V_x and performance takeoffs - POH discrepancy

[Beefitarian]

Sorry, I don't know how many of you saw my other post but..

What I meant was basically the POH probably added the 59 knot thing because it would actually result in the most height from a sloppy pilot instead of Vx which would be a "perfect scenario" sort of speed.

I stand by, "Cut down those trees." or use a different airport.

[Cat Driver]

On the subject of using Vx for obstacle clearance, both Cat Driver and Colonel have the right perspective.

photofly: If a Pilot Examiner is fussing about a 2 or 3 knot deviation from the book figures, that individual should be taken behind the wood shed and given a thrashing.

Thanks for chiming in TC Aviator, if a DFTE asked me to demo such a takeoff I would taxi back to the ramp and find another DFTE with a functioning brain.

[2550]

Somebody answered the original question early on I think:

59 KIAS gets you from ground effect to clearing an obstacle (normally 50 ft.) most effectively. 64 KIAS is best angle when a takeoff over obstacles is not directly involved. You would eat up more forward distance accelerating to 64 than you would accelerating to 59. So, distance eaten up during acceleration actually penalizes your obstacle clearance on takeoff.

Seems to me you should be able to do aproximatley the same thing by feel, as CS and Cat say, but hopefully with more margin for error.

In the real world, its pretty near impossible for a normal human to judge DA, feild length, how soft it is, how much the mud puddle will slow you down, how high the tree is, (not 50 feet!) how much gas is left in the tanks, how much your preformance will be degraded by the worn out prop and high time engine in front of you, how fat your passengers are, how strong that little breeze is, and how much your judgement, reflexes and preformance are degraded from drinking beer and arguing about preformnace numbers on avcanada last night.

One or two knots dont matter, as long as its not one or two below the stall speed as you crash into the trees.

[Cat Driver]

One or two knots dont matter, as long as its not one or two below the stall speed as you crash into the trees.

Exactly, which brings us back to the most important part of this subject.....

....why would any teacher or flight test examiner choose to fly that close to the edge on take off?

....soon we will have people deciding to practice stalls and spin entry recovery at fifty feet...

[Beefitarian]

Moderators, please lock the thread to prevent constructive conversation since the question was answered.

[CpnCrunch]

One or two knots dont matter, as long as its not one or two below the stall speed as you crash into the trees.

Exactly, which brings us back to the most important part of this subject.....

....why would any teacher or flight test examiner choose to fly that close to the edge on take off?

....soon we will have people deciding to practice stalls and spin entry recovery at fifty feet...

Is 10-12 knots above stall "close to the edge" or am I missing something?

[Cat Driver]

I guess it all depemds on how things go.

A sudden power failure at fifty feet close to the stall can get serious real fast.

My position on this subject is why increase the risk factor if you don't have to.

[iflyforpie]

I wouldn't call 59 knots (or 60) in the 172 dangerously close to the stall. I fly it at 45 knots clean when doing slow flight and it is just fine to stay like that all day.

But yes, personally I'd rather have a surplus of airspeed on takeoff.

[Colonel Sanders]

It's not the stall that concerns me.

What I worry about is people inadvertently entering
the back side of the power curve. By the time they
recognize their mistake, it's too late - even with full
power, they can still mush into the ground, pulling
back as hard as they can on the control column,
trying to instinctively raise the nose and climb.

Low time pilots should not operate on the back side
at low altitude. Their instincts will kill them if
they make a mistake. They simply don't have
enough stick time to cerebrally recognize what
is happening, and to do the right thing.

I have the same problem when i am teaching
people to fly tactical jets. On takeoff the back
side of the power curve must be avoided - see
the Sacramento ice cream parlor in 1971 (F-86).

And this is the biggest problem on approach in
a tactical jet - controlling airspeed to avoid the
back side of the power curve like the T-33 that
crashed short of the threshold at Hamilton last
year.

Neither the F-86 or T-33 pilots above were
low-time by AvCan standards.

[Cat Driver]

Getting behind the power curve is obviously a possibility, and yes 59 knots is not by its self a great danger.......but ....how accurate are these airspeed indications on these small airplanes and should the engine quit at fifty feet what are the chances the average low time pilot will react quick enough to get the nose down and maintain lift on the wings?

As far as slow flight is concerned how often do you fly at low airspeeds with full power and a high angle of attack at fifty feet above the ground?

[Shiny Side Up]

There's two ways of looking at this. Either the student at this point is trained so poorly that yes, 59 knots near the ground is scary ass shit or the student is trained well enough that it isn't. The main purpose of doing the whole short field exercise isn't to tell your new students "hey go out there and see how close you can get to trees", its about shining up that precision level of flying. If you want to use 59 knots or not is up to you as the instructor and probably depends on how up to the task you feel your student is. Keep in mind though that its precision with the attitude that's important that will translate into precision with the speed. Personally, a long time before we get to this exercise we've already made the student fly the thing around just above the stall, so its not like suddenly its a new thing, now we're just doing it near the ground. Maybe if one skipped that step earlier in a big race for the student to go solo, then maybe they won't be good at it.

should the engine quit at fifty feet what are the chances the average low time pilot will react quick enough to get the nose down and maintain lift on the wings?

Why worry about engine failures now? There's licensed pilots out there who would botch it up if they had hundreds or thousands of feet to play with. Depends all on how much you demand from them and how much they demand from themselves. Its actually pretty easy to learn and do, but you just gotta care. But that's a running theme in the flight training forum here. This should be a piece of cake for anyone who's concerned with being a decent stick in their training, not sure when this became "advanced" stuff. Frig, in something like a champ or cub you're always flying around at slow speeds, best rate, best angle and your cruise speed are all within, what, 10 knots of one another?

As far as slow flight is concerned how often do you fly at low airspeeds with full power and a high angle of attack at fifty feet above the ground?

Technically its only for a few seconds, then you got more than 50'.

[Colonel Sanders]

how often do you fly at low airspeeds with full power and a high angle of attack at fifty feet above the ground?

Hopefully never! I am presuming when you say "High AOA"
you mean near CLmax and hence quite a way up the back
side of the power curve - not a good configuration for 50 AGL.

This implies significant drag, which rises exponentially as
AOA is increased. Perhaps more drag than the thrust
available - even with full power - which implies speed decay,
which results in a higher AOA.

In that situation, you have no choice but to maintain full
power and lower the nose, even though you are only at
50 feet. Few people in the heat of the moment make
that correct decision - to lower the nose, when they
badly need altitude.

You have to lower the nose, to reduce the AOA, to
lower the drag, to allow the aircraft to accelerate out
of the back side of the power curve. If a wing drops,
you CANNOT pick it up with aileron - see adverse yaw -
but instead must walk it down with rudder.

This is all pablum to 99% of everyone reading this,
but 99% of everyone reading this will NOT instinctively
do the right thing in the heat of the moment.

Guaranteed.

A low-time pilot will consume a large percentage of
his brain (think CPU), simply operating an aircraft
during takeoff or landing. Student pilots are often
at 100%, especially during challenging conditions,
such as a crosswind, when they get overloaded
during the flare and forget about the sideslip, and
start moving downwind towards the downwind
side of the runway.

As a pilot gets more and more experience - there
is simply no substitute for practice, to burn the
electrochemical tracks in his brain - his percentage
of brain required to perform a particular task will
drop.

The excess brain power available will allow him to
think when things go wrong - assuming he does
not emotionally clank up due to stress. I call this
"deer in the headlights". Some people have an
emotional reaction to stress and they simply lock
up and sign off. I have experienced this during
inverted and accelerated spin training. Not a good
personality characteristic for a pilot. As an instructor
this is something that you have to carefully watch
for in a student.

The above sounds like a heap of steaming mumbo-
jumbo, but I might remind you of NASA's study of
airline accidents, which at the end concluded that
pilots that started to fly younger had fewer accidents,
presumably because they used less of their brain
to fly, and had more left over during an emergency
to figure out what was going on, and to do the
Right Thing (tm).

Sound familiar?

[CpnCrunch]

When I was doing my PPL my instructor would regularly cut the throttle at about 400 feet and expect me to immediately push the nose down and maintain 70 knots. It was very useful, as it makes it an automatic response - you don't need to think about it at all, just nose down, 70knots, land straight ahead.

[Cat Driver]

When I was doing my PPL my instructor would regularly cut the throttle at about 400 feet and expect me to immediately push the nose down and maintain 70 knots. It was very useful, as it makes it an automatic response - you don't need to think about it at all, just nose down, 70knots, land straight ahead.

You can do that after an engine failure at fifty feet climbing at 59 knots?

[pdw]

The 70kts was light and for training purposes only.

Wasn't 59kts only for gripping the climbrate early at V1 ... then gradually to 65kts by 50ft ?

[Shiny Side Up]

You can do that after an engine failure at fifty feet climbing at 59 knots?

Again, why the preoccupation with the engine failing? Nine times out of ten this is going to be disasterous anyhow if the engine fails in this phase of flight. How much safer is the pilot at 62 knots at this junction? 65 Knots? 70 knots? If the engine stops and you don't get the nose down here, the difference is really fractions of a second. Only the best trained are going to perform well here anyhow. Like the Colonel said, 99% of pilots are going to panic, pull back and write the climax of their personal story. This tendancy is unlikely to change unless the pilot does lots of practice, far past what either a PPL or a CPL equips them with. A good student who's had a good instructor will have a shot of doing the right thing just as long as the panic impulse doesn't set in. No matter the emergency, if someone panics, all is lost. No sense worrying about it because you won't generally know who will step up when the time comes and who will become a basketcase until that moment.

[Cat Driver]

This has come down to preferences it would seem.

I prefer to smoothly lift off and accelerate to a chosen climb speed in ground effect......usually above the normal recommended speed...it gets me where I am going faster and it is easier to see ahead and in hot weather gives better engine cooling.

But that is only my way of doing things.

[Beefitarian]

This has come down to preferences it would seem.

I prefer to smoothly lift off and accelerate to a chosen climb speed in ground effect......usually above the normal recommended speed...it gets me where I am going faster and it is easier to see ahead and in hot weather gives better engine cooling.

But that is only my way of doing things.

So more or less,

Cut down those trees or use a different airport.

yeah?

Because flying out of a place where your tires will touch trees and the only question is, "How hard?" is a bad decision.

[CFR]

This has come down to preferences it would seem.

I prefer to smoothly lift off and accelerate to a chosen climb speed in ground effect......usually above the normal recommended speed...it gets me where I am going faster and it is easier to see ahead and in hot weather gives better engine cooling.

But that is only my way of doing things.

Geez I was jut going to say that! I set the trim for takeoff (in my A/C it is slightly nose up with T/O flaps set) make sure the RPM is where I want it during the initial roll, check that the ASI is alive and climb into ground effect with the slightest of stall warning burps when it feels, sounds and looks (how far down the runway I am) right. Accelerate to climb speed (at exactly 77 kts see note below re airspeed needle) and climb out till safe altitude ... flaps up (slight thumb action on the electric trim to maintain climb speed ~80kts), fuel pump off, rest of post takeoff completed, and on my way!

IF I am with an instructor then I follow the book, raise the nose at EXACTLY 40kts and lift off at EXACTLY 53 kts with flaps or EXACTLY 59 kts without. I can do the airspeeds this accurately since I have sharpened my ASI needle to a fine point!

[Shiny Side Up]

This has come down to preferences it would seem.

No its not, don't play that game. Don't make it out like everyone else but you does something different on a regular basis. We're talking about practicing something in training that confers a skill we want the pilot to have. Just like spin recoveries, we're not doing them all the time for shits and giggles. The point of the whole obstacle take off to practice a lot of applicable things. Part of it is the whole PDM discussion on when to use and when not to use. Obstacles after all are rarely the textbook 50' tree at the end of the runway. Iflyforpie's home is a good example, depending on the wind and a lot of other factors. Is it dangerous to fly out of there because of that? Frig no, but sometimes you can do stuff to stack the deck in your favour.

Other things to think about. If you're really concerned with engines failing at low altitude all the time, climbing closer to your obstacle performance, really gives you a better chance of pulling off the "impossible turn." Again, something one might consider given one's circumstances. I'll assume that everyone here remembers the answer to the question that sometimes appears on the written test: What do you do if a birdstrike seems imminent? Hint: it might be useful for other things than clearing trees to know how far you can have that nose up.

[Beefitarian]

I don't think Cat was going the, "Don't learn and practice difficult techniques." But maybe he was. I just figured he was suggesting we try to not paint ourselves into a corner.

[CFR]

I don't think Cat was going the, "Don't learn and practice difficult techniques." But maybe he was. I just figured he was suggesting we try to not paint ourselves into a corner.

I agree with you Beef. Knowing HOW to do it is important but not as important as knowing WHEN to do it (or more likely when NOT to do it). Pulling out the POH and applying all the charts (temp, altitude, length of grass on runway, is it wet or dry, tailwind or headwind,etc) and arriving at the conclusion that you will clear the obstacle by 3 feet may not end well. I have used the charts on my A/C before flying INTO the field I tow gliders from. If the conclusion is wait for another day, I drive over.

[Cat Driver]

Note:::

I spent a life time teaching people how to fly and by the end of my career I was able to make a thousand dollars a day, therefore it stands to reason that I understood the importance of flying accurate attitudes / airspeeds.

When teaching manouvers that requred low airspeeds / high angle of attack I did it at a safe altitude for the lesson being taught.

Could I safely fly the airplane under any and all conditions / attitudes / airspeeds?

The people who did my annual re rides for my airdisplay authority to fly in the airshow circuit in Europe thought I could because they passed me for the eight years I flew the airshow circuit.

Like I said a while ago I do things the safest and easiest way but make sure I can perform to a high standard if a situation leaves me no choice.

Pretending there is an obstacle a thousand feet down a six thousand foot runway is not my style.

[Shiny Side Up]

Pretending there is an obstacle a thousand feet down a six thousand foot runway is not my style.

Fair enough, you can't expect that students have a good enough imagination for such stuff, and it seems silly anyhow. Playing lets pretend isn't really the point of it though. We don't need to have an obstacle to clear to make an excersise out of transitioning to a steeper rate of climb from take off. Of note as someone has saw fit to PM me about the 59 number is only for the airplane at max gross, it gets less the lighter the airplane is. What should become apparent to the student during the exersise is that regardless of the speed asked for, the nose should be in the same position in relation to the horizon during such a departure from the ashpalt/gravel/grass. If you do have said 6000' to play with to yourself its an interesting demonstration if on over rotation (but not banging the tail) that the airplane just won't leave the ground. It struggles against all that drag. Throttle off, stop backtrack, try it again. Again if you have the luxury of space and time. Done correctly she rotates, claws her way skyward and when you're done pretending with obstacles (or not) a smooth transition to a normal climb. I should say the 170 demonstrates this better since she crow hops as the poor little beast tries to get airborne, which usually freaks people out when they try to yank it off the ground too soon, to which of course they yank back and it gets worse. Let pressure off of the yoke (since it doesn't have a stick) keep straight and she'll pick up speed again, tail's up then evenly rotate to where you want it and off she goes.

[CpnCrunch]

When I was doing my PPL my instructor would regularly cut the throttle at about 400 feet and expect me to immediately push the nose down and maintain 70 knots. It was very useful, as it makes it an automatic response - you don't need to think about it at all, just nose down, 70knots, land straight ahead.

You can do that after an engine failure at fifty feet climbing at 59 knots?

Obviously you won't want to glide at 70 knots in that case (and hopefully even the most dimwitted student will realise that). The point is that putting the nose down to maintain speed is ingrained in the student's mind, so it becomes automatic. Now I haven't had an instructor do this with in over 10 years so who knows what I'll actually do if the engine does quit, but I think/hope I have a fairly good chance of doing the right thing because it has been etched pretty well into my brain from my initial training.

I should say that I've flown with 10 instructors since my PPL (7 of those in Canada) and not a single one of them has practised an EFATO with me, even when I had a fairly extensive flight review at a very picky flight school after not flying for 3 years. I'm not sure if it's because it's just something that isn't normally done outside PPL training, or whether it's just not done at all in Canada.