Height velocity aviod curve - for fixed wing?

[PilotDAR]

It occurs to me, with the recurring discussions about speeds, that there is an important speed (Vy) which is regularly ignored - at great peril. For those who are familiar with helicopter flying, Helicopters are provided with the Height Velocity Curve (AKA "avoid curve" or "deadman's curve"). For convenience for those not familiar, here's one:



You can notice the reference to the recommended takeoff profile. It occurs to me that where this to also be presented for fixed wing aircraft, and pilots to take it seriously, accidents could be prevented.

In essence, that curve tells the pilot the flight conditions from which a safe autorotation landing is not possible. An important thing to know! So no, you can't sit up hovering at 500 feet, and expect to land safely if the engine quits. Similarly, you can't blast along at 60 knots, and ten feet up, and expect to land safely after an engine failure.

So why does PilotDAR think this applies to airplanes too? Though I have never seen a height velocity avoid curve for a fixed wing aircraft, I think that they might be equally applicable, and important. The airplane Flight Manual will state a Vy, and a Vx, and a recommended glide speed. Think about your plane, which is the slower speed - Vx, or glide speed? Ah, Vx you say - there in lies the problem.

Pilots find that the plane will climb away at Vx, and it's cool to look like you're flying a super STOL departure, 'cause the nose is way up, and the increase in altitude verses ground covered is very good. But, hanging up there, on power, but with lesser forward speed, you're in the middle of the curve avoid zone for your aircraft. The engine goes blat, and you're gliding at that slower speed, you're going to have to trade altitude for speed - just to get going a fast a glide speed! Then you glide! Depending on your beginning altitude, you may have reached the ground before you reached glide speed. If you descend at a speed slower than glide speed, you've reduced the reserve you have with which to flare, pull to flare, you just continue down, with even less control.

People ask me what's the most scary thing about test flying modified aircraft. Well, the testing I did, which I most thought would lead me to wreck a plane was required engine failures from 50 feet, and glide back to the remaining runway. 'Thing was that as I had reduced Vy from 87 knots to 80, TC required a demonstration of a land back from that slower speed - I did it, but it was scary! Now I know why Cessna errs to the faster speed for climb out. Not so much for good climb performance, but simply so you can glide and land reasonably after an engine failure at that altitude.

So, the climb speed in the flight manual might be established more based upon safe return to earth following a low altitude engine failure, rather than the optimum performance. I would be pleased to see a height velocity curve published for planes too, so a pilot understands why that Vy speed is a little faster, and actually embraces that speed for normal operations, to improve safety.

There are many recommended operational practices, with hidden underlying rationale. I wish that pilots were given the "full story" so they could make more informed decisions about how they fly. An HV curve in the emergency procedures section of a flight manual would be one way of presenting this information to the pilot.....

[photofly]

I suspect a lot of pilots obsess about "best glide" speed. Best glide speed is only good for one thing: making distance. If you want to carry out an immediate landing on the remaining runway, making the maximum distance is exactly the wrong thing to think about.

With an engine failure at 50 ft I'm going to throw down those flaps and transition immediately to my "short final" speed, and screw best glide.

[Cat Driver]

Excellent subject PilotDar, like you I have always wondered why more people do not think about this very demanding phase of flight.....watching some pilots lift off and climb at a high nose attitude and low airspeed is scarey at best.

Speed is your friend at low altitude.

[photofly]

With a bit of luck nobody will have to report flying into trees at the departure end of the runway because they were afraid they might scare Cat Driver if he watched them takeoff.

[PilotDAR]

"Best Glide Speed" and the Flight Manual "recommended glide speed" might be very different speeds. Of course pilots would glide at the recommended glide speed, not the "best" speed. The recommended glide speed might not be that for the greatest distance traveled, or minimum descent rate, but rather the speed which allows you the best control during landing. This would be similar to a helicopter autorotation at the minimum rotor RPM, rather than the maximum. You're going to stay aloft longer and travel further at the minimum rotor RPM, but you're going to have very little reserve left to flare to land.

Is was practicing glide approaches in the Teal. It will happily glide at 70MPH, and I did. I went to flare, and there was no reserve energy. Only a rapid application of power prevented a hard landing. I tried 75MPH, and similar but less dramatic result. I tried 80 MPH, and that was adequately safe. Hmm, a rather fast glide speed, I thought to myself. I pulled out the Flight Manual, and sure enough, recommended glide speed is 80MPH. The Flight Manual did not provide a glide speed for maximum distance or minimum descent rate. I assure you that the 80MPH glide in the Teal is like an autorotation!

When I first started demonstrating STOL modified Cessnas to owners, I found that they would glide at and even slower speed with the STOL kit installation. But, when you went to flare - nothing! It would just settle through in the stall. I would then train that once the power was off, STOL kitted Cessnas are to be flown at the Flight Manual speeds.

For the Caravan, Cessna states that 97 knots is the "best glide speed" and that speed is also referred elsewhere in the Flight Manual as the "maximum glide speed" (so I'm going to ask Cessna to clarify that). I can tell you that a Caravan will glide at a much slower speed than this. It states "normal climb out" speed of 85 to 95 knots, which sounds appropriate relative to the glide speed. But it also states a short field takeoff speed of 83 knots, and a 20* flaps balked landing speed of 80 knots. I can tell you from experience that an engine failure at low altitude at 80 knots in a Caravan is a startling event, which is going to require a lot of skill fast.

So I opine that it would be helpful, in the absence of published height velocity curves for single engined aircraft, that pilots consider what that chart would look like for the plane they fly. Once airborne, and beginning to climb away, are you flying at a speed which will allow you to transition to the recommended glide speed with little altitude having to be surrendered to get to that speed?

report flying into trees at the departure end of the runway

[because of higher speed and lower altitude] I presume.

If the runway and obstacles are that tight, we're discussing rather extreme operations. For the two really tight runways I occasionally use, the takeoff technique is to get airborne, accelerate to a comfortable speed, while aiming deliberately for a point two thirds up the trees. When I have attained Vy or better, I will pitch up to clear the trees. I know that I can allow the speed to decrease to Vx if it has to to clear the trees, but usually the inertia the aircraft has obtained will carry me the last third plus over the trees. I would far rather pluck off a few leaves while carrying extra speed, than to attempt to clear the trees by a hundred feet, and be hanging on the stall doing it. Yes, altitude is a friend, but speed is a better friend. It's safer to give up some speed to obtain altitude, than to attempt to recover speed at the expense of altitude.

If you have just crossed the line of trees, picking off a leaf along the way, at a speed between Vy and Vx, and your engine stops, you have a hope of a glide to a landing ahead. If you have crossed that line of trees with 150 extra feet, but now you're flying at less than Vx, and at full power, it is doubtful that you will recover that required speed with the extra 150 feet to trade when the engine stops. You'll probably loose 75 of those extra feet during the settling just recognizing the engine failure. Now you're scrambling to attain enough speed to glide and flare, and it might not work well.

[photofly]

"Best Glide Speed" and the Flight Manual "recommended glide speed" might be very different speeds. Of course pilots would glide at the recommended glide speed, not the "best" speed. The recommended glide speed might not be that for the greatest distance traveled, or minimum descent rate, but rather the speed which allows you the best control during landing
...

For the Caravan, Cessna states that 97 knots is the "best glide speed" and that speed is also referred elsewhere in the Flight Manual as the "maximum glide speed" (so I'm going to ask Cessna to clarify that). I can tell you that a Caravan will glide at a much slower speed than this.

I feel reasonably confident that any speed listed by Cessna as a recommended engine-out glide speed is the speed that gives you furthest distance with no wind. Terribly basic PPL training hammers into your head that this isn't the speed that gets you the furthest given a wind, or the speed that gives you the slowest descent.

I've never flown a Caravan, but I've flown some other Cessna singles (hasn't everyone?) and they all glide slower than the recommended best glide speed, quite nicely, right down to the stall. It sounds like you're trying to invent another "recommended speed" for engine out scenarios based on a new mysterious assessment of handling characteristics. The regular target speed of 1.3Vs0 (or 1.3Vs1) works nicely as a speed to aim at, at 50 feet, for both a glide descent and powered descent. Why do you want to complicate things further?

So I opine that it would be helpful, in the absence of published height velocity curves for single engined aircraft, that pilots consider what that chart would look like for the plane they fly. Once airborne, and beginning to climb away, are you flying at a speed which will allow you to transition to the recommended glide speed with little altitude having to be surrendered to get to that speed?

You're making it way way too complicated again. Vy is typically fast enough to get the nose down on power loss and to perform a safe emergency landing, as long as you do it promptly. If you're climbing out more steeply than a Vy climb you're doing it for a valid reason like an obstructed takeoff path. In which case, do exactly what the book says, which is something like climb at Vx (or the recommended performance takeoff speed/configuration) until the obstacle is cleared. Then lower the nose and accelerate back to a normal climbout speed.

I would far rather pluck off a few leaves while carrying extra speed, than to attempt to clear the trees by a hundred feet, and be hanging on the stall doing it.

Allowing for some poetic licence on your part, I hope most pilots wouldn't accept actually hitting an obstruction just as long as they don't hit it actually hard enough to stop flying altogether. If I was plucking leaves off my undercarriage I'd be having a stiff drink and a reconsideration of my procedures.

I would opine that for most pilots, it's very unwise to start inventing your own obstructed field departure procedures. Fly the procedure in the book. Hitting the obstruction is a much more present danger than losing the engine immediately after takeoff. Minimize your actual risk factors, not the ones your mother is scared of.

[PilotDAR]

they all glide slower than the recommended best glide speed, quite nicely, right down to the stall.

Very true. Every plane I have glided (glid?) will glide nicely right down to the stall, but trying to flare for a power off landing at that slower glide speed could result in a very hard surprise when the plane does not flare at all.

I feel reasonably confident that any speed listed by Cessna as a recommended engine-out glide speed is the speed that gives you furthest distance with no wind.

I do not have that same confidence. I'm opened to thinking that the stated speed might be a compromise between best distance, and best flyability close to the ground. Rotorcraft Flight Manuals do not recommend autorotation at the slowest rotor RPM (which would be the longest glide), your training will tell you that you might use that as a "trick up your sleeve" if you have a long autorotation (glide) to do.

Vy is typically fast enough to get the nose down on power loss and to perform a safe emergency landing, as long as you do it promptly. If you're climbing out more steeply than a Vy climb you're doing it for a valid reason like an obstructed takeoff path. In which case, do exactly what the book says, which is something like climb at Vx (or the recommended performance takeoff speed/configuration) until the obstacle is cleared. Then lower the nose and accelerate back to a normal climbout speed.

Absolutely agreed. "Until the obstacle is cleared", though not by a high margin which compromised a safe recovery from an engine failure un-necessarily.

It sounds like you're trying to invent another "recommended speed" for engine out scenarios

No, not a speed, but an awareness of a speed vs altitude range in which one might have to fly, where recovery to a safe forced landing is uncrertain. The "Avoid" curve. It is not a speed, it is a condition - to be avoided if possible.

Hitting the obstruction is a much more present danger

Absolutely. Leaves and tiny branches are not obstructions. Lots of other things are, and of course we're going to aim to give them a safe margin.

If I was plucking leaves off my undercarriage I'd be having a stiff drink

I don't drink, but I have plucked leaves the odd time.

based on a new mysterious assessment of handling characteristics

No, no! Not new, and certainly should not be a mystery! That's the problem! Pilots can be seen to fly steep slow climbouts for reasons other than necessary obstacle clearance at Vx. It seems to me that pilots doing this are unaware of the risk they are creating for themselves, in the event of an engine failure. Indeed, I can personally attest that these steep attitudes can be the cause of the engine failure (I've done it in a C185 during flight testing). So, it's a double whammy, as you just caused the circumstance from which you cannot now recover.

It is this behavior I seek to spot light and discourage. New pilots find that the plane will takeoff and climb more slowly than the book says, and they have never been taught the "mysterious" underlying reason why doing this is extra risky.

[Skyhunter]

I find it hard to believe this discussion is happening without anyone mentioning L/D max. Have things changed that much since I learned to fly that L/D max is not relevant to best glide or or best angle of climb.

Been an exremely long time since dug deep into cessna books, but do remember much about the Tutor which just did all the same things just faster. L/D max was 130 knots, funny that was also "best glide" and "best angle" (number was just slightly less than 130 for the purists). Strange how they were the same isn't it? Again, I don't remember the cessna details, but is not best glide (for range) and best angle of climb the same number both being L/D max? Incidently best rate was 220, which was also "normal" climb speed.

Again, it has been a while, so if I am out to lunch on the L/D max please feel to say why, but ask that you use a little respect

PilotDar, some points I don't know if you have considered,

1. I would rather do a hard landing in a field that I made, gliding for max range that have lots of energy to flare and go into the trees cause I didn't glide for max distance. Already know you can make the field the fill your boots of course and hold a little extra NRG for the flare if you like.

2. Clearing obstacles at greater than best angle of climb wastes NRG. Yes you can zoom and use the extra NRG from the increased speed. What you can't use is the NRG you wasted away to access drag due to flying faster. That is gone. In a truly tight field with obstacles that you are worried about clearing best angle is the most efficient way to go period. Clear the obstacle, accelerate. Personally I haven't flown into that many tight spots like that, but it is the same as many of the engine out procedures/speed used in case of an engine failure on twin flying when I was flying is some tight fjords up North.

3. As far as the power loss vs ability to lower the nose... decaying airspeed is sometimes the first indication of power loss. As attitude controls airspeed, you should already be lowering the nose to maintain airspeed as you realise the power loss. In fact if trimmed correctly it should already be lowering a bit helping you. I remember doing a lot of these years back prior to the military, and pushing the nose over to land ahead (trained on a long runway where could safely practice). Don't remember it being that hard, after the first couple.

I do believe though, that "speed is life," and where obstacles permit and having a few extra knots is not a bad thing and gives your more optioins.

[photofly]

New pilots find that the plane will takeoff and climb more slowly than the book says, and they have never been taught the "mysterious" underlying reason why doing this is extra risky.

They should be taught (I was) - but sadly people like to behave in 'risky' behaviour even when they know better.

Again, I don't remember the cessna details, but is not best glide (for range) and best angle of climb the same number both being L/D max? Incidently best rate was 220, which was also "normal" climb speed.

Again, it has been a while, so if I am out to lunch on the L/D max please feel to say why, but ask that you use a little respect

Best glide (distance) speed is certainly at the airframe's best l/d ratio. But Vx is a different speed. If you sketch some performance graphs, you can see why.

For a C182 (from memory) best glide (distance) is 80mph (70kts) and Vx is 68mph at sea level (59kts).

One way to see that the airspeeds are not the same is to note that Vx depends on engine power: the same airframe with a bigger engine will have a reduced Vx (and climb more steeply); whereas best glide speed depends on the airframe only and not engine size.

Alternatively (and somewhat equivalently) note that for a normally aspirated engine Vx (indicated) increases with altitude because of the reduction in engine power, whereas the best glide (distance) speed is at a fixed indicated air speed regardless of altitude.

[iflyforpie]

I think I'd feel safer in a light airplane using Vx to clear some trees at the end than in a helicopter at the end of a 200ft long line for hours on end or a turboprop under 12,500 without a balanced field.

In each case, you are taking a risk that is necessary to what you are doing. If you have to clear obstacles by using Vx... you probably aren't going to have many places to glide below an altitude where you will have enough to convert it to best glide or minimum sink.

[PilotDAR]

In each case, you are taking a risk that is necessary to what you are doing.

This is the key. Sometimes it is necessary to operate the aircraft in the "avoid" range. It's "avoid" rather than "prohibited" because the manufacturer recognizes that sometimes you have to be there. The key is to realize that for each aircraft, there is an "avoid" range. Do not go there without good reason, get out of it as quickly as practical, and otherwise mitigate risk the best you can while you're in that range.

[pdw]

For the Caravan, Cessna states that 97 knots is the "best glide speed" and that speed is also referred elsewhere in the Flight Manual as the "maximum glide speed" (so I'm going to ask Cessna to clarify that). I can tell you that a Caravan will glide at a much slower speed than this. It states "normal climb out" speed of 85 to 95 knots, which sounds appropriate relative to the glide speed. But it also states a short field takeoff speed of 83 knots, and a 20* flaps balked landing speed of 80 knots. I can tell you from experience that an engine failure at low altitude at 80 knots in a Caravan is a startling event, which is going to require a lot of skill fast.

Would be interesting how the POH differentiates for the podded fuselage or if 97kts is all-encompassing for the other configurations. Would not be surprised if the fuselage drag co-efficient for a non-podded Van is less-than / a-fraction-of the basic Van, so that its glide distance would be a lot more impressive at that "maximum glide speed" than for instance a fully-equipped Cargomaster at MTOW (so can see/ask if the sleek/low-drag versions of the C-Van family would glide even farther if "slower" ?). Which Caravan-config are we using ?

Gliding at MTOW with a pod, one main reason I can think of for keeping 97kts across the board, given it's the speed for both "best" and "maximum", is for maintaining extra speed for the light airspeed decays when at the maximum POH wingloading of a Van's high-lift wing. The power-off glidespeed on the MTOW-Van can dip too-low more quickly when "slower" (more bleedoff-susceptible in light shears when gliding or takeoffs at MTOW) even where the increased power-curve is already backing the greater drag of the pod configured fuselage. IMO "slower" speed when gliding exponetially reduces the extra drag curve so not a surprise if the greater glide-distance is evident when at those slightly slower test-speeds, .. and then again, exponetially more glide-distance achieved if flying so much lighter than MTOW.

As for the balked landing at 80 knots, or the 83 knots for the shortfield at point of rotation, both happen near ground effect at almost zero AGL. Any "low altitude" maneuvering above that must be flown at those higher speeds given.

What's it like transitioning to glide at low altitude from the Vx climb or the balked landing at "80kts" ? ... think I got the general idea.
Was often heard said: "it flies like a big 172", the Cessna I am (many are) used to; so often at MTOW, but no additional cargo-hold.