Turbulence due to heavy winds in the lowest layers

ThomasLineal · December 23, 2014

Hi everybody! I am Thomas, i work as a forecaster for the navy in Germany. This is my first post in this group. I've looked around in the FAQ's and poked around in google but can't find my question covered there.

The question is:

How is turbulence caused by heavy storm or hurricane force winds in the lowest layers, say below 2000ft gnd, affecting helicopters? Are there any rules of thumb forecasting severe turbulence for helicopters in strong wind situations?

For fixed wing a/c there is a rule of thumb which is: If the vector difference between surface wind and wind at 2000ft gnd exceeds 20 40kt, there will be severe turbulence.

So i'm wishing you all a happy christmas and always happy landings in 2015 and i am looking forward to see your comments.

Sincerely

Thomas

Edited December 29, 2014 by ThomasLineal

Wally · December 23, 2014

There's airspace above 2000 feet???

As to high winds and turbulence, helicopters are mostly affected by orographic and similar effects on winds. It gets real sporty in the mountains and around some structures, some places and some winds you just can't operate with acceptable risk. "Wind shadows", reverse and turbulent flows, mountain pass 'venturis', low level operations can get real interesting.

Flying Pig · December 23, 2014

One issue with helicopters is that they normally operate in environments where a weather forecaster isnt able to provide anything accurate beyond what an airplane pilot would need to know. Its up to a helicopter pilot to take that information as well as their understanding of the area and other affects like stated above. I did a decent amount of FW mountain flying for SAR and looking for weed.....and many times the helicopters could operate with ease in the same areas while I was holding on for dear life in an airplane. What would be a no-go for an airplane pilot may be completely doable for a helicopter pilot in the same place. Thats why I sorta chuckle when airplane only pilots attempt to speak with authority about helicopter operations.

Edited December 23, 2014 by Flying Pig

iChris · December 23, 2014

Hi everybody! I am Thomas, i work as a forecaster for the navy in Germany. This is my first post in this group. I've looked around in the FAQ's and poked around in google but can't find my question covered there.

The question is:
How is turbulence caused by heavy storm or hurricane force winds in the lowest layers, say below 2000ft gnd, affecting helicopters?

Are there any rules of thumb forecasting severe turbulence for helicopters in strong wind situations?

For fixed wing a/c there is a rule of thumb which is: If the vector difference between surface wind and wind at 2000ft gnd exceeds 20kt, there will be severe turbulence.

So i'm wishing you all a happy christmas and always happy landings in 2015 and i am looking forward to see your comments.

Sincerely
Thomas

Try reading Chapter 9 on turbulence and Chapter 11 on Thunderstorms in AC 00-6A (Aviation Weather For Pilots and Flight Operations Personnel) it’s one of the PTS references on this subject. See link below.

Below is a quote from Ray Prouty on this subject with respect to helicopters.

AC 00-6A - Aviation Weather For Pilots and Flight Operations Personnel

Turbulence- In meteorology, any irregular or disturbed flow in the atmosphere.

Does a helicopter behave any differently than an airplane when flying through turbulence? Yes and no.

From the standpoint of ride quality, a helicopter flying through turbulence has an advantage over an airplane because of the looser attachment of the rotor to the fuselage compared to a wing. This allows the inertia of the blades to absorb some of the gust-induced variations in thrust before they are transmitted down to the airframe.

A classic demonstration of this difference was made many years ago by the NACA (NASA's predecessor), which flew two aircraft of the same size, a Sikorsky S-51 and a Cessna airplane, in formation through rough air. Each aircraft carried an instrumentation package that recorded the cockpit's vertical acceleration in Gs. The data analysis showed that the airplane's ride was nearly twice as bouncy as the helicopter's.

While making a landing approach, an airplane is particularly vulnerable to horizontal wind shear, especially if the headwind that it was depending on for airspeed suddenly quits or even momentarily becomes a tailwind.

If this reduces the speed of the air over the wing to less than the stall speed, a crash landing is the probable result, and lately several well reported examples have occurred involving commercial airliners. Strong windshears are usually connected with thunderstorm activity.

For a helicopter, the existence of a horizontal windshear is inherently less dangerous. However, if the horizontal shear were also accompanied by a strong downflow, the helicopter might be driven into the ground before the pilot could react. I have heard of at least one accident where this might have been the case.

The same local meteorological phenomena that produce a change in wind with altitude can also cause a change with time so that a wind reported to be out of the north a few minutes ago may now be out of the south and if the direction is important in making piloting decisions, the pilot may be now making the wrong one.

Flying into a vortex left by the wingtip of a large airplane can upset a helicopter just as easily as an airplane. One of these invisible whirlpools can persist for several minutes before becoming unstable and self-destructing. Up until then, it is strong enough to roll any small- or medium-sized helicopter or airplane unfortunate enough to try and fly down the center-or even close to it.

Some people have said that a helicopter rotor will destroy the vortex by chopping it up. Don't you believe it! Even if it were true, the destruction would come too late.

The vortex pattern left by helicopters has, on occasion, been blamed for upsetting small aircraft. However, the more-likely culprit was the downwash in the immediate vicinity of the rotor. Inherently much less stable, the complex pattern of rotor-tip vortices contains the seeds of its own rapid conversion into unorganized turbulence that quickly dissipates.”

Wind blowing over mountain ridges and peaks generates flow patterns that can surprise an inexperienced helicopter pilot. For instance, approaching a landing site by flying upwind is usually good advice—but if that site is on the top of a mountain ridge, the approach is through air that is tumbling down into the valley below so the helicopter has to climb just to maintain altitude.

If the power available is marginal, as it often is in hot-and- high conditions, the landing might be in jeopardy. Experienced mountain fliers cautiously approach such spots along a crosswind path over the upwind side of the ridge, turning into the wind just at touchdown.”

On a smaller scale, the same type of flow disturbances are present near the ground whenever a building, wall, or tree line presents an obstruction to the wind.

Flight In Turbulent Air; Helicopter Aerodynamics I; Ray Prouty

Edited December 24, 2014 by iChris

apacheguy · December 23, 2014

Thomas,

What kind of helicopters are the German Navy flying?

To answer your question - most helicopters aren't permitted to fly in 'severe' or 'extreme' known or forecast turbulence. So as far as you forecasting severe turbulence for helicopters, you basically need to know that if you forecast severe (or extreme) turbulence that all helicopters need to land (or never take off) prior to the forecast taking effect. It's usually better to be conservative in your forecasts, as you probably know.

Viel Spass.

ThomasLineal · December 29, 2014

Thomas,

What kind of helicopters are the German Navy flying?

To answer your question - most helicopters aren't permitted to fly in 'severe' or 'extreme' known or forecast turbulence. So as far as you forecasting severe turbulence for helicopters, you basically need to know that if you forecast severe (or extreme) turbulence that all helicopters need to land (or never take off) prior to the forecast taking effect. It's usually better to be conservative in your forecasts, as you probably know.

Viel Spass.

Hi, thank you for your reply.

At present the German Navy is flying Sea Lynx Mk88a, Sea King Mk61 and some EC135.

What i am looking for are some "hard meteorological facts" because arguing with airmen is easier for both sides. At this point, i know, that i have to distinguish between planes and helos, but there is still a lot room for discussion.

Thomas