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Blackhawk VS Cirrus


RkyMtnHI

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Hi Folks,

 

I sincerely apologize for not posting much lately, we have been very busy and it's hard for me to keep up. I am also a bit stymied by the negative stuff here on VR, seems to go in cycles.. the thing is, the negative nancys keep really smart folks from posting.. it's a bit depressing. Thanks to the folks that keep being positive and are a part of the solution. The rest of you should consider the damage you are doing to a great platform for safety.... just saying.

 

The pilot in this video was not hurt critically; he had some scrapes and bruises, was shook up, but basically came out of it in good condition. I spoke with him yesterday and he was happy to share his experience for the good of safety. This event took place at a non-controlled airport.

 

In the video, you can see that a Cirrus SR-20G2 aircraft was flipped on final by a Blackhawk helicopter’s downwash. The pilot of the Cirrus said that as soon as he realized that he was in turbulence he went to full power with ailerons and rudder full right, but to no avail, the turbulence forced him over and the wing tip struck the runway.

 

The Cirrus was on left downwind, heading South and about to turn base for a full stop landing, the helicopter was on the numbers. The helicopter pilot departed to the North, he also side-stepped to the parallel taxiway to get out of the fix-wing’s way. The winds were about 3kts at the time. What is truly remarkable is that the event happened ~28 seconds after the helicopter departed (the turbulence was still there).

 

While speaking with the pilot of the Cirrus he said that there was another similar incident a few years ago here in Colorado, he is sending me info on that and I will pass it on as well. In that situation a very high timed Air Force Instructor pilot departed in a Diamond 20 and encountered the same issue, only it was ~75 seconds after the Blackhawk had departed and was at 40ft on departure. Even tho the diamond crashed, I believe both pilots in the Diamond were ok then as well.

 

Please pass this video on to others and discuss it with your students and fellow pilots.

 

 

Fly safe,

 

dp

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Almost every flight school teaches pilots in training about wake turbulence but what schools actually make it known in this sense?

 

This should be in next weeks safety/CFI meetings and SMS hazard recognition enlightenments everywhere!

 

DP, send this to our USHST (Scott, Nick & Steve) for Reel Safety production.

 

Mike

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I'm a little dubious of the vortex explanation. Watch the dust blowing in the direction of landing after the Cirrus crashes- he's downwind.

The airplane rolls left well before the point where the Blackhawk pitches down to accelerate. The vortex should have moved farther down the runway (downwind departure) instead of upwind.

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Interesting... I recently almost flipped a C150. I was doing some Mx hovering in the Huey and I saw a plane taxiing towards me. I got on the radio and asked ground if he could hold the 150 for about 30 seconds, so ground said "Cessna 1234 the helicopter suggests you hold for about 30 seconds." Our pad is just feet from the taxiway. The pilot said "No thanks, he's not paying my hobbs." Ground then said "Cessna 1234, hold in place." I love flying at an airport with a bunch of crotchety old retired guys. It was somewhat satisfying watching him taxi past with me at idle and seeing his wings rock :)

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Im sad for my airplane only brothers and sisters. Interesting to read about this on FW discussions and seeing how many FW pilots have no concept of how a helicopter flies. Things like "No way wing tip vortices could do that, helicopters wings have very little surface area compared to an airplane ." I guess my fellow FW brethren are not aware that a helicopter is a gigantic air pump/wind machine.

 

As an airplane CFI... and as someone who started this journey in airplanes, I can testify that the vast majority of airplane pilots know absolutely nothing about helicopters. I even recently heard an airplane pilot locally bark at a Forest Service Huey pilot about not flying a standard traffic pattern at a local uncontrolled airport. When I was in CA, I was landing on the designated "H" and a guy came out waving his arms at me from a hangar nearby. His comment to me was that I needed to land with less pitch in my blades as to not blow dust. The stories go on and on.

 

I do believe a portion of this was a pilot being caught completely off-guard. The pilot would have known a UH60 had just departed. Turbulence, yes... probably made worse by improper response.

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Although I have been known to stir some stuff up, it was certainty not my intentions here.. Rather to promote safety and get pilots thinking. I will agree that the title on the video could have been better stated and I will try to get that changed.. something like ‘Blackhawk downwash may have been a contributing factor in Cirrus crash’.

 

That being said, it seems that there are more than a few similar situations, some even documented by the NTSB. Here is what I found after a quick search.

 

http://www.ladieslovetaildraggers.com/blog/karen-croskell-blackhawk-rotor-wash/

 

 

http://www.kathrynsreport.com/2014/01/diamond-da20-fort-carsons-butts-army.html

 

 

https://ntsb.gov/aviationquery/brief.aspx?ev_id=20140130X20427&key=1

 

 

 

I have also been contacted personally since the post by people that have been witness to these exact situations.

 

What ever the case, at the least, this is a great teaching tool, and in my humble opinion is an issue we all need to be more aware of.

I have been promised a 33 page report from the gov about these issues, i will post it when it shows up.

 

I was hoping Chris would have some additional info.

 

dp

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We're taught all about their wake turbulence are they not taught about ours?

There's probably a general level of ignorance for fixed wing guys operating near us. Truth be told, a blackhawk can f*** up a small plane and even a big plane.

 

We do displace a large volume of air however, I'm skeptical whether the turbulence coming off the rotor system was the sole reason for the crash rather than being a contributing factor.

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  • 2 weeks later...
  • 2 weeks later...

The cirrus starts to roll hard left from level flight at 15:28:19. At 15:28:22, three seconds later and just prior to impact, the wings are 90º from level. That’s 90º in 3 seconds (30º/sec roll rate). Unaware of the hazard and caught by surprised, it would take at best 2.5 seconds to acknowledge and react to the situation. By that time he would have already passed 75º of left roll (2.5 x 30º/sec roll rate). He only has .5 second remaining to level out. Not enough time or altitude, game over.

 

Wake and Vortex Related issues have been a problem for some time. NASA technical paper references back as far as 1956. The following quote is from NASA technical paper TM-81920 in1980. The color figures are from NASA's 2008 technical paper titled, Rotorcraft Downwash Flow Field Study to Understand the Aerodynamics. The B&W figure is from REF 1.

 

The wake velocities from the UH-60 are show in the figures below. You can see why the wake is a hazard. Calm winds or an issue too. Stronger winds help dissipate the wake. The velocities can be approximated for sea level as:

 

v1 = 14.5 √(disk load) = velocities at the rotor

 

v2 = 29 √ (disk load) = velocities in the remote wake

 

Disk load = gross weight/rotor disk area (π x r2)

 

For the UH-60 with a disk load of 9.7 @ 22,000 lbs. gross, computes out to 90 ft./sec or 60 mph velocities in the remote wake.

 

Over the Past twenty-five years, NACA and NASA have conducted numerous studies of the induced velocities near a lifting rotor. The results have been presented in papers aimed at design and research engineers rather than operators; as a result, the operational consequences of these studies are not widely known among helicopter operators. The present paper reviews a number of these fundamental studies and attempts to draw out the operational implications and restrictions of these studies in a form specifically aimed at the user.

 

Wind-tunnel measurements show that the wake of a rotor, except at near hovering speeds, is not like that of a propeller. The wake is more like that of a wing except that, because of the slow speeds, the wake velocities may be much greater. The helicopter can produce a wake hazard to following light aircraft that is disproportionately great compared to an equivalent fixed-wing aircraft. This hazard should be recognized by both pilots and airport controllers when operating in congested areas.

 

The evaluation of vortex hazard is a complex business involving many factors such as span; span wise load distribution, and wake roll-up. Nevertheless, the simple order-of-magnitude analysis of figure 4 indicates that helicopters produce wakes of such significant strength that they must be treated with respect. Helicopter pilots and air traffic controllers should be constantly aware of the unseen hazard in the wake of a helicopter. Fatal accidents involving light planes intercepting helicopter wakes have already occurred. Indeed, the experimental study of reference 31 was initiated as consequence of one fatal accident twenty years ago in Chicago.

 

A gross indication of the magnitude of the induced velocities behind a modern rotor can be obtained from momentum theory. Wh (v1) being the average induced velocity at the rotor when hovering out of ground effect. This average velocity doubles in the remote wake at about three-quarter of a rotor diameter below the rotor disk.

 

Wh = √ (T⁄2ρA) ft/sec

 

REF 1

REF 2

REF 3

 

Note: the color-coded bar on the right below shows the average flow velocities in ft./sec. {89.3 ft./sec = 60 mph = 53 knots}

Screen%20Shot%202015-02-24%20at%2010.36.

Pages%20from%2019810007470_zpsqe2xalpe.j

Edited by iChris
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Taken together, the rotor downwash and rotor tip vortex produce disturbance in the air near the ground. The magnitude depends both on the disk load, which governs the downwash velocity and the gross weight that governs how much air is being moved. The end results are sometimes unpredictable vortices influencing the entire flow throughout the wake. Wind generating capabilities of a few helicopter below.

 

The disk loads of the R22 Beta and R44 II are 2.7 and 2.9 respectively. The disk loads of the Bell 206B3 and MD500E are 3.84 and 5.48 respectively.

 

Scan-1_zpsf9cqzkmb.jpg

 

Pages%20from%20wake-related%20operationa

REF 1

Edited by iChris
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I'm a little dubious of the vortex explanation. Watch the dust blowing in the direction of landing after the Cirrus crashes- he's downwind.

 

The airplane rolls left well before the point where the Blackhawk pitches down to accelerate. The vortex should have moved farther down the runway (downwind departure) instead of upwind.

 

However, we don’t know from the video were the UH-60 started it’s hover from. The wake and vortex related velocities begin in the hover once the rotor is supporting the full weight of the helicopter. Did the helicopter start it’s hover near the approach end? They say it was on the numbers. Was it hovering over the numbers or wheel taxing? How far did the UH-60 hover down the runway before takeoff?

 

Yes, he's downwind, watch the dust blowing. Also note how the calm winds are slow to dissipate the dust. Again, calm winds or an issue too. Stronger winds help dissipate the wake. Most of these vortex related accidents occurred in calm wind conditions.

 

Moreover, had they been aware of the hazard and not caught by surprise, this accident may have been avoided.

Edited by iChris
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I'm a little dubious of the vortex explanation. Watch the dust blowing in the direction of landing after the Cirrus crashes- he's downwind.

The airplane rolls left well before the point where the Blackhawk pitches down to accelerate. The vortex should have moved farther down the runway (downwind departure) instead of upwind.

 

Landing runway 33 with winds 110 at 3 per http://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20141208X21730&key=1&queryId=3e594c4a-2ea0-4590-9c53-64c5b914f4e1&pgsize=100

 

From the AIM - https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0703.html

A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. Thus a light wind with a cross runway component of 1 to 5 knots could result in the upwind vortex remaining in the touchdown zone for a period of time and hasten the drift of the downwind vortex toward another runway. (See FIG 7-3-6.) Similarly, a tailwind condition can move the vortices of the preceding aircraft forward into the touchdown zone. THE LIGHT QUARTERING TAILWIND REQUIRES MAXIMUM CAUTION. Pilots should be alert to large aircraft upwind from their approach and takeoff flight paths.

 

 

So, basically - the exact condition to cause the vortex to remain on the runway. (emphasis added by FAA)

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Here are some thoughts that could go both ways on the issue, but I'll also follow with some personal experience that puts me in the doubters catagory.

1. That Blackhawk doing a little training jaunt to a civilian airport was not at 22,000lbs. Probably more like 15,000. If they just took on fuel they might be around 16,000

 

2. Ground effect will dissipate vortices so much that they really are more like turbulence then a clean constantly rotating vortex like we see from a dirty airliner on approach that weighs 200,000lbs.

 

3. The quote above talking about a vortex remaining in the touchdown zone has to do with a large fixed wing landing not a medium helicopter taking off.

 

ON THE OTHER HAND:

4. Wake turbulance of any kind is subject to so many factors that it can be quite unpredictable.

 

5. The Cirrus in question appears to pitch up sharply prior to rolling. Obvioulsy something crazy happened.

 

PERSONAL EXPERIENCE: Before I was flying Apaches I spent five years as an enlisted soldier whose job it was to not only work on the armament systems of apaches but also to load them at the FARP. I've been everywhere from right underneath to a couple hundred yards away from heavy apaches taking off. They do produce a lot of wind and turbulance but it doesn't take very long to dissipate. Even in a no wind environment.

 

It is certainly possible that the wind from the blackhawk caused this accident. I'm just not quite there yet.

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Here are some thoughts that could go both ways on the issue, but I'll also follow with some personal experience that puts me in the doubters catagory.

 

1. That Blackhawk doing a little training jaunt to a civilian airport was not at 22,000lbs. Probably more like 15,000. If they just took on fuel they might be around 16,000

 

2. Ground effect will dissipate vortices so much that they really are more like turbulence then a clean constantly rotating vortex like we see from a dirty airliner on approach that weighs 200,000lbs.

 

3. The quote above talking about a vortex remaining in the touchdown zone has to do with a large fixed wing landing not a medium helicopter taking off.

 

It is certainly possible that the wind from the blackhawk caused this accident. I'm just not quite there yet.

 

The tip vortices are the byproduct of lift. The tip vortices always remain as part of the wake dynamics, in or out of ground effect. The flow is mostly rotating on an axis line that occurs either on a straight-axis or a curved-axis.

 

The magnitude of the wake depends both on the disk load, which governs the downwash velocity and the gross weight that governs how much air is being moved. Newton law governs the conditions at the rotor:

 

Force = (Mass) x (acceleration).

 

For the rotor to support the weight of the helicopter it must accelerate a mass of air on a continuous basis equal to the weight of the helicopter:

 

Weight = Thrust = Lift = (Mass flow per second) x (total change in flow velocity)

 

L= (m/sec) x (Δv)

 

In order words, you need to take air from above the rotor and accelerate it down through the rotor at a mass flow rate of 15,000 Lbs. per/sec. That a lot air moving at 25 - 50 MPH in the wake.

 

Screen%20Shot%202015-02-27%20at%2010.02.

 

UH-60 Gross wt @ 15000 Lbs.

Rotor Radius 26.83 Ft.

Disk Load 6.632847939

V1 = 37.34375824 Ft./Sec. = 25.45 MPH

V2 = 74.68751647 Ft./Sec. = 50.9 MPH

 

Don’t forget the NASA test….

 

Wind-tunnel measurements show that the wake of a rotor, except at near hovering speeds, is not like that of a propeller. The wake is more like that of a wing except that, because of the slow speeds, the wake velocities may be much greater. The helicopter can produce a wake hazard to following light aircraft that is disproportionately great compared to an equivalent fixed-wing aircraft.

 

The evaluation of vortex hazard is a complex business involving many factors such as span; span wise load distribution, and wake roll-up. Nevertheless, the simple order-of-magnitude analysis indicates that helicopters produce wakes of such significant strength that they must be treated with respect. REF 1

 

Safety Work shop

 

DOT-VNTSC-FAA-95-04

 

Scan-1_zps1azwjxdm.jpg

 

 

Screen%20Shot%202015-02-27%20at%2012.46.

 

Screen%20Shot%202015-02-26%20at%2012.44.

Edited by iChris
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I'm a little dubious of the vortex explanation.

 

 

I wonder where the Blackhawk started their takeoff. He had quite a bit of speed already when the video starts.

 

 

We do displace a large volume of air however, I'm skeptical whether the turbulence coming off the rotor system was the sole reason for the crash rather than being a contributing factor.

 

 

Here are some thoughts that could go both ways on the issue, but I'll also follow with some personal experience that puts me in the doubters catagory.

 

It is certainly possible that the wind from the blackhawk caused this accident. I'm just not quite there yet.

 

Again, we don’t know from the video were the UH-60 started it’s hover from. The wake and vortex related velocities begin in the hover once the rotor is supporting the full weight of the helicopter. Did the helicopter start it’s hover near the approach end? They say it was on the numbers. Was it hovering over the numbers or wheel taxing? How far did the UH-60 hover down the runway before takeoff?

 

​The fact that this was only the pilot's second solo flight is an issue. How much turbulence was really encountered? Was the pilot just overwhelmed and lost control?

 

The hazard from helicopter wake turbulence is real; however, awareness is key. In most cases recovery is possible. Need not end in an accident.

 

 

The solo student pilot stated that he entered the traffic pattern at FNL for a full stop landing on Runway 33. He observed a Sikorsky UH-60 helicopter on downwind and delayed his turn to base until the helicopter was on final, abeam his position. While on final, the student pilot adjusted his aim point to land long, as he was concerned with wake turbulence and wanted to land beyond the helicopter's touchdown point. Just prior to landing, he encountered turbulent air and attempted to go around. The airplane subsequently impacted terrain and cartwheeled, which resulted in damage to the fuselage and wings. NTSB Identification: CEN15LA069
Edited by iChris
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  • 3 weeks later...

Based on nose attitude at the beginning of the video, I'd guess the Blackhawk either lifted off the ground after ground taxing or took off from a hover somewhere from the left side of the screen, but not off of it. At the beginning of the video it looks like 5ish degrees nose low, then moves to more like 10ish degrees nose low as it accelerates. A Blackhawk with a more aft CG (HH-60G, UH-60M) hovers about 5 degrees nose up. Lowering the nose to that 5ish low starts accelerating, as you go through ETL one typically lowers the nose to about 10 low to favor acceleration. That would be pretty close to a "normal" or "marginal power" takeoff profile. All that said, I'm not sure how different a light weight UH-60L with a more forward CG compares., and I certainly can't tell what model 60 that is in that video.

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