Black Hawk Accident was NOT settling with Power [Archive]

rjsquirrel

1st Jun 2002, 17:37

The speculating is terrific, but often self defeating. We "learn" what happened before we know what happened, then we carry that "knowledge" around as truth while the real cause is announced weeks later, and we ignore it.

Some facts:

It was not settling with power. The rate of descent needed to get into vortex ring state is over 2500 fpm for a Hawk, and I saw nowhere near that on the tape. See RW-1's page for an explanation:
http://www.dynamicflight.com/aerodynamics/settling_power/

It was not LTE. A Black Hawk can hover in a 45 knot wind at 10,000 feet DA. The tail rotor is notoriously good, unlike many light helos and Bells.

It could have been a bunch of things, from failures of engines or systems, to loss of reference to simply "over-pitching" or running out of power.

In fact, the coning we see could be from a drop in rpm, and if great enough, it could get the tail rotor on the stops (not LTE, because the rpm drop is the cause). The gentle right turn is consistant with that rpm loss scenario, and the aircraft could start settling against the slope, set off the pilot's personal anxiety alarm bells (thus the cyclic wiggling) and then the main blades hit the slope and that was all she wrote.

One thing, that intact fuselage when the dust settled shows a safe machine, which held its shape and protected its occupants. In a Huey, the transmission would have unplugged and landed in the laps of the pax to help stir things up. Says a lot about US Military crash requirements.

The Sultan

1st Jun 2002, 19:32

rjsquirrel

Your post must be a joke.

Helicopters get in VTR in a low speed situation at much lower sink rates than 2500 fpm.

As to LTE, that is why the 60 is not seen in Afghanastan.

One last thing, a Bell 407 filmed the crash of the 60 from above. This must mean that it is not too bad to be in a Bell.

The Sultan

:rolleyes: :rolleyes: :rolleyes: :rolleyes:

rjsquirrel

1st Jun 2002, 20:00

the sultan,

You obviously don't know what settling with power is. As described in the texts (such as the one pointed to) the downward motion of the rotor must get close to the velocity of the downwash to enter vortex ring state (also called settling with power). Some pilots confuse running out of power and then settling as a result as being "settling with power". They are wrong, and their mistake could lead some people to believe that true VRS is common as an accident cause. It is not common, it cannot occur at low rates of descent, unless you are in an ancient helicopter with very low disk loading, like an R-4. For a Black Hawk, the VRS boundary descent rate is about 2500 fpm, for a Huey, about 1200 fpm.

The H-60 is used all over Afghanistan, just not where you were, I guess. There are dozens of H-60 types used regularly over there. The incident that made all the press was an overloaded 60M, operated way above its flight manual limits (a genuine combat emergency, not a crew error).

One would have thought that you might be able to differentiate between a news helicopter cruising at 40 or 50 knots and a heavily loaded rescue helicopter in an OGE hover. You don't understand, obviously. Work on it, OK?

It is not too bad to be in a Bell. I've got a thousand hours of combat in one. The reference to a Huey is correct, I've seen enough of them toss their transmissions after a blade strike to know. The older helicopters, Bell included, are not designed to the same tough standards as the more modern ones. I understand that the newer Bell models are quite tough.

Steve76

1st Jun 2002, 20:09

Has anyone considered an engine failure?
The little kick to the right got me thinking?

ALSO, lets all remember that this is the PPRUNE and if you don't know what that stands for then you should not be here.

Lets speculate until the cows come home!

:p

Nick Lappos

1st Jun 2002, 21:18

Steve,
Anything's possible! That's what mr squerril is talking about!

A slow power failure could explain alot. The investigators will iron it out.

Nick

donut king

1st Jun 2002, 22:39

Don't speculate!!!

That does nothing but create rumour and gossip. Go to the ENQUIRER website for that.

We are supposed to be professional, thinking individuals.....

Let's just be glad those 4 crewmembers survived. That Sikorsky fuselage gets my praise.

I will wait for the investigation and actual account from those involved.

D.K

Gibbo

2nd Jun 2002, 01:21

Why not Speculate Donut King ???

Speculation here will not influence a BOI or investigation. In terms of professionalism it is entirely professional, IMHO, to hypothesise about these cases; it can be very educational and may even prevent an occurrance in the future.

There is an age old tradition in aviation of going over situations informally, either in the crew room or the bar, why not continue it here?

As long as there is no finger pointing or disrespect I see problem.

Gibbo

BlenderPilot

2nd Jun 2002, 03:09

I also don't think it could have been SWP but, I strongly disagree with the statement that you need a rate of descent in excess of 2,500 fpm to encounter one (even in the "hawk"), especially at higher altitudes, my home airport is at 7,340 FT AMSL and you can easily get into a SWP situation with say a 700 fpm true vertical descent (no wind), in any heavy helicopter at this altitudes.

When it comes to SWP its mostly about weight, disk load, and power, and there is not a lot of differece between say a 412 or a UH60, actually the 60 may be even more vulnerable due to higher weight and a lot more power.

I think its irresponsible and ignorant to make such affirmative statements, considering many newer pilots are reading this forums to build up their "decision making criteria" and settling with power is affected by so many variants.

2,500 fpm descent to have SWP, come on. In the position they were a good strong downdraft was all that was needed.

Anyway I personally think it was more like engine related.

Nick Lappos

2nd Jun 2002, 03:32

Blenderpilot,
I have to agree with squirrel. You and sultan are mixing up the fundamental problem of having too little power with the other aerodynamic condition specifically related to reingestion of the downwash known as Vortex Ring State or settling with power.

What I think you are concerned about is a true issue for pilots - the behavior of the machine when you are severely performance limited. That causes most helicopter accidents that occur on landing, is especially a problem at altitude and high temperature, and has nothing at all to do with settling with power.

It is a very common mistake for pilots to lump these things together, and it is passed from instructor to student as gospel, but it is still wrong.

In the case you describe, the engine power available is very little more than the hover power required, so there is almost nothing left to maneuver. Any disturbance, or any downdraft can absorb all the extra power that you have, and you will descend. That is because you don't have the power to climb, since there is no more power left. Most landing accidents at altitude are caused by this. You are right to be concerned if it is glossed over.

However, that is NOT Vortex Ring State, and it is NOT settling with power. To get into VRS, the rotor must descend fast enough to catch up with its downwash. That speed DOES depend on the disk loading, and that rate of descent IS around 2500 feet per minute for a Black Hawk. The downwash speed of an H-60 is about 45 knots, which is about 4500 feet per minute. At about half that rate of descent, the inboard sections of the rotor are eating up wash, and not producing any down wash. No downwash, no lift. THAT is settling with power. For a Huey, with half the disk loading of a Hawk, you get the first nibbles of VRS at about 1200 fpm rate of descent.

The reason why this is important is that you can't go around teaching folks that some mysterious stuff happens to the rotor at altitude, with moderate rates of descent. You must teach them that at altitude with moderate rates of descent, if you don't have enough power, you will get painted into a corner you can't climb out of, and the earth will smite you.

If you think a downdraft can cause VRS or settling with power, you are on the wrong page! That downdraft is dangerous, I agree, if you are in a heavy helo with little excess power, but that has nothing to do with settling with power.

I can mail you some charts and pictures and stuff, or post them on the web to discuss this more fully, if you'd like.

BlenderPilot

2nd Jun 2002, 04:40

Well it seems you are truly informed on the subject, and have certainly clarified the situation to me at least! I was stupid of me to say that "all that was needed was a downdraft", honestly, I know better than that!

You mentioned a downwash speed of 45 KTS in a UH60, I suppose this varies with power power right?

I've been in situations doing ENG in an B206L4 at 8,000 FT AMSL, was hovering with about 80% TQ (no wind), I decided to descend vertically (slowly), so the camera could have a better view of an accident under a bridge which was about a mile away, about 5 seconds later with about 500 FPM the thing was "clearly" in VRS, it shook like crazy, it started wobling slightly, and applying power didn't help at all, on the contrary. At least I think it was VRS.

The helicopter aerodynamics book I have right here states that the actual critical rate of descent to achive VRS is dependant on GW, HD, and "other pertinent factors" but in any case you should consider anything of more than 300 FT/MIN, and nearly 0 airspeed a probable VRS area, I used to think this was excessive even for say 8,500 FT 25 degrees celsius we commonly operate here, now I really don't think this is excessive except when I have lots air below me.

I will definately like to have more info on this subject since I obviously have much to learn, I only have about 2,500 hrs in helicopters, but I have only flown little below say 7,500 FT, this has probably led me to be extra careful with collective application, and helicopter operation, up here the things just tend to drop to ground much faster!

Thanks for your input.

SASless

2nd Jun 2002, 10:21

Nick,

Do you recall the crash of a hovering Canadian H-3 which occurred at an airshow....the video has been shown all over several TV programs. The incident occurred when the aircraft was being repositioned from one parking spot to another and appeared to start from a relatively stable hover.....at least in the vertical plane. The aircraft was being turned .....dropped like a stone, impacted terra firma and rolled over. Interviews of the crew as shown on the video were consistent in confirming no mechanical malfunction occurred and the crew stated they had encountered vortex ring state or settling with power....key point being no descent or very slow rate of descent involved at the onset of the problem.

The question in my mind is if it matters how one begins to recirculate the downwash....be it while in descent...or at a hover....it would seem so long as you find yourself in a downward moving column of air and application of additional collective (power) only makes the situation worse....then you have arrived at the point of settling with power/vortex ring state or some very similar phenomenon however you want to name it.

In an ideal test, while in VRS, would not rate of descent vary with application of collective pitch? Higher rate of descent for increase in collective pitch applied and reduced rate of descent with decrease in collective pitch applied. Which is directly opposite of the reaction in normal flight conditions?

Nick Lappos

2nd Jun 2002, 11:16

BlenderPilot,

The stuff you were taught about the factors that cause VRS are correct, because they all affect the downwash speed, which directly affects the VRS condition.

Power required to hover is prime. As the weight grows, you need more power (and more downwash) to support the machine. Remember that Bernoulli is interesting, but it is the stream of air you are blowing downward that makes the lift to hold you up. More weight means more downwash velocity, so the VRS rate of descent actually increases slightly.

In your case with the 206, what was your approximate weight? How much maximum torque did you have available (as compared to the 80% you needed to hover)? What was your maximum available vertical rate of climb?

I published a web page that explains this, and I used a 3,500 lb Bell 206 at 8,000 feet for the example. See this pointer:

http://mywebpages.comcast.net/llappos/

I hope this helps explain things.

SASless I added some discussion of what probably happens in many cases when a pilot falls through on approach. I believe that this is what happened to that legendary Sea King. I recall that the machine was spinning right as it descended, probably because the pilot had drooped the rotor to try to hold the OGE hover, and he ran out of pedal. This is appears to be a classic case of "Settling with too little power" and not settling with power (as in VRS).

Regarding the old legend that in VRS more collective makes you go down faster, I don't think that is correct. More likely, if you are in VRS, the collective doesn't do much, and that is disconcerting enough. If you increase collective, you could droop the rotor rpm, and that would be a bad thing, as well.

In truth, real VRS is hard to get into, and hard to stay in. In a powerful helicopter, you can actually muscle your way out by just increasing collective (I was told that the test pilots in the Sky Crane when flying with no load could just climb up out of VRS). This is not true of any working helicopter, so it is not recommended for recovering.

Note from the chart that you can clean up the VRS flow entirely by DESCENDING faster until you get out of VRS, and into autorotation. I have done this countless times in an S-76 during testing. It is a bad idea if the ground is within 2000 feet or so, since the rate of descent gets to 4000 to 6000 feet per minute, a truly sporty fall!

The best cure for VRS is to nudge yourself forward to gain the forward velocity to clean things up, in a jet ranger, by 12 knots you are entirely out of it.

There are two big reasons for making a point out of this distinction:

1) If you try to hover with too little power, that is a chargable foul and indicates poor planning and headwork. This is too common, and is too easy to prevent. To call it VRS allows too easy of an excuse for the pilot.

2) The VRS accident for the V-22 was classic, full blown VRS, and was increased by simply back tilting the nacelles. In tests after the accident, crews were able to induce VRS in LEVEL flight, no descent, while back tilting the nacelles (so much for pilot error, huh?) If we in the helicopter world toss about the definition of VRS improperly, someone in tilt rotor land will say that VRS is common, and such incidents in a tilt rotor are "just like helicopters" so why all the fuss.

Nick

Helinut

2nd Jun 2002, 13:28

Nick,

With reference to your interesting last post and your second paragraph, would it also be the case that at height, as density altitude increased downwash velocity for a hover would need to increase, requiring an even higher RoD to get VRS??

Nick Lappos

2nd Jun 2002, 13:55

Helinut,
Yes, although the effect is not too large. The downwash velocity varies with the square root of the density, so a 25% reduction in density would result in a 15% increase in the downwash velocity. For a jet ranger, downwash velocity at sea level would be about 1600 ft/min, at 8,000 feet altitude, it would be 1800 ft/min.

On the other hand, the power the rotor needs to hover goes up by about 1% per thousand feet, so the aircraft needs 8% more power to hover. Also, engine power falls of sharply with altitude (not easy to predict in a rule of thumb) but if you lost 10% power on the engine, then a good hovering helo at sea level could have a 15 to 18% power deficit at 8,000 feet.

This would lead some folks to say that it is more prone to "settling with power" but in truth it is more prone to "settling without enough power"

Flare Dammit!

2nd Jun 2002, 16:19

I usually never disagree with what the esteemed Mr. Lappos says, but in this case, I think he might be slightly off-base.

We tend to think of helicopter rotors as being absolutely and repeatedly predictable in their behaviour, much like the plank of an airplane is. However, when it comes to what is going on within a rotor system in flight, things get less precise. Ray Prouty has published charts in Rotor&Wing Magazine which show the data points to be all over the place, with a line more or less arbitrarily drawn through them at an "average" to give us a baseline for predicting performance. But what this tells us is that there are many variables - perhaps an infinite amount - which affect the way a particular rotor (or indeed, individual blade) will perform at any given time.

When it comes to VRS or SWP, we tend to focus solely on rate-of-descent of the aircraft, as if it were independant of other factors. Well, the air moves too, eh what? And sometimes that air moves vertically, not just horizontally. Suppose a helicopter developed a small ROD just as it encountered a slight updraft. The net result would be an increase in effective ROD, which might or might not be noticeable unless we were studiously watching the VSI, a gauge not known to be in our instrument scan during hovering ops (mine wags up and down considerably in a OGE hover). Therefore, I personally believe that making declarative statements like: "A UH-60 needs a ROD of 2500 feet before it can ever get into VRS" is the height of irresponsibility. Bollocks! I propose that in addition to the "hard" numbers that we know will aggravate or excite VRS, there are other factors which are quite nebulous that figure into the equation.

Did that UH-60 crew on Mt. Hood get into VRS? Who can know for sure? Even if the charts said (which we usually translate into "prove") that the aircraft should have been capable of the job, perhaps the rotor was closer to the "edge" than we know. Perhaps the rotor's reserve of lift at that HOGE altitude was borderline, and perhaps they experienced just enough of an updraft [in the mountains? come on, mate!] to cause them to fall off the bubble and into incipient (maybe not full-blown) VRS.

Then again, it appeared that they were hovering nose-in toward the slope. Is this true? Maybe I didn't see the whole accident sequence. Why not turn ninety-degrees to the slope? Would that not have given them better visual cues? And would that not have given them an easier "out" of they needed to lower the collective and bail from the situation for any reason? I wasn't there, but I sure wouldn't want to try to hold a stationary hover with a wall of white filling my windscreen.

Contrary to popular belief, gradual engine failures in a twin-engine helicopter do not cause a similar yaw-snap to the right (Yank birds) as they do in a single. As one engine falls off, the other goes to full power or beyond to take up the slack. We do not see any evidence of a catastrophic engine failure from the video (smoke, flame, etc), so I would move away from that line of thinking. Although the extreme coning of the blades does indicate (to me, at least) a reduction in MR rpm - or perhaps a over-application of collective pitch. Whatever DID happen, I believe it to be aerodynamic in nature and not mechanical - we'll soon find out.

Finally, Nick surmises that the Canadian Sea King at that airshow did not get into a setting-with-power situation, but rather a settling-with-too-little-power, owing exlusively to the right-yaw of the nose as the ship came down. Well, old boy, you're entitled to your opinion. I propose that it was indeed SWP (VRS), judging by the incredible rate-of-descent that built up so quickly, and the extreme coning of the blades as the poor pilot yanked everything he had to stop it, to no avail. The right-hand yaw was simply either his failure to fully mash the left pedal all the way (focused as he was on other, more pressing things in that brief second of time he had between "stable hover" and "Crikey, that hurt!"), or the inability of the tail rotor to hold the nose straight with the collective lever on the up-stop.

It is a myth to think that VRS is "hard" to get into, or that it must be excited deliberately. Anyone who would make such foolish statements has obviously never gotten into it inadvertently in the real world. Those of us who have know that all the "hard" numbers in the world don't mean a thing when the bottom falls out when you least expect it. Anytime you're in a HOGE situation you are susceptible, chaps.

Nick Lappos

2nd Jun 2002, 17:08

to Flare Dammit:

Your disbelief is earned of experience, and there is nothing wrong with behaving at all times OGE as if the sky was going to fall at any minute, but that still does not mean it will. It also doesn't mean that you won't find trouble, because you are looking in the wrong place for it if you believe VRS is common. It is power deficit that is common, and that is what I contend we should stamp out. For you to blanket all effects as one, ignore the actual physics and press on can seem quite safe, but it is opening yourself up to problems because you really don't know what causes the problem, or how to prevent it. I suggest, Flare Dammit, that you open up a bit.

I should note for you that purely vertical rejected takeoffs in twins from heights up to 100 feet are routine, and the descents can result in 1000 feet per minute drops without VRS. I have done at least 1000 such maneuvers.

The fact that you can conjecture a reasonable down draft causing VRS, or that that wildly spinning Sea King doing so because the pilot was too busy are examples of such compartmentized thinking. As far as real VRS data are concerned, I will post some on my brand new web site, if you'll look at it!

Vertical maneuvers are easy, and safe when there is enough regular ordinary available power. One does not need magical rotor VRS bugaboos to get in trouble if there is not sufficient power reserve in the OGE hover.

The US Army specifies that the Hawk must have 500 feet per minute climb rate OGE at 4000 feet and 32 degrees C while at full mission gross weight. That helicopter can be maneuvered vertically with impunity, down to rates of descent as I described.

My suggestion is to try the dreaded maneuver in question at safe altitude, and see. Use a light aircraft with gobs of extra power. Begin to descend carefully, in stages, about 250 fpm at a jump and see what indicated rates of descent you get, and how the trimmed power changes. If things get flakey, note the ROD and nose out of the maneuver, then post the results here.

As for the Bollocks, tried them once and didn't like them! I started off "slightly off base" and ended up "foolish" but that is par for this course, I'm afraid!:D

Flare Dammit!

2nd Jun 2002, 19:17

Nick, old boy,

Such thin skin for a Yank! I do believe you were only slightly off-base. The "foolish" comment was directed at the other person who initially postulated that a UH-60 needs a vertical descent rate of 2500 fpm to get into VRS. What rot!

And speaking of rot, check out the video of your "wildly spinning" Sea King. It did no such thing, Nick, and I'm quite surprised at your efforts at hyperbole or perhaps faulty memory. The bird settled straight down with only a slight right yaw moment set up. It hit the ground, if you recall, at about ninety degrees to the right of its initial heading.

And I did not speculate that a *downdraft* causes or caused VRS, what I said was that an UPDRAFT could cause it! Below ETL, an updraft does not have the same effect as it would when the aircraft is above ETL.

It's lovely that the UH-60 has such good vertical maneuverability at a hover even OEI. You must consider it SuperCopter! But what you're assuming - where you go wrong, in fact - is that the performance of the rotor is as predictable as the performance of the engines. Silly boy.

And as for your suggestion to go out and try to excite VRS, my response would be the same as it's always been: I have done it. But what might excite it on one day might not on another. There are too many variables. It's not like an airplane wing, where we know that it will always stall at predictable speeds.

That is my only point.

ethereal entity

2nd Jun 2002, 19:42

I have never flown a blackhawk, or a bell, for that matter. But I do have a LOT of time in military helos in europe and an awful lot of that has been in the mountains. I don't know if the unfortunate crew suffered an engine failure/rundown, or whether they were hit by a dowdraft (Cold slopes make katabatic winds - I've seen them descend at 40 kts). What I do know, is that as soon as I watched the aircraft approach the slope (still under (apparently)full control), I wondered aloud what the pilot thought he was doing. Where was his escape route? He needed to be higher than he was from the survivors (His winch was long enough - I've checked), and his heading needed to be at least 30 deg left of where it was. Yes, I realise that I wasn't there, and that I couldn't possibly know all this for sure, but it was obvious to me. When I went to work (military sqn) one of my fellow drivers said 'Did ya see that blackhawk on the news...was that guy begging to crash or what'. Callous and unfeeling I know, but the sentiment is not lost on me, or others I suspect. I sincerely hope they are all ok, it looked at least like the fuselage stayed together, so fingers crossed.

I expect this will generate some negative comments, but if anyone can learn from this, then good. Mountains Bite, so watch out.

Nick Lappos

2nd Jun 2002, 20:47

That's it, Flare Dammit, you are scratched off my Christmas Card list!

Seriously, stop by for a beer sometime and we can complete this discussion. My fingers hurt as it is.

Nick

Draco

2nd Jun 2002, 21:38

Mountain flying and its effects on the helicoper are always something of a mystery for low-time pilots like myself, so thanks for all of the warnings and tips.

BlenderPilot

3rd Jun 2002, 02:25

Nick,

HOoooly Cooow! This sure has been an enlightening post! At least for me, this VRS topic is very interesting, and I can now see how it could easily be confused with "settling with too little power".

I read all the info on the website and it seems to make sense, in the case of Bell 206 I was hovering and started to settle, I had extra power to climb, the L4 (with "high altitude kit") does pretty good at altitude in terms of engine power, TOT or N1 will not be reached before TQ, but for some reason applying the extra 15 or 20 % TQ I had did nothing, I had to dive and get out of it. I just don't know what happened.

(although the real limiting factor, in L4's without "high altitude TR kits" is the TR authoritiy, you will hit the left pedal stop at about 85% TQ, with 9,000 HD, so if you keep pulling TQ, you'll keep turning, unfortunately few L4's have this kit)

Trust me I know the difference between hovering with too little power and developed VRS, I have flown most of my 2,500 hrs. at HD's above 9,000 FT and I have rarely taken off with enough power to hover OGE, then sometime after burning some fuel I always end up doing it, I understand the line is thin and sometimes the helicopter might just be a bit heavy and the helicopter just sinks due to a lack of power but then again it just sometimes sinks unexplicably even with extra power!

Now there is still some strange things I don't fully understand, according to the chart you posted "VRS Cond. for Bell 206" its actually easier to get VRS with a little forward airspeed, and it is not possible to get the worst cases if you are descending truly vertical, this is strange.

Now in the case of the Canadian Sea King, I used to think of that as the perfect example of settling with power, I have the video, it hovered level for several minutes OGE, doing pedal turns to show off, then all of the suddenly it just started sinking slowly and then the rate of descent increased tremendously and with a slight yaw of about 80 degrees it slammed into the ground.

Correct me if I am wrong, the RoD requiered to encounter VRS is directy related to disk loading right?, and disk loading is related to GW? Could it be possible that HD was low, the helicopter really light, which in turn gives a low downdraft velocity, and as result the RoD required to meet VRS was met with a much lower than expected RoD? I read somewhere that the official cause for the crash was VRS, do you have any more info on this particular accident?

Now pulling out of VRS by just going thru it and into autorotation, that must be a ride! You say you have done it several times in an S-76, that's even more interesting, I have only flown the 76 (arriel powered) once, here at 7,500 ft, and it was scary, with an 1 1/2 hours fuel, 4 on board, it was over the edge, a real pig, my boss wanted to trade it for our Bell 230 which with its small 250's was a lot better, actually that same 76 ended up having one incident and afterwards finally crash landed one day after an engine failure during cruise flight, after that the remaining engine had enough power to sustain a very negative rate of descent..... I have some friends who've flown the 76 offshore a lot and they talk positively about it, except for the time when the 250 engines on 6 brand new helicopters started blowing up after a while, they told me Sikorskys solution to the problem for the moment was to somehow wrap or surround the turbine sections of the engines with "armor" so when/if they blew up they wouldn't throw the wheel thru the fuselage or have the same wheel mess something up.

Well and now I am now wandering aimlessly, I want to say thanks for the time, it has been enlightening and interesting, it has changed my view of VRS and hopefully will help me in not getting it mixed up in the future. I will have to dig deeper into the subject to fully understand it.

Also I remember calling someone at the beginning "ignorant", and I am sorry for that.

helmet fire

3rd Jun 2002, 03:02

we seem to get into heaps of discussion about VRS dont we?

My two bobs:

The Pave Hawk was not a VRS accident. See my verbal dihorea on the Black Hawk accident thread. An updraft causes you to reduce power to stay at the same hieght, thus it would REDUCE your likelyhood of VRS wouldn't it? Also, (see my other thread) the conning angle DOES not appear to get excessive despite claims above.

I agree with Nick - the Black Hawk is extremely difficult - but not impossible - to get into VRS. There was an incident in an Australian one practicing unusual attitude recovery in IMC when it got into VRS (or incipient). It fell thousands of feet before recovery (through LSALT too :eek: ).

I believe way too many accidents are blamed on VRS, particularly in the USA. I agree with Nick on the Sea King. Too many pedal turns, not enough power. Rotor bleed, tail rotor looses effectiveness, aircraft goes in. Another two video accidents that always stick in my mind for VRS blamming is the Skycrane that landed on a road along the side of a wide valley. As it takes off, it drops of the side of the road and tries to climb out of the valley. IMHO This too is a lack of power accident, not a VRS accident. Lastly there was the US Army (Gaurd I think) Black Hawk giving the rappel demo doing a quick stop, falling through, bouncing off the ground and rolling into the tree line. Again, IMHO, not a VRS accident, but a merely rotor droop issue.

I have believed the gospel according to my instructors - when below 30 KIAS and above 300 fpm RoD BE CAREFUL OF VRS. And so I try to be.

:cool:

Nick Lappos

3rd Jun 2002, 03:45

This is a good post, I learn from each one.

I found that Sea King video, at least the last few seconds of it. It is certainly possible that the thing started out as a low power event, and it could have become VRS, but that is not necessary to explain the mess. I would guess that it hit at about 10 feet per second, maybe 12, which is only about 600 to 700 ft/smin. That could easily be explained if the engines were topped out and the rotor drooped down to the low 90's or less, where the tail rotor would run out of poop as well as the main rotor. Once on a torque limit (does the Sea King in UK have a dreaded torque limiter? I seem to remember a fancy fuel control with torque matching was developed and fitted) then the actual power of the engine is being reduced as the rotor droops and torque is held constant, so the situation goes to worms real fast, rate of descent builds and the crowd screams "VRS!" Is my memory wrong, or was there a more full version with lots of hover turns before the dramatic thump?

BlenderPilot - The forward speed comment is quite right. You need some forward velocity, or else the VRS bubble doesn't form and stay. The smoke picture on my web site shows that perfectly. I did get some real VRS on a single engine landing once, with 8 knots airspeed and about 35 feet per second rate of descent (yea that's per second!) It was a real eye opener (and landing gear spreader) I try not to spout theory here, most of my observations are based on practical experience, with some theory to explain it (got that, Flare Dammit!? No Christmas cards.)

Your description sure sounds like VRS, if you had 15% torque margin and applied it and nothing happened. What was your ROD when that was happening?

I did the shakedown and structural flying on the S-76, and we had to do zero knot auto entries, increase the ROD and keep going down to full auto. About 6000 ft/min descent when trimmed. The A and A+ model are not altitude machines, they are optimized for sea level and do nicely there. The engine bursts are (thankfully) ancient history. They seemed to be associated with rebuild techniques, IIRC.

Regarding "Ignorant" - we all are, we each have a piece of this puzzle, and when we all listen, we all learn. I do, every time.

helmetfire - The 300 fpm rule of thumb is not bad, especially for lightly loaded helos. A tad conservative for heavy big turbines, but what's the harm? I remember the Black Hawk TV demo where he screamed in, flared, landed hard and went into the bushes! Dramatic footage, as they say.

RJSquirrel, what say you? You started this mess. I don't agree with your 2500 feet per minute, that's the theoretical VRS at 1.0 times the downwash speed. I think the UH-OH point is half the downwash velocity. For a Hawk, that's about 1500 fpm as the max hover descent without creating a smoking hole.

Nigel Osborn

3rd Jun 2002, 03:51

VRS. Veryemotive subject. I think one problem could be that many years ago, the British called it Vortex Ring and the Americans called it Settling with power. In the same way, the British called a fast stop into wind a"into wind fast stop", whereas Bell called it a "Whoa boy"; bit more expressive than the Brits!
Coming in to land, pulling full power and thudding in ,is not being in VRS but a combination of not enough power available, bad planning and bad flying. VRS has specific requirements:-
1. Very low airspeed, around 8-10 knots, less than transational lift for that helicopter.
2. ROD of at least 300 fpm. This has many variables; altitude,oat, type of helicopter, weight,etc.
3. Power applied. Doesn't have to be full power.
Getting out of VRS involves removing those conditions. Increase speed, remove power. This can cause large loss of altitude, ROD of 6000 fpm is common. A Puma ditched and the last 500 feet took 16 seconds.
Before VRS is fully established, you will get some vibration warning while in the insipient stages. Once established, the main rotor is basically stalled but the tail rotor isn't and I have found in demos that a boot of pedal can speed up the recovery process.

Oh sh@t, I think I might have disagreed with Nick!:eek: :rolleyes:

Nick Lappos

3rd Jun 2002, 04:06

Nigel,
Anyone who has slung loaded a moving motor vehicle is someone who is not to be trifled with!
Nick

helmet fire

3rd Jun 2002, 05:11

Yeah - I dont want to mess with him either....:D

In fact I agree with you Nigel. I think VRS is poorly named in the US, although settling with power may descibe the end result, I would hardly call it "settling" - perhaps "unsettling" is closer? Either way, I believe the terminology has always caused confusion.

Nick - I agree about the 300 fpm being conservative. I use that for lighties such as 206, EC120, etc, and I use 40 KIAS/500 fpm for the heavier aircraft such as 205, 412, UH-60, etc.
Me LIKE conservative though!!:D
Remembering that UH-60 demonstration, the Black Hawk again showed what an incredible aircraft it is by plowing into the tree line and staying perfectly intact, and it probably could have been backed out again for a drive train change. It was only when the pilots pulled off the engines and the un-powered blades drooped into the uncut trees that it started coming apart. Even so, I dont think anyone was seriously hurt.

Flare Dammit!

3rd Jun 2002, 05:11

Nick Lappos writes:

"This is a good post..."

Well, we all are our own harshest critics, eh what?

"I found that Sea King video, at least the last few seconds of it. It is certainly possible that the thing started out as a low power event, and it could have become VRS, but that is not necessary to explain the mess. I would guess that it hit at about 10 feet per second, maybe 12, which is only about 600 to 700 ft/smin. "

Do you mean 600 to 700 feet per minute? I find it amazing that a Sea King - a bird designed for shipboard duty - can hit the ground at a paltry 600-700 fpm and have the landing gear collapse. Blimey! Doesn't Sikorsky design them any better than that? Heh- some of my 206 landings have probably been that hard ;) I can't imagine that the Sea King was all that heavily loaded (it only had crew aboard). It would be troubling if a bird like that had such marginal power for HOGE at such a relatively low DA.

Finally, if you watch the video of the Black Hawk that went into the bushes after the hard landing, as the camera pulls back you can see some nearby flags - all pointing directly away from the camera. Downwind approach! Even the mighty Black Hawk is evidently not immune.

And as for the Christmas card - well, my mate Lou Zuckermann and I will just have to somehow survive.

Interesting thread indeed!

helmet fire

3rd Jun 2002, 05:31

Flaredamnit! - Is it a possibility that the landing gear collapsed due to the twisting moments applied as it touched down?

Also, I conceed that that there is a possibility that the Sea King, and the UH-60 demo, may have been caught in turbulent air that they had created themselves (ie their own downwash), but that is NOT VRS. Nor is it incipient VRS. It is a lack of power in trying to outclimb the turbulence. having done downwind approaches in the UH-60, I still cannot attribute that accident to VRS. I still believe it was excessive NR droop due the power required to terminate BECAUSE it was downwind quickstop. I think the pilots even claimed they had a stabilator failure which would make sense as the NR can droop to the point that the generators pop off line momentarily, tripping the stabilator offline and activating the audio.

RotorDroop

3rd Jun 2002, 08:51

HI all,

The usual disclaimers here, speculation is bad....

I have a few hours in the 60 and based on what I could make out I wonder if the crew was a victim of the 60 protecting itself from them.

The crew was hovering in an area of very limited visual reference, OGE at 11000 or so feet. Pave Hawks are, as a rule, heavier than UH's given the blue suit propensity for ordering all the options.

If memory serves me correctly the chances of doing immediate and fatal damage to a 60 by over-torque are nil but an engine over-temp or over-speed are both possible and bad so the aircraft prevents you from doing both.

Given the conditions of the rescue I find it likely that the crew got into TGT limiting and weren't looking inside so they did not know of this until they got a low rotor audio and light. If they failed to react appropriately, immediately and the droop worsened they would lose most cockpit indications when the GCU's dropped the generators from the AC busses.

Rather confusing in the best of circumstances when you have time to figure out what's going on.

In the simulator when doing autorotation training most crews turn on the APU to preclude this loss of AC power should they allow the RPM to droop. The caveat is the APU isn't certified for normal flight and may only be used in flight during an emergency.
These folks didn't know they had an emergency till it was too late and even so the appropriate procedure does not call for the APU.

Sorry about the APU tangent.

Um..... the issue is TGT limiting. Most folks have never seen it in flight because they don't get that high and the engines are marvellous. Most crews haven't seen rotor droop either. I'm speculating on this as the cause because of the gradual onset of both the descent and the right yaw, both of which I have experienced in a 60 in rather unusual circumstances and I was very surprised because you just don't expect to run out of anything in a hawk and because the onset is very benign. If you don't pay attention you'll miss what the aircraft is telling you till it really needs you to aviate.

The result of bumping a limitation in a 60 is usually bad because the pilot's skill level is often far below that needed to extract all the helicopter can deliver and when it needs help he's out of options.

Fortunately, no matter whether of pilot, mechanical or environmental cause, the airframe will keep you alive through impacts fatal in any other craft.

Thanks to Nick and the rest of the fine Sikorsky team for a fantastic machine.

My prayers and wish for a speedy recovery to the folks involved.

Best regards,

Brian

Thomas coupling

4th Jun 2002, 00:45

For my sins, I had a part in training the pilot who commanded that CH-124 in NY. I also subsequently flew with the observer who was thrown out of the passenger door in the crash.
The Canadians prefer to call the mishap: SWP as the phrase VRS does not appear as regularly in their literature, as it does here in the UK. For that matter, the reverse is evident here (VRS common, SWP not).

Very few people have experienced VRS, unless they have been demo'd it. It takes some considerable time to manifest itself. When established, the pilot has no control over the a/c, if it comes out of VRS it will do so of its own volition. What most pilots experience (should they be so unlucky), is Incipient VRS where there is still room for manouevre :rolleyes:

Per Ordure Ad Asti

8th Jun 2002, 10:58

Nick,
I have to say that I agree that it doesn't look like VRS. I can't see any reason why the disk should cone that much unless they had run out of power and the Nr had drooped (I've done that, it's not fun).
Thanks for the excellent page explaining VRS. I couldn't get the formula to work though, it may have something to do with the RAF being metric. I tried to work out the downwash velocity for a heavy CH47 - 50000lbs with a rotor area of 5600 square feet and came up with 43. I was expecting something more like 100mph or 10000fps, what am I doing wrong?

Nick Lappos

8th Jun 2002, 12:47

Per Ordure Ad Asti,
You got it right, the answer is 43 feet per second, or about 2580 feet per minute rate of descent. At half that rate, the rotor begins to get substantial VRS (1290 ft/min). The downwash velocity depends on the disk loading, and the Chinook is quite kindly in that way, even for its size. Pilots tend to believe that helicopter size alone drives downwash speed, and that is a common misconception.

I will put the units on my web page ASAP. Thanks!

My wife is horrified that I loaded up the web page with helicopter crap, and not pictures of the family and such. Oh well!

One of the reasons for the skeptisism of some of the posters on this thread (where experienced, knowledgable pilots insist that any descent could cause VRS) is probably due to their vast experience flying heavily loaded machines where OGE hover was all that is possible (or even IGE only).

It is different for those of us who fly and test aircraft where the maneuverability is the driving parameter, and the load is reduced to allow vertical maneuvering.

The shame of it is that an entire generation of helicopter pilots has grown to believe that when you get in trouble OGE it must be VRS, and so they feel that VRS is always with us, and it is a fundamental helicopter limitation in any OGE situation.

This vastly restricts their ability to use their machines as they can be used, and it affects their understanding of what it is that gets them in trouble OGE.

helmet fire

8th Jun 2002, 13:44

Per Ordue,
I have a copy of the footage and have watched the clip very carefully, but I dont see any sudden or large increase in the conning angle until after its begins striking the mountain - which may be expected (I'd be pulling like hell to get away from the slope too!!).

As I said in the other thread, the aircraft does not appear (appear is the critical word here) to strike the mountain until it has turned GENTLY right toward the mountain, pitched slightly nose down (only a few degrees if at all) whilst the same hover height. It then pitches rapidly nose up to what looks like about 40 odd degrees before sliding backward and begining a descent. At about a 50 ft height loss, it then pitches rapidly forward and only now does it appear to have the first strike (fuel probe followed by main rotor) off the nose of the aircraft as it is pointing into the hill (Did anyone see any evidence of a strike before this?) and THEN the conning angle can be seen to increase.

Nick Lappos

8th Jun 2002, 16:22

For Per Ordure Ad Asti,

I have posted some data from the V-22 tests for those doubters who question the rate of descent estimate of 1200 feet per minuteas the trigger point for VRS in OGE flight:

http://mywebpages.comcast.net/llappos/

Let the debate rage!

Flight Safety

9th Jun 2002, 08:28

I finally found some time to look at this accident. I looked at the video, and I looked at some performance data for the Pave Hawk.

First the video, the only one I've seen is the Sky News video, which appears to be a CBS news video. That video has a cutaway scene to the climbers in trouble, that perhaps seems to miss the very beginning of the problem as it developed. Anyway, the helo is hovering nicely over the climbers, winching up the 3rd climber, when suddenly the hoist cable is cut (stated in the news articles) and the pilot does an abrupt left cyclic manuver to both steer the helo away from the climbers and to position the helo over a ravine. I've looked at the video a number of times, and it's apparent that the altitude loss begins to occur at the start of the event, even before the pilot makes the abrupt left banking manuver (which also includes the right yaw). The altitude loss continues until ground impact. It looks like the pilot tries to keep the helo in the center of the ravine (presumably to try and keep the ground away for as long as possible), but he seems unable to get the nose turned around away from the mountainside. He contacts the ground just left of the center of the ravine after trying to decend down the center of the ravine. The helo then rolls more or less down the center of the ravine after the impact.

Regarding the yaw, maybe the pilot was trying to perform a right yaw (easier with reduced power) to get the nose turned away from the mountain. When the event started, the nose was left of directly facing the mountian, but he turned through directly facing the mountain, and managed to turn about 120 degrees total (more or less) to the right before impact. From the start of the event until impact seems to have taken about 6 seconds (from the video), so that's a yaw rate of about 20 degrees per second (give or take).

Now for the performance data. Pave Hawks have a standard empty weight of 12,330 lbs. The news reports indicate that 6 people were on board, so add about 1,200 lbs. Then there could have been a maximum of 2,433 lbs. of normal internal fuel on board (I'm assuming that the Pave Hawk's cabin aux fuel tanks were empty for a high altitude mission), minus what was burned on the trip to the mountain. So we could have had a gross weight of anywhere from about 14,000 lbs, to maybe a max of 15,500 lbs.

I have to make assumptions about weather, as I cannot find any data on the net regarding weather on the mountain at the time of the crash (1:40PM PST 5/30/02). So I'll assume standard baro, and a temp of 0 degrees C (unless someone else knows). The altitude of the climbers was about 10,430 ft (they were about 800 ft below the 11,230 ft peak), so the hover height before the problem developed should be about 10,500 ft (in round numbers).

For these conditions, the HOGE for the Pave Hawk at 14,000 lbs is about 16,200 ft, and at 15,500 lbs it's about 13,600 ft (10 minute ratings). I could not find a chart indicating the HOGE for OEI, but the service ceiling OEI at these weights is about 14,900 ft (14,000 lbs) and about 13,000 ft (15,500 lbs). I wish I could see a chart for OEI HOGE, but I'm pretty sure the helo could not hover at 10,500 ft on just one engine.

Based on the fact that the hoist cable was cut immediately, and the pilot's abrupt left cyclic move away from the climbers and towards the center of a ravine, seem to indicate to me that the crew of the H-60 knew they were going to loss altitude as soon as the event started. I don't know what that means, but I guess I'm still leaning towards the idea that an engine failure caused this accident. I also agree fully with Nick, no way was this a VRS or LTE accident.

Per Ordure Ad Asti

9th Jun 2002, 09:28

Nick,
Thanks for the update, the explanation of VRS given in groundschool was sketchy to say the least and we were never given any equations to play with. We are taught to use 30kts (the lowest reliable ASI reading) and 500fpm as a safety margin for the Chinook, it's nice to know that there is plenty of room for error.
An old and bold pilot once told me that, many moons ago, he put a Chinook into VRS on purpose (with difficulty) and managed to power out of it. I was taught that this couldn't happen but your explanation is much better.
I dont doubt your maths at all, but I find a 2500fps (25mph) downwash figure for a heavy Chinook to be a bit surprising. On the conversion course they make you hook up loads for the experience. I have seen strong men blown flat by the downwash and stories of destruction and mayhem to vehicles and buildings are common.

Nick Lappos

9th Jun 2002, 12:35

Per Ordure Ad Asti,

The discussion on the web site is the first time I have ever seen it all put together in one place, a reason for doing it. Lots of tribal knowledge, but little explained factually, I think. More to go, I am doing some correlation with low speed descent power increases to help explain the legitimate points raised by several posters (Flare Dammit! for example) that something is happening in descent that is not healthy. They are right, the descent power is rising even at low rates. More to follow.

The downwash velocity we are calculating is for the disk average, and assumes a constant velocity stream the diameter of the rotor. It is quite reasonable to assume that there are higher velocities at the tips, so the crew will feel gusts and peaks at perhaps 50% higher than that average (maybe more, I am looking for some papers of measured wake velocities). If 50% higher, than the 40 mph gets substantial enough to toss stuff around, I think. Also remember that the total energy is awesome, so the sheet of wind is a large one that reaches far.

The Chinook is particularly blessed with low wake velocities as it is a very low disk loading machine, a virtue of the tandem design. That is why it outlifts the tilt rotor with so very much less power. It has so very much less empty weight. At 50000 pounds total gross weight, it has an empty weight of 23,000 which is 46% empty weight fraction (the same as that of the 53E, 32000 empty for 69000 gross). Compare those excellent numbers to the V-22's 69% empty weight fraction is a real eye opener - 33,300 empty for HOGE of 47500 at operating altitude).

virgin

12th Jun 2002, 16:00

This thread read with the 'BlackHawk Accident' one were facinating. Better than any handbook on the topic. Brilliant!

John Bicker

11th Jul 2002, 21:50

Interesting stories all of them.

Funny on how no-one has touched on the APPARENT or RELATIVE ROD that the aircraft could have experienced. I have no idea of the geography or the met conditions that were experienced but if you are in a situation approaching the side of a mountain or hovering there with the wind coming up from below and it seems to be hovering for "free" get that s***t eating grin off your face and watch your arse. Ask a Llama pilot. The Llama being so lightly disc loaded sometimes and being able to easily get it self into places where many other machines can only dream about is a common candidate. If it looks like you are getting something for nothing - you probably will. I dont know how many times as a sprog pilot in a H300 I charged into a sloping face with a minimal tail wind and holy s***t. The 30 knots BS and 300 fpm BS is fine and only relavent if you are operating without any existing vertical flow components - (read wind) but if the wind is from behind and as you approach the slope the vertical component increases to maintain these relative figures you would likely find yourself climbing. Most pilot's only think about downdrafts unfortunately. Think about it as a normal approach and add the vectors for a tail wind that is coming UP behind you. The comparison of IAS versus groundspeed is going to indicate the tailwind, provided you have taken into account the TAS at high altitudes. The vertical component will be apparent (possibly) in the fact that the VSI is telling the same story but where did all that excess power come from? 2500 fpm vertical gust is not much but if you are coming down at 500 fpm you can subtract that from the figure - 2000 fpm or 20 knots vertical component is not much and it increases as you get closer the slope. Trust me you can get into OGE hover situations next to a slope where you are just sitting in an updraught that is going to eat you alive with VRS. This make sense to anyone yet?

As for these air velocities being able to be achieved in the natural world - I have had the needles split in effectively zero forward speed conditions in a B 205 at 10500lbs and been climbing. Dumped the water load and it went up after it left the tank. Corsica - top spot. The FW guys all stayed in the pub, joined them soon after.