ETL during a crosswind.

[SBuzzkill]

I was on an approach on Thursday into one of our stagefields that is surrounded by trees. There was a stiff crosswind (about 15-20kts) from the east and I was approaching to the south (putting the crosswind off my left side). As I got lower and slower (fwd airspeed below 15kts) I noticed no shudder or increase in power required until I got within 20 feet of the lane (below the trees), which resulted in me dropping and having to make a large increase in power and yawing right due to me not expecting it and not putting in enough left pedal. My descent rate was around 150 feet per minute at that time, maybe a little less, it's hard to tell with the instrumentation we have.

 

Thinking about it later I feel I didn't pay enough attention to the winds. I suspect that the crosswind was keeping my rotor disk above ETL even though I had near zero FWD airspeed. Is this a correct assumption? It's something we never really talked about in academics or at the flight line.

Edited March 5, 2011 by SBuzzkill

[iChris]

I was on an approach on Thursday into one of our stagefields that is surrounded by trees. There was a stiff crosswind (about 15-20kts) from the east and I was approaching to the south (putting the crosswind off my left side). As I got lower and slower (fwd airspeed below 15kts) I noticed no shudder or increase in power required until I got within 20 feet of the lane (below the trees), which resulted in me dropping and having to make a large increase in power and yawing right due to me not expecting it and not putting in enough left pedal. My descent rate was around 150 feet per minute at that time, maybe a little less, it's hard to tell with the instrumentation we have.

 

Thinking about it later I feel I didn't pay enough attention to the winds. I suspect that the crosswind was keeping my rotor disk above ETL even though I had near zero FWD airspeed. Is this a correct assumption? It's something we never really talked about in academics or at the flight line.

 

Each knot of incoming wind gained by horizontal movement or surface winds improves the efficiency of the rotor system. As the incoming wind enters the rotor system, turbulence and vortexes are reduced and the flow of air becomes more horizontal. Therefore, your helicopters performance was much better above the trees, in 15 knot winds, than below the tree line in decreased winds and possible turbulence caused by wind over and around the trees.

 

The wind that your rotor system knows is the air in motion with respect to the airfoil, relative wind. In a stationary hover, zero ground speed, with 15 knot surface winds, the rotor sees 15 knots of relative wind. Thereafter, each knot of forward ground speed into the wind increases the relative wind by that same amount.

Edited March 5, 2011 by iChris

[Darren Hughes]

iChris pretty much summed it up.Your rotor disk doesn't care where the horizontal airflow is coming from. Once you get about 5 knots or more of horizontal flow through the disk your power required is drastically reduced with each knot of increased speed, until you get into proper ETL, so to speak. After that your power required reduces further at a slower rate with increasing speed until you reach Vy, which if memory serves me in a R22 is 53kts.

 

After a while you just get the feel of it, when you're gonna completely loose ETL, especially if you're landing at the same confined areas on a regular basis. It's just a matter of bringing the power in early so as to prevent that sudden loss of altitude. With practice you'll find yourself applying pedal without thinking as you raise the collective, to keep the nose straight.

 

In a lot of situations you'll find yourself approaching a pad that has some tall obstacles around it blocking the wind, so at a certain point over the ground your airspeed will drop from 30-40kts to zero within a split second. The stronger the wind the quicker this will happen. At that point you're pretty much in an OGE hover making your approach to the spot. In a lot of cases like this, the obstacles blocking the wind near the pad, will cause the air on the lee-side to start to flow backwards in towards the base of the obstacle due to a loss of air pressure in that area causing you to not only drop out of ETL, but to have to deal with a pretty strong tail wind on short final approach. Now you have to be quick on the pedals too to prevent the weather vane effect on the tail from causing un-commanded yaw which will require more power to counteract. I've even found my self having to reduce power sometimes due to such a strong tailwind at the last 100ft AGL of the approach putting me back into ETL, except from behind. They're the fun ones!!

 

Performance planning becomes critical when you're flying in situations like these. Knowing you can hover OGE with extra power in reserve, with your weight and the prevailing conditions is the key. You'll need the extra power in reserve because of the turbulent conditions these obstacles can cause the closer you get to them. Being smooth and deliberate with control inputs by expecting or at least forecasting what's coming next in the approach always requires less power than yanking on the collective at the last second. This will help keep you from an over-torque or exceeding manifold pressure limitations while making for a smoother ride for your pax.

 

A slow steady approach into the wind is always favored but not always possible because of obstacles or noise restrictions for local areas. If the approach isn't perfect, fix it early, or go around. If you think that the downdrafts caused by the obstacles, or your final approach power check indicates the power required at the bottom is more than you have, go around. If there's anything you should take away from confined area or pinnacle training, it's that go arounds are your friend, and will keep you alive/employed!

 

There are some great books that hit on this subject, such as Cyclic & Collective by Shawn Coyle, and The Helicopter Pilot's Handbook by Phil Croucher are just 2 of them.

[SBuzzkill]

OGE hover power was 92% when we did the OGE hover power check with a max available of 100%. Granted we burned off about 400lbs of fuel after doing the check.

 

What I learned from this situation is to think about the wind and it's effect on lift no matter what direction it's hitting the disk.

 

I think the right yaw was also a result of me holding right pedal in to align the aircraft with the lane. No left X-wind resisting the pedal input would combine with my increase in power to cause the nose to go right.

Edited March 6, 2011 by SBuzzkill

[Darren Hughes]

Ok, so i'm now guessing your lessons are in an OH-58 or bigger having burned 400lbs of fuel during a lesson. I assumed you were a civilian student, hence the reference to the R22 and manifold pressure.

 

All the rest applies really, except when you violate the noise restrictions for an area when setting up an approach, instead of the locals calling up and complaining in a whiney bitchy tone, they shoot at you!!

 

I think the right yaw was also a result of me holding right pedal in to align the aircraft with the lane. No left X-wind resisting the pedal input would combine with my increase in power to cause the nose to go right.

 

You're right, your nose would have started to yaw right slightly anyway by taking the left crosswind out of the equation as you dipped below the level of the trees. By pulling all that collective to reduce the rate of decent you added to the rate of yaw. To stop the momentum gained by the delayed pedal input, you needed a larger input than you would have needed if you had applied a little left pedal as the yaw started.

[SBuzzkill]

Something I will keep in mind next time.

Edited March 6, 2011 by SBuzzkill

[palmfish]

Something I will keep in mind next time.

 

Also the 3 LTE wind regions. You were certainly operating in the Vortex Ring State region, and possibly the Disc Vortex region.

[Wally]

I was on an approach on Thursday into one of our stagefields that is surrounded by trees. There was a stiff crosswind (about 15-20kts) from the east and I was approaching to the south (putting the crosswind off my left side). As I got lower and slower (fwd airspeed below 15kts) I noticed no shudder or increase in power required until I got within 20 feet of the lane (below the trees), which resulted in me dropping and having to make a large increase in power and yawing right due to me not expecting it and not putting in enough left pedal. My descent rate was around 150 feet per minute at that time, maybe a little less, it's hard to tell with the instrumentation we have.Thinking about it later I feel I didn't pay enough attention to the winds. I suspect that the crosswind was keeping my rotor disk above ETL even though I had near zero FWD airspeed. Is this a correct assumption? It's something we never really talked about in academics or at the flight line.

 

 

OGE hover power was 92% when we did the OGE hover power check with a max available of 100%. Granted we burned off about 400lbs of fuel after doing the check. What I learned from this situation is to think about the wind and it's effect on lift no matter what direction it's hitting the disk.I think the right yaw was also a result of me holding right pedal in to align the aircraft with the lane. No left X-wind resisting the pedal input would combine with my increase in power to cause the nose to go right.

 

 

You have to consider how the surrounding terrain affects the wind, trees included. Trees block the wind pretty well, easily making 15-20 knots difference, near and directly downwind of a dense stand. That's enough to make significant difference in an approach, especially steep and fast, or an autorotation would be very busy if you hadn't planned ahead. If you don't have any choice but to fly into the 'wind shadow', keep as many options open as you can. Slow down before you make entry into the area of turbulent wind speed change (just above the trees in this case) until you're through the transition, in the slower air. Keep an abort in mind, available, and use it if you need to- although trees usually don't complicate a slow approach that much. That is, if the trees aren't on hill or honkin' big trees right up against you landing point... then just continue the slow approach once you're below the trees. Do this slow, because a high rate of descent or speed makes it hard to change your mind and not much time to do so.

[Goldy]

I think the right yaw was also a result of me holding right pedal in to align the aircraft with the lane.

 

While I don't disagree with anything said, who says you have to hold alignment to the runway? You're in a helicopter, crab in, keeping the wind closer to head on, and keeping it out of the LTE areas. Personally I think that is easier, and often safer, than trying to maintain a straight in heading and allowing the wind to come from an angle that puts the aircraft in greater jeopardy.

 

As you know, really easy to overtorque when the bottom drops out from under you!

[SBuzzkill]

That's the way they tought us to shoot an approach, crab until 50 feet then align with the lane.

 

From the Aircrew Training Manual entry for VMC Approach:

 

"Maintain entry airspeed until the rate of closure appears to be increasing. Above 50-feet above ground level (AGL), maintain ground track alignment and aircraft in trim. Below 50-feet AGL, align the aircraft with the landing direction."

 

I ran into the same situation yesterday but due to my anticipating it I was better able to control the aircraft. It was also my safety tip during our brief.

Edited March 11, 2011 by SBuzzkill