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constant speed or variable speed approach

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constant speed or variable speed approach

Old 30th Jun 2020, 02:40
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constant speed or variable speed approach

Never could get a good answer on that: when is it better to do a variable speed approach or a constant speed approach.

Variable speed: you establish your approach angle and decrease your speed gradually in proportion with your distance to the pad, all the way working the collective to stay on the glide slope. Finish with very gentle leveling off. [This allows you to climb on the back of power curve super progressively]

Constant speed: establish your approach angle and keep your speed 60 knots or above as long as practicable (collective hardly requires any work because all things remain equal) finish with a solid flare and leveling off. [The later part requires you to climb on the back of power curve very quickly]

passenger comfort, risk profile, landing situation, weather, type of aircraft. what factors come into play?
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Old 30th Jun 2020, 04:34
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Have you ever heard of the term "stabilized approach"?

The idea behind it is that you establish the aircraft on the glide path, then do as little adjustments as necessary until you reach your DA/MDA.

In case of a missed approach procedure, all you need to do is raise the collective rather than pitching up..

Pitching up will result in more control I puts than just raising the collective at Vy......

But this is just my take on it...

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Old 30th Jun 2020, 05:07
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I think he is asking about a visual approach to a landing site, rather than an instrument approach.

I always did it at an apparent walking pace over my toes. At 300' it equates to 60kt, having selected the hover attitude. Hover attitude, walking pace over the toes, controlled rate of descent.
As the descent continues, the speed bleeds off.
Around 30kt the power starts to come in, a gradual change to the ah-send of the power curve, holding the attitude steady, not letting the secondary effect of collective kick the nose up. Hold the hover attitude, same walking pace over the toes, and you end up in ground effect at walking pace, about to stop, and knowing that you have the power to terminate.

The problem with a 60kt approach is not knowing if you will fall through at the bottom, either through slow response from the engine, or insufficient power available. The pax might not appreciate the rapid flare upwards, the rapid roll-over to level, the rapid increase in engine noise, and the rapid increase of metal tearing itself to bits around them as you misjudged it.

And anyway, if you are on an ILS, your autopilot should be slowly reducing your speed from 120kt to 70kt without you having to activate a brain cell.
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Old 30th Jun 2020, 06:32
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My favorite way to bring it in was to back it off to just above ETL and crawl down riding that vibration.

,...or if I had more room,..

I'd back it off to 40kts, dump the collective and do a nice quiet 180 from downwind to final, then transition to an air taxi for the rest of the way to the pad.

:-)
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Old 30th Jun 2020, 06:49
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Originally Posted by Ascend Charlie View Post

And anyway, if you are on an ILS, your autopilot should be slowly reducing your speed from 120kt to 70kt without you having to activate a brain cell.

Oh the joys of a modern autopilot. Some you have to engage IAS and DECEL for such a smooth approach.


​​​​​​
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Old 30th Jun 2020, 07:33
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Agile - the variable speed, constant angle approach is a good teaching tool as you learn to control the angle of approach with collective. It is good for night flying, mountain flying and confined area approaches but you spend a long time in the H-V curve. It does allow you to monitor power requirement easily and to go around if you start to run out. However, it's really not very tactical.

The constant speed is faster, keeps you out of the H-V curve most of the time, is more tactical but relies on performance planning to ensure you don't run out of puff as you lose ETL and make larger collective and yaw inputs at the end.

It is horses for courses, some prefer one way exclusively to the other but it depends on what you are flying and where. Into an airfield I would use constant speed - into a small HLS I would use constant angle. Stacks of power margin - no problem with constant speed. Minimum performance available - constant angle.

Hope that helps.
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Old 30th Jun 2020, 07:35
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The faster the approach, the quicker in the bar!
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Old 30th Jun 2020, 07:47
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The main danger with a constant speed approach IMHO is that the faster you are going at the bottom, the more flare you are going to need to stop. In some landing sites, you really dont want a big flare because: A) you put the tail closer to obstacles/ground and B) raising the nose can impede your forward visibility towards the landing site and obstacles in front.

That is why most manufacturer-designed profiles for helipads/helidecks/confined areas will take you to a decision point at the minimum possible speed where you are going to still have enough power ( in a twin) to fly away if you gat an engine problem up to that point.
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Old 30th Jun 2020, 07:52
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If you have space, use it.
Bleeding off speed that you could need later seems counter productive.
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Old 30th Jun 2020, 10:38
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Crab, technically he isn't in the H/V curve, which is for level flight with the power to stay there.

This is a low-power descent, a different scenario for which the manufacturers do not produce a graph.Way too many people try to superimpose a descent onto the Curve, when it ain't applicable.

I have to agree that skooching in at 130kt (just slow enough to get the Dunlops dangling) and then doing a smooth 180 quickstop to terminate in a perfect hover over the pad is very satisfying. The Sydney control tower even said "Gee that looked good" on one of my better landings.
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Old 30th Jun 2020, 11:14
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Maybe we should also ask whether we are talking about single engine vs twin?

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Old 30th Jun 2020, 11:29
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heuyracer, why did you have to bring that up
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Old 30th Jun 2020, 11:32
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Same thought crossed my mind, but elected to stay away.

braver sole than me!
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Old 30th Jun 2020, 11:42
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Such an interesting discussion! Other than an evil desire to perform airshow maneuvers, I've never wanted to perform a constant speed approach to a dramatic flare at the end, much less a 180 quick stop All of my instructors always wanted to see a steadily decreasing speed with the ship maintained in a nearly level attitude. While I've certainly had to deal with situations where an OGE hover can't be accomplished, that did not require carrying a whole 60KN, more like 30-40KN, and of course the flare is not so dramatic at that speed.
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Old 30th Jun 2020, 11:46
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Originally Posted by Robbiee View Post
My favorite way to bring it in was to back it off to just above ETL and crawl down riding that vibration.
Except that when you feel that vibration you are already below ETL and what you are feeling is the vibration from the transverse flow effect. That's not a good technique, and certainly not when one can make a normal approach.

When making a steep approach that allows maintaining ETL if you are getting that vibration speed up just a bit as long as descent rate permits.
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Old 30th Jun 2020, 11:52
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Originally Posted by Ascend Charlie View Post
I think he is asking about a visual approach to a landing site, rather than an instrument approach.

I always did it at an apparent walking pace over my toes. At 300' it equates to 60kt, having selected the hover attitude. Hover attitude, walking pace over the toes, controlled rate of descent.
As the descent continues, the speed bleeds off.
Around 30kt the power starts to come in, a gradual change to the ah-send of the power curve, holding the attitude steady, not letting the secondary effect of collective kick the nose up. Hold the hover attitude, same walking pace over the toes, and you end up in ground effect at walking pace, about to stop, and knowing that you have the power to terminate.

The problem with a 60kt approach is not knowing if you will fall through at the bottom, either through slow response from the engine, or insufficient power available. The pax might not appreciate the rapid flare upwards, the rapid roll-over to level, the rapid increase in engine noise, and the rapid increase of metal tearing itself to bits around them as you misjudged it.

And anyway, if you are on an ILS, your autopilot should be slowly reducing your speed from 120kt to 70kt without you having to activate a brain cell.
Hate to reuse someone's entire post, but Ascend Charlie just makes a whole lot of sense.

When I began working offshore for PHI, I was a "field ship" which is one that is assigned to an offshore post for the seven-day "hitch" and bounces around between a specific group of platforms all day long. My "worst" job assignments had me doing 70-100 landings per day. At first, my philosophy was "every approach an autorotation." I'd come screaming in at 60 knots all the way to the flare. Worked great, and they were fun, but my timing had to be exquisite - which it was, through practice. I was young back then.

Eventually I realized that turbine engines are pretty reliable and we don't have to constantly worry about them quitting on landing. Pilots fail more often than engines. I worried about my timing; sooner or later I would screw up. So I got slower and slower. Now, I do it the way Ascend Charlie describes - nice and gentle, stable, no big power changes (if any) at the bottom. My alternate way is to do like Robieeeee and "ride the ETL burble" down. Or just above it. Keep your spot constant, don't look at the gauges, just keep the cabin level, and don't let the nose come up as you squeeze the power in (common mistake). Easy-peasy. If you do nothing, the ship settles onto its ground-cushion without any mucking about by you. If you screw up and get below ETL early...well, so what? Just keep it coming down gently.

In 15 years at PHI, almost all of that offshore, I did six or seven landings per hour to elevated offshore helidecks. Times 5,000 hours, that roughs out to, ohhhh, 30,000 or so landings on oil platforms/rigs. And over the course of that time, I sure learned what *not* to do on an approach.

Lots of great info in this thread.
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Old 30th Jun 2020, 14:15
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I guess this comes from the offshore world teaching inexperienced pilots how to be gentle on the controls but what else do people think of the following type of approach? More a Heli deck type of landing I suppose.
0.5 miles, 50kts, 500’, 0.4 miles, 40kts, 400’, 0.3 miles, 30kts, 300’ which pretty much brings you to your decision point at around 30kts for moving across and down to a landing on the deck. No big collective movements required, no big speed changes required. But I guess it can often be very type specific.
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Old 30th Jun 2020, 18:04
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I’m reading lots of “do it this way...,” but very few answers to the OP’s question, “when is it better to do a variable speed approach or a constant speed approach?”
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Old 30th Jun 2020, 18:07
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In my opinion (and out of my experience):

Constant speed approach is something i use if the weather is bumpy , or when the approach needs to be expedited (due to traffic, or other reasons)-and if there is enough space at the end to "flare to a stop".

Variable speed is what i normally use for all other conditions, or to a heli deck/a confined area (you name it)....


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Old 30th Jun 2020, 18:48
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Originally Posted by JimEli View Post
Im reading lots of do it this way..., but very few answers to the OPs question, when is it better to do a variable speed approach or a constant speed approach?
It is better to do a constant speed approach when practicing autos. It is better to do a variable speed approach when a check pilot is sitting next to you and you want him to see that you know what you're doing.

Outside of that, you're the pilot, you decide what works best for you.
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