NTSB Safety Alert SA-062 - Loss of Tail Rotor Effectiveness in Helicopters

Loss of Tail Rotor Effectiveness in Helicopters

Be alert for un commanded yaw so you don’t get caught off guard!

The problem

 In helicopters, l oss of tail rotor effectiveness ( LTE), or unanticipated yaw, is an

uncommanded rapid yaw that does not subside of its own accord . LTE can occur in all

single -engine, tail rotor-equipped helicopters at airspeeds lower than 30 knots and,

if uncorrected, can cause the pilot to lose helicopter control , potentially resulting in

serious injuries or death .

 Various factors can contribute to LTE, including varying airflow from the main rotor blades

(particularly at high power settings) or from the environment, wh ich can affect the airflow

entering the tail rotor; operating at airspeeds below transl ational lift; operating at

high altitudes and high gross weights; operating near large buildings or ridgeline s, which

can cause turbulence; and the relative wind direction (see figures 1 and 2) .1

1 On US -manufactured single -rotor helicopters, the main rotor rotates counterclockwise as viewed from above.

The torque produced by the main rotor causes the fuselage of the helicopter to rotate in the opposite direction

(nose right). On some European - and Russian -manufactured helicopters, the main rotor rotates clockwise as viewed

from above. In those helicopters, the torque produced by the main rotor causes the fuselage to rotate nose left.

Operating with the relative win d direction within ±15° of the 10 -o’clock position (for counterclockwise main rotor

helicopters) or the 2 -o’clock position (for clockwise main rotor helicopters) generates vortices that directly blow into

the tail rotor. Also, tailwinds from 120° to 240° c an cause high workloads. Finally, crosswinds can create roughness

due to tail rotor vortex ring state (wind from 210° to 330° on counterclockwise main rotor helicopters or from 30° to

150° on clockwise main rotor helicopters).

Figure 1. Relative wind directions that can

contribute to LTE for counterclockwise main rotor

helicopters. Figure 2. Relative wind directions that can

contribute to LTE for clockwise main rotor

helicopters.

SA-062 March 2017  Due to safety concerns, training for LTE is rarely done in an actual helicopter. Simulators

allow pilots to practice recovery; however, the element of surprise —and the rapid yaw that

pilots may experience when the helicop ter encounters LTE in flight —is difficult to

realistically achieve in some simulator s.

Related accidents

During the 10 -year period from 2004 to 2014, t he National Transportation Safety Board (NTSB)

investigated 55 accidents involv ing LTE. In the following cases , the pilots were unable to recover

when the helicopter s encountered unanticipated yaw. All three cases involved helicopters with

counterclockwise rotating main rotor blades.

 The pilot was making an approach to a hospital helipad into light wind at night when he

chose to go around. The pilot lowered the helicopter's nose, added power, and raised the

collective ; the helicopter then entered a rapid "violent" right spin. The pilot appl ied left

antitorque pedal and cyclic but was unable to recover . The helicopter spun several times

before impact ing power lines and t errain. Just before the pilot added power to go around,

the helicopter was traveling about 5 knots groundspeed . At such a low groundspeed, the

tail rotor is required to produce nearly 100 % of the directional control. The pilot likely did

not adequately account for the helicopter’s low airspeed when he applied power to go

around, which resulted in a sudden, uncommanded right yaw due to LTE. (CEN15FA003 )

 The pilot and two passengers were surveying deer, with the helicopter about 50 to 100 ft

above ground level with a 5 - to 10 -knot left crosswind and an indicated groundspeed of

30 to 35 knots. As terrain began to rise, the pilot added power to clear a ridge. The pilot

reported that, when the helicopter was about 100 ft from the top of the ridge, the helicopter

began to yaw to the right. He added power to clear the ridgeline, w hich greatly increased

the right yawing motion . The helicopter began spin ning, crossed over the ridgeline

backward , and continued spin ning before it contacted the ground and rolled over onto its

left side. A passenger reported that, although the wind was a bout 10 knots when they

started the survey, the wind speed increased when the helicopter reached the top of the

ridge, and the pilot had to correct for it twice before the helicopter began spin ning to the

right. The helicopter was operating with wind comin g from the left and at a high power

setting ; the unanticipated right yaw and subsequent spinning of the helicopter are

consistent with LTE. (CEN13TA165 )

 The pilot had planned a Part 91 sightseeing flight around New York City with two

passengers; however, four passengers arrived for the flight. The pilot did not complete

performance calculations before the accident flight , and the helicopter was in excess of its

maximum all owable gross weight at takeoff. Shortly after departure, while the helicopter

was climbing to 60 ft above the water, t he pilot fail ed to anticipate and correct for conditions

(high gross weight, low indicated airspeed, and a right downwind turn) conducive to LTE,

which resulted in LTE a nd an uncontrolled spin. (ERA12MA005 )

SA-062 March 2017 What can you do?

 Include wind speed and direction in your preflight planning because it can greatly affect

your helicopter’s susceptibility to LTE .

 Know your helicopter’s performance limitations , as outlined per the manufacturer, and

adhere to them.

 Be aware of your helicopter’s flight control characteristics, p articularly tail rotor pedal

forces, so that you can quickly recognize and resolve the onset of unanticipated yaw.

 Review the Federal Aviation Administration’s (F AA) Helicopter Flying H andbook for

specific tips on avoiding LTE . Here are a few tips to get you started :

o Conduct a thorough preflight planning assessment with particular attention to the

helicopter’s maximum allowable gross weight.

o Maintain awareness of the wind direction and speed in flight , especially in

high workload areas , when flying along ridgelines and around buildings , and when

hovering in wind of about 8 to 12 knots when a loss of translational lift can occur .

o Avoid tailwinds or crosswinds (the direction depends on the type of helicopter you

are flying) when operating below an airspeed of 30 knots .

o Avoid out -of-ground -effect operations and high -power -demand situations below

30 knots.

o Monitor the amount of antitorque pedal being used. If insufficient pedal is available ,

you may not be able to counteract an unanticipated right yaw.

 Train for and know how to recover immediately from LTE so that you are prepared .

Remember that LTE can be sudden , and pilots have described the onset of yaw as

“violent.”

Interested in more in formation?

The following FAA resources are available on the FAA’s website (www.faa.gov ):

 The Helicopter Flying Handbook (FAA -H-8083 -21A) , chapter 11 , “Helicopter Emergencies

and Hazards ,” provides an in -depth explanation of LTE.

 Advisory Circular 90 -95, “Unanticipated Right Yaw in Helicopters,” explains unanticipated

yaw in helicopters, why it occurs, how to prevent it, and how pilots can respond to it.

The NTSB has produced a video regarding LTE, which includes one investigator’s experience

with LTE and tips on what you can do to be prepared.

The NTSB’s Aviation Information Resources web page (www.ntsb.gov/air ) provides convenient

access to NTSB aviation safety products. The reports for the acci dents referenced in this

safety alert are accessible by NTSB accident number from the Aviation Accident Database link,

and each accident’s public docket is accessible from the Accident Dockets link for the

Docket Management System . This safety alert and others can be accessed from the Aviation

Safety Alerts link.