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LTE

LTE – Loss of Tail Rotor Effectiveness:

What is LTE? LTE is an uncommanded, rapid yaw towards the advancing blade which does not subside of its own accord. It can result in the loss of the aircraft if left unchecked.

Which helicopters are susceptible to LTE?

All helicopters with a single main rotor and tail rotor are susceptible to LTE. Those helicopters equipped with a Fenestron are affected by a similar phenomenon traditionally known as ‘Fenestron Stall’. The Bell Jetranger is statistically the most likely to encounter LTE and it is this type which caused the FAA to look more closely at the problem back in the 1980s.

Is LTE caused by a mechanical defect?

No. LTE is caused by an aerodynamic interaction between the main rotor and tail rotor. Some helicopter types (Jetranger) are more likely to encounter LTE due to the insufficient thrust produced by having a tail rotor which meets certification standards, but which is not always able to produce the thrust demanded by the pilot.

From the FAA:

Unanticipated Right Yaw in Helicopters – FAA ac90-95

4. THE PHENOMENA OF LTE.
CI, LTE is a critical; low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control.
3. BACKGROUND. Unanticipated right yaw, or loss of tail rotor effectiveness (LTE), has been determined to be a contributing factor in a number of accidents in various models of U.S. military helicopters.
The National Transportation Safety Board (NTSB) has identified LTE as a contributing factor
6. LTE is not related to a maintenance malfunction and may occur in varying degrees in a single main rotor helicopters at airspeeds less than 30 knots.
LTE is not necessarily the result of a control margin deficiency. The anti-torque control margin established during Federal Aviation Administration (FAA) testing is accurate and has been determined to adequately provide for the approved sideward/
rearward flight velocities plus counteraction of gusts of reasonable magnitudes. This testing is predicated on the assumption that the pilot is knowledgeable of the critical wind azimuth for the helicopter operated and maintains control of the helicopter by not allowing excessive yaw rates to develop.
c. LTE has been identifid as a contributing factor in several helicopter accidents involving loss of control. Plight operations at low altitude and low airspeed in which the pilot is distracted from the dynamic conditions affecting control of the helicopter are particularly susceptible to this phenomena.
The following are three examples of this type of accident:
(1) A helicopter collided with the ground following a loss of control during a landing approach.
The pilot reported that he was on approach to a ridge line landing zone when, at 70 feet above ground level (AGL) and at an airspeed of 20 knots, a gust of wind induced loss of directional control. The helicopter began to rotate rapidly to the right about the mast. The pilot was unable to regain directional control before ground contact.
(2) A helicopter impacted the top of Pike’s Peak at 14,100 feet mean sea level (MSL). The pilot said he had made a low pass over the summit into a 40knot headwind before losing tail rotor effectiveness. He then lost directional control and struck the ground.
(3) A helicopter entered an uncommanded right turn and collided with the ground. The pilot was maneuvering at approximately 300 feet AGL when the aircraft entered an uncommanded right turn. Unable to regain control, he closed the throttle and attempted an emergency landing into a city park.

6. CONDITIONS UNDER WHICH LTE MAY OCCUR.
a Any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur.
6. There is greater susceptibility for LTE in right turns. This is especially true during flight at low airspeed since the pilot may not be able to stop rotation. The helicopter will attempt to yaw to the right. Correct and timely pilot response to an uncommanded right yaw is critical. The yaw is usually correctable if additional left pedal is applied immediately. If the response is incorrect or slow, the yaw rate may rapidly increase to a point where recovery is not possible.

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Comparison of different types and operating zones:

F28F-R44_Comparison + LTE

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The report in 2011 from ATSB:

Loss Tail Rotor Effectiveness – 2011 R44

Old 6th Apr 2013, 08:28   #38 (permalink)
Join Date: May 2010
Location: More than 300km from SY, Australia
Posts: 355
LTE and Robinson’s

I have had a quick trawl on LTE and the following is interesting:
Helicopter Safety | Loss of Tail Rotor Effectiveness [LTE]

Quote:
What is LTE? LTE is an uncommanded, rapid yaw towards the advancing blade which does not subside of its own accord. It can result in the loss of the aircraft if left unchecked.

Which helicopters are susceptible to LTE?

All helicopters with a single main rotor and tail rotor are susceptible to LTE. Those helicopters equipped with a Fenestron are affected by a similar phenomenon traditionally known as ‘Fenestron Stall’. The Bell Jetranger is statistically the most likely to encounter LTE and it is this type which caused the FAA to look more closely at the problem back in the 1980s.

Is LTE caused by a mechanical defect?

No. LTE is caused by an aerodynamic interaction between the main rotor and tail rotor. Some helicopter types (Jetranger) are more likely to encounter LTE due to the insufficient thrust produced by having a tail rotor which meets certification standards, but which is not always able to produce the thrust demanded by the pilot.

AND [:]Rotor & Wing Magazine :: Safety Watch: Loss of Tail Rotor Effectiveness

Quote:
Thursday, February 1, 2007
Safety Watch: Loss of Tail Rotor Effectiveness Tim McAdamsACCORDING TO THE NTSB, ON July 11, 2000, the pilot of a Robinson Helicopter R22B said he was flying northeast about 10-20 mph at about 230 ft agl when the helicopter began to rotate to the right and seemed as if it was inverted.
He applied full left pedal and lowered the collective. After about three rotations, he heard a horn, which he believed was a low rotor-rpm warning. Initial inputs did not correct the loss of control.
As the helicopter descended toward trees, he applied full left cyclic, followed by full forward cyclic, then full right cyclic. The helicopter seemed slightly more controllable and was no longer inverted. However, it was still corkscrewing to the right. It impacted a swampy area and the fuselage, tail boom, and tail rotor were damaged. The pilot sustained serious injury. The passenger was not injured.
Examination of the wreckage by an FAA inspector did not reveal any pre-impact mechanical malfunctions. Reported winds at an airport about 7 mi from the accident site were from 300 deg at 15 kt, gusting to 21.
The NTSB determined the probable cause as the pilot-in-command’s improper in-flight decision to maneuver at a low airspeed with a left quartering tailwind, which resulted in a loss of tail-rotor effectiveness.
Just over a year later, the pilot of a Bell Helicopter 206B JetRanger and his passenger were not so lucky. They were fatally injured when their helicopter collided with Alpha taxiway abeam Runway 15 Right at Baltimore-Washington International Airport.
The helicopter approached a construction site on the airport in an out-of-ground-effect (OGE) hover taxi, with a quartering left tailwind. The helicopter turned right, and slowed to a stationary hover at about 250 ft with a direct tailwind. Once in a hover, it made a right, rapid, 180-deg pedal turn around the mast, stopped momentarily, then initiated another, rapid pedal turn to the right. The helicopter turned at a faster rate than the initial turn and continued into a spinning, vertical descent to the ground. The FAA’s examination of the helicopter found no mechanical anomalies.
Again, the NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to a loss of tail-rotor effectiveness and his inadequate recovery from the resulting unanticipated right yaw.
According to FAA Advisory Circular AC90-95, any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail-rotor effectiveness in right turns and states the phenomena may occur in varying degrees in all single main-rotor helicopters at airspeeds less than 30 kt.
Bell’s Operations Safety Notice OSN 206-83-10, regarding loss of tail-rotor effectiveness in the 206B and similar airframes, describes the phenomenon as an unanticipated right yaw. It contains the following warnings when maneuvering between a hover and 30 mph:
“Be aware that a tail wind will reduce relative wind speed if a downwind translation occurs. If loss of translational lift occurs, it can result in a high power demand and an additional anti-torque requirement. Be alert during hover (especially OGE) and high-power-demand situations. Be alert during hover in winds of about 8-12 kt (especially OGE), since there are no strong indications to the pilot [of] the possibility of a reduction of translational lift… Be aware that if a considerable amount of left pedal is being maintained, that a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw.”
Included in this notice is a chart that depicts relative-wind directions referencing the fuselage where an unanticipated right yaw can occur. More information is also available in Bell’s Rotorbreeze magazine.
One reason these pilots may have placed themselves and their passengers in jeopardy despite the abundance of warnings and information regarding loss of tail-rotor effectiveness is inexperience. The pilot of the R22B held a private pilot’s license and did not provide any further information to the NTSB or FAA. The Bell 206B pilot was commercially rated, but had less than 500 hr of helicopter flight experience, and 87.2 hr of experience in that model.
Both of these flights were performing aerial photography, the nature of which requires maneuvering at low altitudes and slow speeds. Add to that the distraction of trying to work with a photographer to line up the desired shot and the mission becomes very demanding. Any pilot flying a photographer needs to insure that he understands the aerodynamics and limitations of maneuvering at slow speeds.
Flight instructors should take note because students often misunderstand loss of tail-rotor effectiveness. More emphasis should be placed on this subject and instructors must ensure students fully understand the dangers of loss of tail-rotor effectiveness. Failure to do so can be deadly.
Tim McAdams has more than 9,000 total flight hours, with 7,000 in helicopters. A helicopter CFI and a fixed- and rotor-wing ATP, he flies a single-pilot IFR Agusta 109E for CareFlite in Dallas. You can reach him at rotorandwing@accessintel.com.

And from Australia [http://www.atsb.gov.au/media/3450408/ao2011055.pdf]

Quote:
ATSB TRANSPORT SAFETY REPORT
Aviation Occurrence Investigation AO-2011-055
Final
Loss of control, VH-ETT
4 km south-east of Kilmore, Victoria
30 April 2011
Abstract
On 30 April 2011, the owner-pilot of a Robinson Helicopter Co. R44 helicopter, registered VH-ETT, was conducting a local flight from a private property located near Kilmore Gap, Victoria. During low-level maneuvering at low speed around a dam, the pilot lost directional control and landed heavily in the water. The helicopter was seriously damaged; the pilot and passenger sustained minor injuries.
The investigation found that the helicopter was probably serviceable and that the loss of directional control was likely to be a result of a loss of tail rotor effectiveness.
The emergency locator transmitter (ELT) activated on impact and prompted an effective search and rescue (SAR) response through a broadcast on the 121.5 MHz frequency. However, the 406 MHz transmission that was monitored by the SAR agency did not trigger an alert or provide identification information. As a result, there was no assurance of an immediate and effective response from the SAR agency.
The investigation found that the ELT could be programmed with identification information either directly or (if fitted) by input from a component (dongle) in the ELT wiring connector. In this occurrence, the ELT had been inadvertently reprogrammed with incorrect information from the dongle.
A minor safety issue was identified in that there were only subtle cues to distinguish programmable dongles from the standard-type wiring connector. There was also variability in the conduct of post-installation ELT testing.
In response, on 6 June 2011, the Civil Aviation Safety Authority (CASA) published Airworthiness Bulletin 25-018 to alert maintenance organisations to the risk of programming dongles transferring potentially invalid details to the memory of ELTs. CASA advised that an article in Flight Safety Australia would also highlight the issue.
The helicopter manufacturer advised that they were introducing measures to increase awareness of programming dongles in their new helicopters.