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Normal Approach

This maneuver is used to transition from forward flight to a hover or a landing. A JPEG and GIF sequence of photographs of a normal approach are available.

Maneuver Description

Approach Angle

We usually consider a normal approach to be a 10 degree approach. More than 10 degrees is considered to be a "steep" approach, and less than that is considered to be "shallow". For reference, 10 degrees is about what you get in a Cessna 172 with engine at idle and 40 degrees of flaps hung out.

We initiate the maneuver by intercepting the 10 degree approach angle. Normally a collective pitch adjustment will be required to start the helicopter descending on the 10 degree angle. The exact power setting will depend on things such as wind, density altitude, and helicopter weight.

The way that a helicopter pilot judges whether he is maintaining the desired angle is similar to what an airplane pilot does. There are multiple cues which will tell you whether you are changing approach angle. These include:

Closure Rate

Unlike an airplane, helicopters do not fly constant airspeed approaches. That's partly because they don't have to. If an airplane attempts to decelerate too much on approach, it stalls. A helicopter doesn't have that problem. Normally, inside of a mile of the landing zone the helicopter is decelerating at the same rate it is losing altitude so that by the time the altitude of the helicopter approaches zero, the ground speed will also be approaching zero. One way for helicopter pilots to judge this is to look at apparent ground speed. From high up, the ground seems to be going by very slowly. As we descend, the ground appears to speed up. Helicopter pilots simply hold the apparent ground speed to approximately a jogging pace, and that will insure that as they approach the ground they will be moving forward at a jogging pace. The last few knots of ground speed can be killed as the helicopter transitions into a hover.

Power Requirements

During the deceleration from approach speed to minimum sink airspeed, less power is required as the helicopter slows. This will require the helicopter pilot to be decreasing collective initially. However, from minimum sink airspeed until reaching the LZ, the power required will be going up, because the helicopter is on the back side of the power curve.

During the last portion of the approach, typically begining around 40 knots of airspeed, the helicopter is on the part of the power required curve where power requirements are going up very quickly. The pilot will normally notice a sudden tendency for the helicopter to sink below the approach angle. The pilot will have to increase collective substantially to maintain angle.

Because the helicopter is in a slightly flared attitude, this increase in thrust will increase the deceleration force (because the rearward component of thrust will be increased). If the pilot does not push forward on the cyclic at this time, the helicopter will generally come to a stop well short of the LZ, typically at a height of 25-35 feet. By adding forward cyclic, the pilot will keep the helicopter moving forward at a slowly declerating rate, losing altitude at the same time, until the helicopter reaches the LZ at the desired hover height.

Termination

The maneuver can terminate either in a hover, which is the usual case, or can be flown right to the surface. Approaches to the surface are typically used when the pilot wants to minimize downwash, or does not want to hover for one reason or another (poor visibility is one reason that comes to mind). The pilot will find that a large amount of left pedal is required to maintain skid alignment as the helicopter decelerates through translational lift.

Whether the maneuver is going to terminate in a hover or to the ground, the pilot should align the skids with the ground track at approximately 100' AGL. This prevents a rollover accident should the skids touch down with forward speed.

Common Mistakes

Failure to maintain approach angle

Until the pilot can judge approach angle accurately, he may deviate from approach angle and not realize it. While overshooting onto steeper angles certainly occurs, the more common problem is failure to increase power to account for the back side of the power curve. Especially below 40 knot, the power required increases quickly, and the pilot may simply not raise collective fast enough.

Failure to decelerate properly

Airplane pilots who solo in helicopters are famous for flying very fast approaches (because of their trained in behavior to avoid low airspeed which can stall an airplane). Even non-pilots have this problem, though. Maintaining too high an airspeed until on short final causes a couple problems. Usually a flare is required to decrease ground speed, and if this is done too low a tail rotor strike is possible. Another problem is the pilot who flares and reduces his airspeed while keeping a high descent rate may encounter settling with power.

Failure to reach the LZ

As described above, on short final the power increase required will cause the helicopter to stop short of the LZ unless the pilot adds forward cyclic at the same time. Indeed, it's not unusual for student pilots to come to a hover well short of the LZ at about 35 feet, and then begin spinning around because as translational lift is lost the tail rotor loses substantial thrust, and a large increase in left pedal is normally required to counter this effect.
Paul Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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