Wake Turbulance & Helicopters

[DynamicrolloverHecknoo]

I would like to start a dicussion regarding wake turbulence with helicopters. Many aviation books and FAA test commonly lump fixed-wing and rotor together. I was trained that wake turbulance is a serious threat and how to avoid it. I understand that wake turbulence is a serious threat to fixed wing and have experienced it first hand, but is it a significant danger to helicopters? I fly around large commercial aircraft daily and have never felt more than a burble due to wake turbulance. A respected CFI and I were discussing the aerodynamic involved and he felt wake turbulance is not a factor to helicopters. He felt the rotor system chops through the turbulance and a helicopter does not have the large horizontal surface area like a airplane needed to roll the aircraft. I took this a step further and researched accidents, but was unable to find one incident where wake turbulance was determined to be the primary factor in a helicopter accident.

 

I'm not going to go looking for wake turbulance, but this thinking could influence some of my departures and approaches.

[nightsta1ker]

I operate out of Boeing Field in Seattle, so we have a lot of heavies coming and going. I have experienced wake turbulence so bad that I almost crashed. Another danger is jet-wash. It can and will affect helicopters. Sorry to say your instructor is mistaken. Helicopters do deal with turbulence better than fixed wing aircraft, but it does not mean we are immune to the effects of a vortex or strong gust of wind.

[SBuzzkill]

I've been rocked around sitting on the ground in a 10,000 gallon fuel truck by wake turbulence from a landing 767. I would definitely not want to experience it in any sort of helicopter.

[Wally]

The rotor "chopping wake turbulence" and rendering it ineffective doesn't hold together logically. There's a lot of energy in that wake, and flying down it or too close across, behind and below would be bad.

 

What does help, I think is the fact that blades have higher wing loading, are more flexible than most airplane wings and gyroscopic plane rigidity, precession will affect the aircraft reaction to wake vortices. What doesn't help is the usual limited control authority in most helicopters. Helos just don't do well aerobatically because they don't have the roll or pitch rates, not mention needing postive G.

Edited July 15, 2012 by Wally

[Retreating Brain Stall]

Oh, you'll feel it if you push the envelope too close! You've been operating in the green which is good- ATC must be doing there job and your keeping your distance from the large/heavies. Don't become a test pilot or mislead because you haven't experienced or felt anything while being cautioned wake turbulence and never felt anything. The wind may have been in your favor many times but it only takes once for that wind to be going the right way for that turbulence to find it's ways to your rotor system. So here's a question for you, if your thinking wake turbulence is nil for helicopters than, what about regular turbulence or wind shear then? Do I magically chop through the air and never become prone to it?

Don't become complacent:)

[tracon]

Even though rotors create wake turbulence. They will not prevent you from becoming a victim of wake turbulence.

 

Here's a link for more information about wake turbulence.

http://www.faa.gov/training_testing/training/media/wake/04SEC2.PDF

[nightsta1ker]

Tack on the fact that wake turbulence avoidance is a critical task in the Commercial/CFI PTS. In fact, I almost failed my check ride because I neglected to point out a heavy that was landing parallel to me while on my checkride. I took the necessary precautions to avoid the turbulence, but I wasn't TALKING about what I was DOING, and the examiner spanked me pretty hard for it. We got back and before he got out of the aircraft he said "I almost failed you back there... we'll talk about it later". Turned out it was the wake turbulence avoidance thing. He showed me that it was a critical task in the PTS, and we discussed what I did wrong and what I should have done. So now teaching wake turbulence avoidance is pretty high on my list of things I teach my students, including the mandatory 2 minute wait if a heavy just landed.

[Mikemv]

From FAA-S-8081-15A Private Pilot Rotorcraft PTS 2005

Special Emphasis Areas

Examiners shall place special emphasis upon areas of aircraft operation

considered critical to flight safety. Among these are:

1. positive aircraft control;

2. procedures for positive exchange of flight controls (who is flying

the aircraft);

3. collision avoidance;

4. wake turbulence avoidance;

5. runway incursion avoidance;

6. CFIT;

7. wire strike avoidance;

8. ADM and risk management;

9. checklist usage; and

10. other areas deemed appropriate to any phase of the practical test.

Although these areas may not be specifically addressed under each TASK,

they are essential to flight safety and will be evaluated during the practical

test. In all instances, the applicant’s actions will relate to the complete

situation.

This PTS is currently involved in an upgrade/revision and Wake Turbulence will remain in the new edition!

While thinking about Wake Turbulence, pilots should be taught about flying thru rotor downwash and rotor wash courtesy also.

One last point, CFIs should not make things up and teach them!!!!!!!!!!!!!!!!

In my Seminars, I state that there are no "Mikeyisms". I can document or give references for all info I put out.

So, for me, the well respected CFI just went to "needs remedial training". I hope he/she will accept it and re-teach this to his students.

Mike

[tracon]

Here's an NTSB report where an airplane crashed and the pilot was killed as a result of the helicopters wake turbulence.

 

http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20001212X20445&ntsbno=ATL00FA026&akey=1

[aeroscout]

Here's an NTSB report where an airplane crashed and the pilot was killed as a result of the helicopters wake turbulence.

 

http://www.ntsb.gov/...L00FA026&akey=1

 

That seems to be the opposite of what the thread is about.

[DynamicrolloverHecknoo]

Thanks for the discussion. For the record, I'm not a student and this was just a descussion on wake turbulence with a friend/CFI. He is generally a very knowageble guy and I respect him for questioning everything while trying to dig deeper than just "because thats what the PTS says." I'm sure he is following the PTS when instructing students.

 

His argument made sense to me, but I wasnt fully buying in and hoped to get some firepower to bring to the table. I was really looking for some firsthand helicopter wake turbulance war stories or NTSB reports...1 out of 2 isnt bad…Thanks again

[tracon]

That seems to be the opposite of what the thread is about.

 

The thread was concerning wake turbulence. So if you feel flying behind an s76 isn't an issue in another helicopter then so be it. The bottom line is wake turbulence is wake turbulence regardless of which type of aircraft has caused it, because it can and will kill you when created by another helicopter just as easily as when caused by a fixed wing aircraft.

 

Aircraft other than a "Heavy" can and do cause wake turbulence, a point students reading this thread need to be aware of and according to the original post just might not be happening.

[superstallion6113]

I got in the wake turbulence of a C-17 once, in a CH-53E, 79ft main rotor, 3 engines, 13,000hp. You better believe the pilots had their hands full controlling it. If it effected a 45k lb helo, I'd hate to see what it'd do to an R44.

 

Flying section flights with the VH-3s, and the CH-46s, they were always cautious to stay out of our wake turbulence, because it would effect them when we were grossing 70k lbs with cargo and full fuel. Helicopter wake turbulence is def a factor when flying around larger helicopters.

Edited July 16, 2012 by superstallion6113

[Mikemv]

Thanks for the discussion. For the record, I'm not a student and this was just a descussion on wake turbulence with a friend/CFI. He is generally a very knowageble guy and I respect him for questioning everything while trying to dig deeper than just "because thats what the PTS says." I'm sure he is following the PTS when instructing students.

 

His argument made sense to me, but I wasnt fully buying in and hoped to get some firepower to bring to the table. I was really looking for some firsthand helicopter wake turbulance war stories or NTSB reports...1 out of 2 isnt bad…Thanks again

 

Dyna-Hecknoo,

 

The PTS does not "Say" what Wake Turbulence is, it only states it is an area of special emphasis that the CFI you are talking about should have learned a long time ago during the time he attained his many Certificates. He should not be questioning it at this point in his career!

 

Also, a CFI should not follow the PTS but rather an established syllabus as the PTSs are not meant to be training standards but are minimum testing standards.

 

Maybe you have portrayed him incorrectly but not much respect for him from what you stated here.

 

Mike

[tracon]

Posted for those who are interested in learning more about wake turbulence.

 

The area "contaminated" by the helicopter wake turbulence is clearly larger than that of an airplane of comparable size and weight, especially at speeds below 70-80 knots.

 

Data indicate that the strength of wake vortices generated by large rotorcraft cannot be accurately predicted by the fixed-wing vortex strength equation for helicopter airspeeds below about 50 knots. The data show that the wake vortex will reach its maximum strength as helicopter airspeeds approach the 40-60 knot range and that this strength will fall off as airspeeds drop below about 40 knots. LDV data collected for smaller rotorcraft showed the same phenomenon, but shifted downward in airspeed; maximum vortex strength for these helicopters was observed at airspeeds around 40 knots. LDV data collected for the tandem rotor CH-47 showed a wake vortex strength that was approximately 2/3 that which would be generated by a single rotor machine of equivalent weight under identical conditions. Probe tests indicated that rotorcraft wake vortices resemble fixed-wing vortices. This resemblance became especially evident when rotorcraft airspeeds were 80-100 knots and above.

 

Operations at uncontrolled airports present potential wake vortex hazards when rotorcraft are mixed with small fixed-wing traffic. Wake vortex problems have been noted when medium sized rotorcraft operate at airports where relatively low performance, light, single, and multi-engine general aviation aircraft are operating. Typical separation distances between light aircraft at uncontrolled fields can be as little as 3000 feet, roughly 30 seconds elapsed time at standard pattern airspeeds.

 

Medium weight helicopters, such as the S-76A and UH-1, can easily fit into the traffic pattern at smaller uncontrolled airports and can leave active, potentially hazardous vortices for up to 90 seconds. Separations for small aircraft behind these rotorcraft should therefore be in the 90-second range.

 

Larger helicopters such as the CH-47D and CH-53E can also fit into these uncontrolled fields and were observed to have longer hazard times. A 120-second separation

should be adequate for operations behind these rotorcraft. Likewise, takeoffs for small aircraft behind helicopters should use the same time as currently specified for operations at uncontrolled airports.

 

An indepth study covering helicopter wake turbulence is available @ http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA318103