandably, approach and landing operations to a ship deck are especially challenging at night. On a dark night, the pilot can’t discern the horizon to steady his orientation; all he can see are lights on the ship, and these may be heeling over while it pitches and rolls.
Some newer ships are equipped with a “light bar” — known officially as the HRS or horizon reference system — installed over the hangar door. The HRS is gyrostabilized to remain horizontal and take the place of the lost horizon. Pilots who have flown this system say that while it helps, it does not completely take the place of an actual horizon.
Nighttime takeoffs are also more challenging than daytime ones. This is primarily due to the sudden departure form the well-lit shipboard environment into the complete and utter darkness, and the urgent need to transfer form flying with external cues to flying on instruments.
In some conditions, operation from a ship will be too dangerous to try. The limiting conditions will depend on a combination of the characteristics regarding the ship, the helicopter, the visibility and the weather.
For this reason, the Navy conducts shipboard-compatibility trials in which a given helicopter is flown on and off a given ship both in daylight and at night. These are done in sea and wind conditions that are progressively worse, until the experienced test pilot can say that he would not ask an average pilot to go any further into deteriorating conditions.
The safe combination of the velocity and direction of the wind over the deck with limits on the allowable amount of roll motion of the ship are then published and adhered to for that particular helicopter-ship combination.
Figure 43-3 illustrates such a combination for launch and recovery of the Sikorsky SH-60H operating from a .S. Navy frigate during the day. The entire envelope can be used for both launches and recoveries with RAST. But without RAST, the recoveries are only permitted when the wind conditions are in the hatched areas. Note that the patterns are not symmetrical. This is apparently due to the way the wind at the fantail is affected by the ship’s configuration.