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?

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...

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.

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.

:-)

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


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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.

The faster the approach, the quicker in the bar!

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.

If you have space, use it.
Bleeding off speed that you could need later seems counter productive.

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.

Maybe we should also ask whether we are talking about single engine vs twin?

heuyracer, why did you have to bring that up

Same thought crossed my mind, but elected to stay away.

braver sole than me!

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.

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.

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.

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.

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?”

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)....

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.

AC - I know where you are coming from but you can't disregard the H-V curve just because you aren't in level flight - you are slightly better off being in a descent as the power is a bit lower but not in the latter stages as ETL is lost.

aa777888 have to disagree with that - the vibration is ETL not transverse flow and you can tell by the lower power riding that vibration that you are not below ETL. You can show a demo of transverse flow in a transition into forward flight - it starts much earlier than ETL and there is no vibration until just before ETL (you are powering through the vortices to get through ETL) and then the nose pitches up due to flapback.

As for single vs twin - it doesn't matter unless you are doing PC1 profiles in a twin.

Crab: according to the FAA Helicopter Flying Handbook, page 2-23:

This vibration happens at an airspeed just below ETL on takeoff and after passing through ETL during landing. The vibration happens close to the same airspeed as ETL because that is when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.

Great feedback, the point related in the answers have concentrated what has been running in my head for a long time.

Good point I concur from Ascend Charlie: Let the collective-up increase your flap back and kill your speed as you keep apparent speed above your toes constant, keeping the apparent speed constant is effectively a gradual deceleration because the closer you get to the ground the faster is the apparent speed is. I got that well practiced and it looks gorgeous, so when I want to play it super safe (family is on board or whatever) I request the runway and make it to the number in variable speed, I can abort at any time by a gentle push forward. But there are those few second where the speed is below the green arc and the height is too high for an engine misfortune at that speed (single).

Good point I got form FH1100 Pilot, So I have been practicing hours of variable speed approach and yes the timing has to be exquisite, my interpretation is that when it is perfectly balanced it is indeed very close to an autorotative approach and could bring you to touch down safely more or less regardless of power reserve. But then there had been time when the wind changes direction or the tarmac on a hot summer day keeps a few meters of high temperature air by radiation of the black surface. And then your great timing yet sends you sinking and clenching your teeth.

Robbiee: I had that figured out too, check pilot always nod at a fine variable speed approach, the other always prefer to save 2 min on the odometer.

aa777888 - is that the same FAA handbook that called settling with power and VRS the same thing?

Go and try a transition to forward flight on a still wind day from a 15' hover - don't change the power but move the cyclic very slightly forward. You will start to descend as you have tilted the lift vector, then as the speed starts to increase you get inflow roll (transverse flow) which makes the aircraft roll towards the advancing side - correct that with lateral cyclic and you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash - then the nose pitches up and the aircraft climbs as the rotor experiences cleaner air.

Transvers flow is a flapping to equality in roll that exists throughout the speed range but is most noticeable just after the disc is tilted forwards to initiate a transition. Not associated with vibration.

Agile - if you have your lift vector tilted backwards - a decelerative attitude - raising the collective increases that vector and slows you down as well as reducing your RoD. As you have probably discovered, if you get it right, you can hold the same decelerative attitude all the way from the top of descent to just before the hover and control the angle of descent with lever.

Thanks for the clarity of the sequence, just realized that the nose pitch up is also induced by the aircraft as well as the by the pilot.

yes! --- of course the key really is to stick with a high rate of small collective corrections, falling below the slope = overcorrecting on the collective = losing more speed than anticipated = nosing forward again = everything getting sloppy --> very easy to get that way on shifty windy days
best is to manage to couple collective movement with longitudinal cyclic twick, not there yet.

Agile - the nose pitch up is flapback which has to be overcome with forward cyclic to continue the acceleration.

Controlling angle of descent with collective while reducing speed gradually is one of the more difficult things to get students to do properly - there is a strong temptation to keep the landing point in the same place by moving the cyclic which just changes the speed.

As you say, in anything but smooth conditions, it requires constant small adjustments of cyclic and collective to fly the approach accurately.

Attitude = airspeed
Power = Rate of descent.

Applies to all phases of flight. With enough power, the ROD becomes ROC.

To answer the original question: It depends on where you are, what you're trying to achieve and who's watching!

Flying a variable speed approach to the numbers of a runway, then taxiing 1000m at a walking pace is pointless and expensive (unless your instructor asked you to) so a constant speed approach/air taxi/quick stop might be more fun and more appropriate.
Flying over forest into a small confined area requiring a near vertical descent from a high hover would obviously be much safer with a variable speed approach.

I'm guessing most flights are somewhere in between. It's a magic carpet so put it where you need it?

I think there's more to it than that. All phases of flight are a combination of pitch AND power. You can have full climb power, but if you're pointing down you will still go down. If you set constant power for level flight you can still climb and descend just using the cyclic. If you want to do it at a constant airspeed then you'll have to adjust the power as well. Which control is 'controlling'? That's for us to argue about for the rest of time...

I think the Attitude=Airspeed/power=ROD simplification stems from the fact that we can operate on both sides of the power curve, but it sure is a whole lot easier to level-off with cyclic, then set the speed with collective!

Interestingly in a Jet, speed is throttle and pitch maintains altitude, but climbing or descending speed is pitch and throttle is ROC. Unless you select V/S when speed is throttle again, and ROC is pitch!

Ah Crab - I'm straight back to Shawbury about 30 years ago!

Exactly what I said - you are pointing down, so your airspeed will be high. Full climb power will require a VERY low nose attitude to be going down, and your airspeed will be off the clocks. Attitude IS airspeed.
Still true. Raise the nose, your airspeed will decrease as you climb at a reducing rate. Lower the nose to descend, and your airspeed will increase.

For any airspeed, there is one power setting that gives zero ROD = level flight. But for one power setting, there are usually 2 airspeeds for level flight. White man magic, ain't it?

That's how we are taught to fly planks on finals.

Also works quite well in a helicopter on long finals for a decelerating speed landing, same principle, just the vectors are pointing differently.
Used to be quite good at setting an attitude and selecting power levels from miles out and getting close enough to the pad with barely any input. Sadly those skills are now not what they used to be.

well you taught it to me:)

So you will throw the baby out with the bathwater, then? I bet you can't name one book that doesn't have either a single error or something you don't agree with. So that's a ridiculous way to imply the entire document is worthless. And they've since updated the handbook to only discuss VRS. So perhaps not entirely correct but getting better.

"you will next feel the vibration of ETL as the rotor passes through the roll-up vortices of the downwash"--that's not ETL. If it was fully developed ETL and there was a vibration associated with ETL the damn helicopter would vibrate all day long like that, and of course it doesn't. And if it's not fully developed ETL then it's not an optimally safe condition to be making the approach in.

Every reference I've read to date says the vibration is due to transverse flow and not ETL. Therefore they are ALL wrong and you are right, or you are wrong and all of them are correct.

Of course I am by no means an aerodynamicist, therefore I must rely on what these references tell me is so. Can you provide a written reference (other than your own!) that does not say that these vibrations are due to transverse flow prior to accelerating into ETL, or the same but after decelerating out of ETL?

Just go and try what I suggested. If you do it smoothly you will encounter transverse flow (inflow roll) towards the advancing side with NO vibration. It is due to the difference in the inflow angles at the front compared to the rear of the rotor disc.

You know the rotor produces vortices at the tips right? Those vortices are what cause the vibration as you transition to and from the hover because the rotor has to battle through them into clear air - that marks the onset of ETL and as the vibration clears the aircraft pitches nose up and climbs due to flapback and the increase in lift. The vibration marks the boundary of ETL and that is why, once you are through it - it doesn't keep vibrating.

If transverse flow was the reason for the vibration, you WOULD feel that throughout the flight envelope whenever you tilt the disc and create an inequailty of lift.

Don't confuse flapback (blowback) with transverse flow (inflow roll).

To get back to the question, I would suggest the approach you use will depend on

1. Experience level of pilot, seen far too often people hacking into the airfield , flash quick stop with tail wagging al over the place
2. Situation eg wind, temp weight of aircraft power margin

Go back to your last lesson when being taught the recce of a confined area ie the 5 s that will then dictate the correct approach for each situation. Basically there is no right or wrong, just degrees !!!

aa777888 - this is from Prouty - note where the vortex is at 15 to 20 kts (ie the onset of the vibration and the boundary of ETL) and then where the vortex is when you experience transverse flow (5 to 10 kts)https://cimg2.ibsrv.net/gimg/pprune....3b47045bde.jpg

Apologies for the large size of the picture - don't know how to resize it on pprune.

And now consider your previous comment about how the vibration is transverse flow as you reduce speed on the approach.

You perhaps will agree that in the transition to forward flight, the disc is tilted forward and the difference in inflow angle (small angle at the front and large at the back) is the cause of the transverse flow effect (I say it rolls, you say it vibrates).

If so, what causes the change in inflow angle in a steady, speed reducing descent when the disc attitude doesn't change and the inflow angles are much the same (but from underneath the disc).

The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.

OK, stipulating that this has nothing to do with transverse flow, nevertheless per your own statement you don't get flapback until the vibration clears. That would mean that during the condition of vibration it is not a fully developed state of ETL, or perhaps not a state of ETL at all, which was my original point despite the aerodynamic arguments I was referencing from various manuals. Also, the figure out of Prouty is not relevant because we were originally discussing an approach, not forward flight in ground effect.

We weren't discussing a speed reducing descent. Robbie said he liked to ride that vibration down, so it's a constant speed descent.

Quote:

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The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.

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But you stated above that the vibration occurred before flapback?