Restricted takeoffs, VRS, and ground effect
 

Hello all! I've come across an interesting situation that could lead to some good discussion. Thanks in advance for any input!

I fly a dual engine Huey out of a large city. We have a helipad sandwiched in the middle of the city at ground level that presents some interesting challenges, both on takeoffs and landings. This thread is specifically related to the takeoffs we do to try to give ourselves decent aborts and single engine options while trying to minimize risk to the surrounding populace.

We regularly have to come in and out of an approx 200'x200' helipad in the middle of the city, heavy weight with about a 5-10% power margin for OGE. The helipad is surrounded by 100' obstacles in the immediate area with taller buildings (up to 200' or so) within about 3-400' in some takeoff directions. On takeoff, we often come straight up vertically (sometimes backing slightly) to about 150' or so before moving forward to give ourselves the ability to clear obstacles OEI in the event of an engine failure. If we had any issues before this committed point, we would attempt to abort straight down, or down and slightly forward, to avoid the nearby populated areas.

If I had an engine failure right at about 150' before moving forward, I would pull into single engine power and attempt to come down to the pad, pulling cushion as necessary at the bottom to try to hit the ground softer. This scenario raises what I think are a few interesting questions.

By pulling single engine power while descending in this manner, could I potentially get into VRS? Would I come out of it at about 50' when the aircraft starts to reenter ground effect, or ride it all the way into the ground?

I'm guessing we wouldn't get to a high enough rate of descent to enter in the first place, but it's still an interesting question IMO. Would the downwash velocity decrease as you entered ground effect, allowing you to decrease your sink slightly but still negating your cushion?

As a sidebar, does anyone else have to deal with similar LZs? What are your preferred methods of dealing with them? Thanks again for the input!

Chucklehead,

By 'twin Huey' you may either be a 212 or 412 driver, but both should have vertical departure options to cover your situation. When coming up vertically you should always back up; not 'sometimes slightly' as you describe unless you have a suitable runway ahead for a SE landing. Essentially from a 4ft hover smoothly increase power and at 35ft commence moving backwards to keep the helipad in view, until at TDP you rotate and gain forward speed.

Best described in the appropriate supplement for your flight manual along with appropriate performance graphs: elevated helipad procedures give the best description but may not be suitable for a ground level operation.

Since a failure would require forward movement to regain the pad that you departed from, VRS should not be an issue.

Back up profile
 

If the RFM contains a back-up profile then there will be an associated WAT Curve to aid with 'mass management'. This may be your Achillies heel.

G.

Thanks for the feedback!

John, it sounds like I should clarify the 'sometimes slightly'. When we have room to back up we'll do that in order to keep the LZ in sight, but in some takeoff directions you have buildings behind you as well and not very much room for backing. Unfortunately, our flight manual doesn't go into elevated helipad ops, and I haven't found any performance charts relevant to the situation. If it makes any difference, I fly a 212.

Geoffers, I'm not familiar with the "WAT curve", at least not by that name. What exactly is it that you're referring to?

@ John E.

He describes the landing zone as surounded by obstacles up to 100`!
So my guess is he cannot climb backwards as you should normally do.

Geoffers is referring to the weight/altitude /temperature limitations for each takeoff/landing profile. It should be a graph in the performance section of the RFM.
So, for a given pressure altitude and outside air temperature, there will be a maximum all up mass that you can safely operate at for performance class one or 2.

From what chucklehead describes of the LZ, elevated helipad profiles don't seem appropriate and unless he reduces his AUM considerably, he is unlikely to get a PC1 profile that works.

Chucklehead - if an engine stops at 150' then I would pull until the Nr starts to decay and then back off slightly so that you are optimising the full power available on the remaining engine but preserving the Nr.

I am lucky to have a training switch to allow me to simulate engine failures during such profiles and the difference in rate of descent between just having normal/beeped up Nr and letting it decay by a few percent is quite marked. Nr is life and life is Nr as several wise people have said on this forum.

Don't worry about VRS - you won't get near the RoD required (probably somewhere around 12 - 1500'/min for your aircraft).

Presumably you do beep up your Nr for these arrivals and departures?

Can't comment on that highbrow multi stuff, but... man it must get interesting there on a windy day. With all the obstacles does it swirl around? How can you even tell the wind direction?

Crab, I've never beeped the rotor above 100% (our "normal" range) for a takeoff, but it sounds like it might not be a bad habit to get into. I'm assuming you just mean to start out at a bit higher so you lose less in case of a failure?

That's definitely some good advice about the beeps. One instructor that I had in particular had a compressor stall and always said that the beeps were the difference between him flying out of it and hitting the ground.

I've considered trying it out a bit lighter weight on a training sortie just by decreasing power to single available, but was a bit worried about getting into VRS which I've never even approached before to my knowledge. I would imagine you'd get some decent vibration before it went full bore but the risk/reward hasn't been worth it for me to try yet.

Krypton, it does get a bit squirrely at times. It's not uncommon for the wind to shift somewhere in about a 90° range while on approach. I would compare it somewhat to mountainous flying, with some valley winds between the buildings and demarcation lines off some of the taller ones. With the exception that I haven't seen significant downdrafts.

There are a few flags and other wind indicators nearby, and there is a wind sock on a building just a bit above the pad itself, so you can get a pretty decent idea of what the winds are trying to do. It definitely makes me feel like a bad pilot some days though, makes me work too hard!

Thanks all for the replies!

Chucklehead - most aircraft have the ability to beep up the Nr for critical stages of flight, either manually or automatically and it would be specified in the RFM (well it is in mine) for PC1 profiles.

As you say, the point is that you start from a higher Nr figure which gives you more time to react to an engine failure. If you have a low Nr horn on your aircraft, that is what to pull to followinfg an engine failure, then lower the lever until the horn ceases again, that should give you optimum Nr and max single engine power.

It is worth noting that the helipad departure (up and back) often has a limited rate of climb specified in the RFM which is to keep the pitch angles lower in order to avoid rapid Nr decay following and engine failure; this is somewhat at odds with the military view where you get to height quickly to reduce exposure time to the reject option.

Makes sense to me. It's always interesting to hear other people's perspectives on these sorts of things. A lot of techniques, such as the RFM vs military viewpoint, seem very much a matter of chance and opinion. Thanks for the input!

No problem.

However, it is worth stressing that if you are required to operate to PC1 (carrying fare paying pax for example) you have to comply with the RFM helipad profiles, including keeping inside the WAT limits for those profiles or legally you are on very thin ice if anything goes wrong.

Interesting thread. It would be rounded off really nicely with a couple of pictures of the pad and the approach. :ok:

Crab et al
 

PC1 is an 'operating standard' and only relevant if your jurisdiction mandates it - most don't, EASA being the exception of course. You won't therefore find PC1 data in the RFM as that is produced to comply with 'certification standards'. Note that PC1 applies to EASA CAT (Commercial Air Transport) flights and not just 'fare-paying' passenger flights. You won't find any of the offshore workers putting their hands in their pockets for their tickets - not just yet anyway. Of course the US and other jurisdictions are a different story.

PC1 uses Cat A data and you will probably find the data for Cat A approved take off and landing profiles in the RFM along with the WAT chart as PC Plod explained.

Mass management is the key to safe operation as everything in our world gets more difficult at max AUW and for those souls that ignore the max-AUW limits it gets even trickier. I wish I had a fiver for every student I met doing 139 TR's that had no idea that mass was even important. After all surely no one would build a helicopter without giving it the capability of carrying full fuel with all seats occupied. :ugh:

If you wonder why the 139 is such a popular machine then the fact that it can cope with everything full means that those that don't do any pre-flight mass calculations are no longer putting their lives on the line to the same extent.

G.

Thanks geoffers - that's why I said 'if you are required to operate to PC1' since I don't know what rules and regs chucklehead is operating to - perhaps I should have said Cat A instead.

Chuckle ehad - don't you have governemnt rules to fly to? What are the national requirements for your type of a/c to fly into and out of public sites?
Geoffers has said everything I want to say on this.

IF you are making this up as you go along because there are no national limits then God help you when you stoof. What does your insurance company think of your profiles old boy. It won't pay out if you don't comply?:mad:

One thing I have never, ever worried about is insurance, and whether it would pay. That's not my problem. If the boss is worried about it, he can dictate flight profiles. I've never worked for a manager who even considered insurance coverage, it was always assumed to be there, but there are other people being paid to worry about that. I worry only about complying with the ops manual and staying in one piece.

I don't think that the Bell 212 (listed in his profile) flight manual allows Nr increase beyond 100%.

John, I'm assuming it is like the Brit Mil ones which can (I think) I know you can beep down the Nr - I'll ask my colleagues.

Sure you can beep the Nr on the 212, usually between 97-100%, but the FM limitations only allow those figures as the power on values.

The 412 allows an 'upbeep' for the Nr, the 212 does not.

Lots of replies! For the legal matters, we are not required to follow a PC1 profile. To be honest I'm not 100% sure what that would entail, I assume it's a specific profile that gives a certain safety margin? For weight/power considerations, we maintain at least a 5% margin over OGE power during our ops. I'm with Gomer Pylot in that I don't know the details beyond that.

whoknows, I've been meaning to do that for quite a while now! I'll try to get a video and take some snips from it or something.

Crab, John is right in that our "normal" range is 97-100% Nr. It's definitely an interesting idea to have in the back pocket though, since the beep range does go up to 102.

I fly two different types of twin, both with PC1 performance, and very different profiles, two areas I regularly see ignored are the max ROC on the vertical section of the take-off and also the torque limits. Both are designed to contain the Nr in the event of an engine failure.

The simulator provides fascinating lessons in this regard, when the book says "apply 10% torque above hover requirement" a lot of pilots just give it the full beans. When an engine stops the full beans guys find themselves with a sudden (and often catastrophic Nr droop) while the guys who fly the accurate proflie find the NR drooping to exactly the right spot for either a reject or fly away.

People with very clever science/maths qualifications who have graduated from complex test pilot courses work this stuff out, I wonder why when they've gone to all that trouble an average line driver ignores their work and puts themselves in a very difficult place to explain away if they have a problem.

In the last 28 years I have only once rejected for real off a PC1 take-off and that was actually when a front seat passenger dropped something in attempting to catch it turned off the hydraulics, so it isn't always an engine failure you're waiting for.

SND

Chucklehead - I have just been perusing my colleague's 212 RFM and it shows a vertical take off profile as follows:

Vertical climb to 40-60ft using a maximum of hover Tq plus 15%,

Continued climb with rearward movement to keep the LS in sight up to CDP of 160 ft with the same Tq limits,

Then rotate to achieve VTOSS (30kts plus windspeed) using not less than 72% Tq,

Climb to 200 ft at VTOSS.

You are right that the beeping up only comes following an engine failure.

I don't know what the certifcation of a 212 entailed but I am guessing with the age of the design it was well before the advent of Cat A and B. Hence there don't seem to be any graphs in the RFM which state Cat A or PC1.

Not sure where that places you in the modern era!

I uploaded this CAA Bell 212 Cat A supplement for a previous discussion some years ago; it is not an approved current copy, but may give some idea of the profiles available.

It is also based on very early PT6-3 engines:

Part 1

Part 2

Part 3

It makes me wonder why backing up downwind to keep the LZ in sight and so enable a forward descent / run on in event of an engine failure has any real advantage when in so doing puts more stress on everything and thus more likely to have an engine / tail rotor failure in the first place!
I cringe when ever I see this, using all that power whilst backing up, when you could be doing a towering take off into wind and on your way in much less time.

Some discussion of why the twin behaves the way Crab and John E have so properly described:


In the vertical takeoff, the loss of an engine creates a power deficit that produces the need to descend, of course. The deficit is measured by comparing the power needed to HOGE as compared to the OEI power remaining. If the OEI power is very close to the OGE power, as is the case when very lightly loaded, the ride down is fairly slow and the landing easily cushioned. Since the speed of that descent is driven by the size of the power deficit, as the takeoff mass is made greater, the OEI power deficit is greater, and the rate of descent becomes greater. If the power deficit is appreciable, the aircraft doesn't stabilize in a fixed rate of descent, it actually accelerates downward, so that the stop at the bottom becomes pretty harsh at higher reject altitudes (higher barriers) and at higher weights. The flight manual WAT curve (Weight, Altitude, Temperature) shows the weights where the power ratio is acceptable to perform the published procedure. For vertical procedures, the WAT curve calls for much less all up mass than a ground level procedure where you can accelerate to somewhere close to VY before the engine failure. Of course, the ground level procedure rejects into a much longer heliport, because you have to bring that fast helicopter to a stop while in the protected runway length. In other words, the various procedures for Cat A swap all up mass for speed/runway length.


VRS is somewhat involved in a vertical reject, but the descent is more governed by the simple application of too little power and too much mass, so rotor thrust does not equal weight, and the helo falls, and accelerates as it falls.


Generally, for a vertical reject from over 100 feet to be successful, the OEI power must be an appreciable fraction of the OGE power required, perhaps 75 to 80%. That is almost exactly what HIGE power is as compared to HOGE, so a rule of thumb could be that if you can barely HOGE OEI, you probably have a darn good vertical reject capability.

Chopjock - you must understand that these profiles are, if flown exactly, guaranteed to permit a safe reject back to the helipad or a safe transition into forward flight and climb to 1000 ft providing the weight and temperature limits are adhered to.

They have been proven in flight testing whereas your pull pitch and pop out vertically has not.

The argument about exposure time is valid to a degree but with higher collective pitch comes more rapid and greater Nr decay in the event of an engine failure.

The profiles are actually quite gentle due, as Nick has explained, to the need to keep the power available and the power required quite close; your vertical departure will use more torque and therefore put more stress on the airframe and TR than the up and back version.

C'mon chopjock......stop it. How long have you been on this forum, now? Stop being a dickhe*d now.............................................:mad:

Without the need for name calling, I believe that CJ flies single engined machines where the performance considerations are slightly different.

What 'stress', chopjock? And how does it create a greater likelihood of engine/tail rotor failure?

Using 'all that power'? SND and crab@ have both explained that it is a maximum of hover Tq plus10/15%. It is actually a relatively gentle application and very controlled, with little stress compared to many SE departures of 'pull max take off torque, nose forward and rotate'.

I've done a reasonable amount of both. I'm the guy that crab@ refers to as an ex-mil driver who would reef in power to get up quickly, and had to re-learn to be a bit more considerate to follow the correct profile.

Ok is an engine or anti torque system more likely to fail / stall (LTE) at high loading or not so high loading.?
Yes I only fly singles and I know on a hot day with a light wind and quite heavy with fuel / pax etc it would require more power to climb out backwards down wind than to lift straight up off a cushion into wind. (I'm not talking about yanking up on the stick here).
I know the twins are doing it at a lighter than max weight to allow for this (helipad style), it just seems if the same power used for backing up was used to get going into wind, the exposure time to the risk would be less, or even be accomplished using less power (or an increased take off mass) and in my mind less risk of something breaking.

Chopjock - the profiles are designed so there is no exposure - at any stage of the process, an engine failure will result in either a safe landing or a successful flyaway.

Single engine is different and I would tend to agree (after 32 years of mil flying) that reducing exposure time is a good idea.

John E, thanks for the references! With those numbers we may be close to cat A in the winter but pretty far out in the summer. That's more or less what I suspected, I've always expected to best case spread the skids if we had a failure up past 100'.

Crab, that RFM profile sounds pretty similar to what we do during normal ops. The only exception is that after 30 KIAS or so I normally climb while accelerating to about 60 KIAS for max rate of climb. Do you know why they would recommend climbing out at VTOSS instead? Does it give a better ROC when you take the acceleration into account? I would assume that this would only be valid up to a certain distance..


My reasons for backing up during takeoff would be a) keeping the landing area in sight, b) increasing your forward run available and therefore your obstacle clearance, c) to allow a bit of forward drift from the nosedown tendency when you reduce collective, and d) to allow for landing with a "skids level" attitude as opposed to nose up due to the mast tilt (at least for Hueys). Am I off base, or are there others that I'm missing?

Thanks for all the replies!

Rejected Helipad Take-off
 

The other advantage of the rearwards departure is that the rejection profile (forward and down) allows a little additional airspeed (airflow over the disc) which improves performance, while the slight flare used to terminate helps with NR maintenance/recovery prior to landing. At higher weights the difference (according to the A109 simulator, at least) is significant.

Chucklehead - I think the VTOSS climb is more to do with angle of climb than rate of climb which, as you say, will be better at 60 kts (I think it says 56 kts in the RFM).

Certainly following an engine failure at or after TDP, the attitude change to capture VTOSS for the climb ensures you get a healthy vector away from the ground.

i think all your reasons for backing up are extremely valid - that's probably why the profile is designed that way.

Helitester - perhaps Nick Lappos has an answer to that apparent anomaly.

Well chopjock another sterling job - reeling those innocent ones in....again tut tut.:D
Helitester, S76 do this because they have to break earth orbit @ 11k/mts/sec.:eek: Simples.

Next Q?

Yes, it's for obstacle clearance, rather than obtaining best rate of climb. For example, the profile for the type I fly requires the aircraft to be flown at Vtoss (30 kts) up to 200' above the takeoff surface, then an acceleration to Vy up to 1,000'

Helitester;

Its' actually quite a smooth upward acceleration, the surprise comes on the reject, where in the words of the RFM you should "go light in your seat" Training for that profile is now done in the sim with occasional demos with both engines running in the aircraft. The problem is the propensity for pilots to hit the tail in the touchdown (RTO managed that a couple of times I believe) Certainly the S76 vertical, which is a true vertical with no rearward part is the most aggresive PC1 take-off I have ever flown (and I do it most days), but the power application should be smooth, yank it in and there are good opportunities for an over torque. Every other PC1 takeprofile I have ever tried is a very gentle and controlled experience,

SND

On the subject of that S-76 takeoff, I'd like to poll the crowd on another "max performance" takeoff issue. I've met some proponents of a rather rapid power pull, vs a smoother, continuous pull into takeoff power. The argument being that the rapid pull can give you a higher vertical velocity vector and get you into a safe regime of flight faster.

I prefer the smoother pull because, in my experience anyway, the rapid pull tends to lead to a more unstable takeoff in which you lose a significant amount of what you gain, with a greater tendency to overtorque (esp due to tail rotor inputs). Any preferences, and arguments for one or the other that I maybe haven't considered?

Thanks again everyone! Interesting stuff all around.