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Restricted takeoffs, VRS, and ground effect

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Restricted takeoffs, VRS, and ground effect

Old 26th Jul 2015, 03:38
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Question 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!
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Old 26th Jul 2015, 09:15
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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.
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Old 26th Jul 2015, 09:34
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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.
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Old 26th Jul 2015, 11:46
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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?
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Old 26th Jul 2015, 12:05
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@ 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.
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Old 26th Jul 2015, 16:48
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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.
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Old 26th Jul 2015, 17:47
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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?
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Old 26th Jul 2015, 22:09
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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?
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Old 27th Jul 2015, 08:12
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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!
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Old 27th Jul 2015, 09:06
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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.
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Old 27th Jul 2015, 11:21
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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!
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Old 27th Jul 2015, 12:47
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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.
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Old 27th Jul 2015, 13:38
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Interesting thread. It would be rounded off really nicely with a couple of pictures of the pad and the approach.
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Old 27th Jul 2015, 15:44
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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.

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.

Last edited by Geoffersincornwall; 27th Jul 2015 at 15:49. Reason: more info
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Old 27th Jul 2015, 17:42
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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.
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Old 27th Jul 2015, 19:12
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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?
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Old 27th Jul 2015, 23:57
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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.
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Old 28th Jul 2015, 01:20
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Originally Posted by [email protected]
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.
I don't think that the Bell 212 (listed in his profile) flight manual allows Nr increase beyond 100%.
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Old 28th Jul 2015, 06:04
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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.
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Old 28th Jul 2015, 06:55
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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.
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