Lte And Bell Flight Manuals

[Coastal]

Hi all, well here's something that came up a while ago and I don't think I've ever gotten a straight answer.

 

We all know what LTE is, or at least we all SHOULD know. Based on the highly circulated

FAA circular 90-95, there are three separate, overlapping zones of relative wind azimuth, from 120 to 330 degrees, that may contribute to LTE (along with all the other factors: high gross weight, low airspeed, pilot inattention etc). This is essentially the entire LEFT side of the aircraft.

 

Conversely, Performance Sections of Bell Flight Manuals for the 206 series discuss the "Critical Relative Wind Azimuth Area" and from my understanding, the aircraft will have decreased hovering performance due to "tail-rotor control margin and/or control of engine parameters". This area covers nearly the entire RIGHT side of the aircraft (from 050 to 210 relative to the nose of the a/c).

 

Most pilots I've asked said that, if required, they would rather accept a right crosswind than a left crosswind (in a Bell or other counterclockwise rotor system) and reference the above FAA circular and LTE as the reason why.

 

My question is how does LTE and relative winds from the left side of the aircraft fit into your interpretation of the Bell Hover Ceiling Charts. Which of the two crosswinds is the lesser evil, from the left or from the right? For what reason(s) does Bell show right crosswinds as detrimental to tailrotor control, and not left crosswinds?

 

Obviously into wind is the best choice, but we all know there are situations where we'll have to accept a crosswind in conditions that may lead to LTE. It seems to me that the information in Bell's Flight Manuals and the information available from the FAA on LTE is conflicting and does not leave the pilot with clear information on a potentially dangerous flight condition.

 

What do you guys/gals think?

 

Coastal

[Cirrus]

I think the critical wind azimuth only applies to the hover performance charts. You don't have to avoid hovering with wind in that area, but you have to avoid the shaded area on the hover performance charts if the wind direction coincides with the shaded area on the CWA.

 

The critical wind azimuth is part of the performance charts and isn't meant to have anything to do with LTE.

 

I don't think the AFM addresses LTE, but I hear what you're saying. I'd rather a cross wind from the right rather than the left. Less wrestling.

[skullcap]

In my opinion you fellas have a good understanding the fm. Wind on right = area of reduced performance due to increased t/r thrust necessary to keep the nose where you want it. Wind on left or aft = area of possible loss of t/r effectiveness(kinda scary if not on top of things).

 

In steady winds the flight manual descriptions are exact and can be demonstrated with safety. It is when the winds are gusty that things get interesting(in a bad way). For example when wind is on right and is gusting the rotor rpm can vary due to moderate collective changes, when this happens the t/r thrust is varied and if the timing happens whereas the t/r thrust is reduced due to lower rpm(collective input) at a moment in time where the wind has increased from right then the left pedal stop could be reached(only have hit it once in JRIII and was at 10,000 with strong wind on right) thus a yaw to right occurring.

 

With gusting winds on left or aft then LTE issues are more drastic and thus fast feet required.

 

Have had good results with hoving with strong winds at 9 oclock on long line obtaining more torque available due to reduced t/r thrust requirements.

 

Have to keep in mind that if shutting down/starting up that blade sail is to be taken into account and that it is better to have wind on right slightly to have climbing blade over tailboom but remember to keep your passengers aware of this as they may be walking forward into a descending blade,,,,it is a good idea to have passengers either inside or away from machine while rotors are slow in any condition though.

 

[Helilog56]

While we are on the subject.....some reminders for us all, from my notes I collected over the years.

 

Four aircraft characteristics during low speed flight have been identified through extensive flight and wind tunnel tests as contributing factors in unanticipated right yaw.

 

For the occurrence, certain relative wind velocities and azimuths (direction of relative wind) must be present. The aircraft characteristics and relative wind azimuth regions are:

 

Weathercock stability (120 to 240 degrees)

Tail rotor vortex ring state (210 to 330 degrees)

Main rotor disc vortex interference (285 to 315 degrees)

Loss of translational lift (all azimuths)

The aircraft can be operated safely in the above relative wind regions if proper attention is given to controlling the aircraft. However, if the pilot is inattentive for some reason and a right yaw rate is initiated in one of the above relative wind regions, the yaw rate may increase unless suitable corrective action is taken.

 

WEATHERCOCK STABILITY (120 to 240 DEGREES)

Winds within this region will attempt to weathervane the nose of the aircraft into the relative wind. This characteristic comes from the fuselage and vertical fin. The helicopter will make an uncommanded turn either to the right or left depending upon the exact wind direction unless a resisting pedal input is made. If a yaw rate has been established in either direction, it will be accelerated in the same direction when the relative winds enter the 120 to 240 degree area, unless corrective pedal action is made. The importance of timely corrective action by the pilot to prevent high yaw rates for occurring cannot be overstressed.

 

TAIL ROTOR VORTEX RING STATE (210 to 330 DEGREES)

Winds within this region, will result in the development of the vortex ring state of the tail rotor. The vortex ring state causes tail rotor thrust variations which result in yaw rates. Since these tail rotor thrust variations do not have a specific period, the pilot must make corrective pedal inputs and the changes in yaw acceleration are recognized. The resulting high pedal workload in tail rotor vortex ring state is well known and helicopters are operated routinely in the region. This characteristic presents no significant problem unless corrective action is not timely. If a right yaw rate is allowed to build, the helicopter can rotate into the wind azimuth region where weathercock stability will then accelerate the right turn rate. Pilot workload during vortex ring state will be high; therefore, the pilot must concentrate fully on flying the aircraft and not allow a right yaw rate to build.

 

TAIL ROTOR VORTEX RING STATE (210 to 330 DEGREES)

Winds within this region, will result in the development of the vortex ring state of the tail rotor. The vortex ring state causes tail rotor thrust variations which result in yaw rates. Since these tail rotor thrust variations do not have a specific period, the pilot must make corrective pedal inputs and the changes in yaw acceleration are recognized. The resulting high pedal workload in tail rotor vortex ring state is well known and helicopters are operated routinely in the region. This characteristic presents no significant problem unless corrective action is not timely. If a right yaw rate is allowed to build, the helicopter can rotate into the wind azimuth region where weathercock stability will then accelerate the right turn rate. Pilot workload during vortex ring state will be high; therefore, the pilot must concentrate fully on flying the aircraft and not allow a right yaw rate to build.

The loss of translational lift results in increased power demand and additional anti-torque requirements. If the loss of translational life occurs when the aircraft is experiencing a right turn rate, the right turn will be accelerated as power is increased, unless corrective action is taken by the pilot. When operating at or near maximum power, this increased power demand could result in rotor rpm decay.

 

The characteristic is most significant when operating at or near maximum power and is associated with unanticipated right yaw for two reasons. First, if the pilot's attention is diverted as a result of an increasing right yaw rate, he may not recognize that he is losing relative wind and hence losing translational lift. Second, if the pilot does not maintain airspeed while making a right downwind turn the aircraft can experience an increasing right yaw rate as the power demand increases and the aircraft develops a sink rate. Insufficient pilot attention to wind direction and velocity can lead to an unexpected loss of translational lift. The pilot must continually consider aircraft heading, ground track, and apparent groundspeed, all of which contribute to wind drift and airspeed sensations. Allowing the helicopter to drift over the ground with the wind results in a loss of relative wind speed and a corresponding decrease in the translational lift produced by the wind. Any reduction in translational lift will result in an increase in power demand and anti-torque requirements.

 

 

RECOVERY TECHNIQUE

If a sudden unanticipated right yaw occurs, the following recovery technique should be performed:

 

Pedal - full left; simultaneously, cyclic - forward to increase speed.

As recovery is effected adjust controls for normal forward flight.

 

CAUTION

COLLECTIVE PITCH REDUCTION WILL AID IN ARRESTING THE YAW RATE BUT MAY CAUSE AN EXCESSIVE RATE OF DESCENT. THE SUBSEQUENT LARGE, RAPID INCREASE IN COLLECTIVE, TO PREVENT GROUND OR OBSTACLE CONTACT, MAY FURTHER INCREASE THE YAW RATE AND DECREASE ROTOR RPM.

 

THE DECISION TO REDUCE COLLECTIVE MUST BE BASED ON THE PILOT'S ASSESSMENT OF THE ALTITUDE AVAILABLE FOR RECOVERY.

 

 

If spin cannot be stopped and ground contact is imminent, an autorotation may be the best course of action. Maintain full left pedal until the spin stops, then adjust to maintain heading.

Note

 

The various wind directions can cause significantly differing rates of turn for a given pedal position. The most important principle for the pilot to remember is that THE TAIL ROTOR IS NOT STALLED. Thus, the corrective pedal position to be applied is always in the normal direction of OPPOSITE PEDAL to the turn direction.

 

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