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Quick Stop (Rapid Deceleration)

This maneuver is used to decelerate from forward flight to a hover. It is often used to abort takeoffs, stop if something blocks the helicopter flight path, or simply to terminate an air taxi maneuver.

Maneuver Description

The maneuver can begin from just about any combination of airspeed and altitude, but is typically practiced from around 25 feet AGL and 40 knots. The maneuver can be broken down into 3 parts: the flare, the deceleration, an the approach to a hover.

The Flare

To begin the maneuver the pilot flares the helicopter by pitching the nose up with aft cyclic. The flare would normally cause the helicopter to gain altitude, but in this case the pilot reduces collective to prevent a climb from occuring. The power decrease will require right pedal to be applied as well. The pilot can adjust how quickly the helicopter will decelerate by how aggresively he flares. The more aggresive the flare, the faster the helicopter will stop. Too agressive a flare will cause a tach needle split (the rotor will spin faster than the powerplant), and this is generally not the way the maneuver is flown, although nothing harmful will result from this.

The Deceleration

The helicopter will decelerate as the pilot holds the flare. Because the airspeed is decreasing, the flare will immediately start to lose energy, and the helicopter will try to settle. Instead of allowing the helicopter to settle, the pilot can increase the flare. This will maintain altitude, and cause the helicopter to decelerate even faster. There is an upper limit to how much the pilot should flare the helicopter. The deceleration becomes so fast it is more difficult to judge, plus the helicopter ends up in an extremely tail low attitude.

Instead of continueing to pitch the nose up, the pilot can start adding power to maintain altitude. He does this by raising collective. This not only maintains altitude, but it maintains rotor thrust and keeps the helicopter decelerating quickly. The pilot should continue to hold altitude and attitude until he is about to lose ETL (effective translational lift).

Approach to a Hover

Before ETL is lost, the pilot pitches the nose of the helicopter back down into an approach attitude using forward cyclic. The attitude is still slightly nose high, as on a normal approach, but is not in a flared attitude. The pilot can simply look ahead of the helicopter and pick a spot aligned with a 10 degree approach angle. He then flys a normal approach to the spot.

Common Mistakes

Descending while in the flare

Many pilots actually allow the helicopter to descend during the deceleration phase of the maneuver. Many instructors teach the maneuver that way. I don't personally like that method for a couple reasons.

One reason is that I don't think the helicopter should approach the ground tail low. Although pilots are generally aware of their tail rotor, occasionally people forget and wipe out the helicopter with a tail rotor strike. I feel that if the maneuver can be flown in a way to minimize this hazard, it should be flown that way.

Another reason I don't like descending in the flare is that it means the helicopter is descending directly into it's downwash as it goes through ETL. This is a classic way to set up settling with power. Pilots normally think of high vertical descent rates as causing settling with power, but a more accurate description is a high velocity aligned with the rotor vortex. You can settle with power sideways if you try hard enough. If you watch a pilot performing a quick stop who descends while in the flare, you can see that he is settling directly into his vortex. Usually the helicopter reaches ground effect in time to avoid power settling.

My belief is that by maintaining altitude during the desceleration, the pilot stays above his vortex (similar to an out of ground effect hover) and eliminates the chance of settling with power, as well as eliminating the chance of a tail rotor strike. The termination of the maneuver is a normal approach, so if the helicopter has enough power to fly a normal approach, it has enough power to terminate this maneuver.

Ballooning

A common mistake is to flare, but not lower enough collective to compensate for the increased lift. This will cause the helicopter to balloon, or gain altitude. If the flare is really aggressive, full down collective will not be enough to prevent altitude gain. Altitude gain is not desirable because it puts you deeper in the HV curve.

Incorrect anti-torque pedal manipulation

One of the reasons instructors love to have students work on quick stops is that it really helps them get the hang of the anti-torque pedals. This is because during the entire maneuver, torque is changing constantly. Failure to do this will cause the helicopter to yaw during the maneuver.

Losing ETL

A pilot who holds the flare so long that effective translational lift is lost may have a problem if the helicopter does not have enough excess power available to hover out of ground effect. If the helicopter loses ETL 30 feet in the air, this is equivalent to attempting a 30 foot OGE (out of ground effect) hover. If the helicopter does not have adequate power for the hover it will descend and very likely hit the ground hard. It may even enter settling with power. If it does that it will hit really hard.

Failure to totally stop

A common mistake is to enter the desired hover height with some forward speed. This often occurs when the pilot does not use sufficient power, and the helicopter descends to a hover height while it still hasn't had time to fully stop. The problem with this is that aft cyclic is now required which is going to cause the tail to drop, and little altitude is available so that too much aft cyclic could cause a tail strike.

Failure to hold centerline

Another problem that student pilots often have is holding centerline during a quick stop. The helicopter goes through transverse flow effect while decelerating in a quick stop, and failure to correct for it with lateral cyclic will cause the helicopter to veer to the side while in the flare.

Paul Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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