Fatal crash Arizona

At 11:49 07 May 2014, James Metcalfe wrote:
60 knots to 70 knots: minus 60 feet
sorry - s.b. minus 70 feet

On Wednesday, May 7, 2014 11:49:46 PM UTC+12, James Metcalfe wrote:
At 06:55 07 May 2014, Bruce Hoult wrote:
[1] handy formula: X knots of kinetic energy is worth (X/5)^2 feet of
gravitational potential energy. e.g. 70 knots = (70/5)^2 = 14^2 = 196 ft.
50 knots = (50/5)^2 = 10^2 = 100 ft. Less drag loses of course.
You'll never turn speed into quite that much height, and you'll need more
height than that to get speed. But the differences are large in a high
performance glider at moderate speeds.

..or you could use my rule of thumb: a change of speed of 10 knots IAS
gives
you (or costs you) the number of feet in height of the speed you arrive at.

For example:

70 knots to 60 knots : plus 60 feet
60 knots to 70 knots: minus 60 feet
70 knots to 50 knots (i.e. 70 to 60, then 60 to 50) : (60+50) = plus 110
feet

It's nearly the same formula. If you made it "every 12.5 knots IAS" instead of 10 then it'd be nearly exact.

The derivative of my formula (X/5)^2 i.e. X^2/25 is X/12.5. So gaining or losing X feet at X knots takes roughly 12.5 knots of speed change.

Doing it using the speed you're changing *to* is clever, as it makes some allowance for drag losses. However your height estimates are systematically biased 20% too large.

If you said 70 to 50 is (60+50)-20% = 110-22 = a gain of 88 ft then that would be good.
And 50 to 70 is (60+70)-20% = 130-26 = a loss of 104 ft, which is pretty good too.

If you're doing something like "how high can I zoom to after a 140 knot low pass and still have 60 knots for the circuit?" then I'm not convinced that adding up eight numbers is easier than calculating two squares and subtracting them :-)

By me: 28^2 - 12^2 = ~800 - ~150 = 650 ft (784-144 = 640 if you do it exact)
By you: 130+120+110+100+90+80+70+60 = 760 ft
By you with my -20% correction: 760 - 152 = 608

(remember folks, that's 140 knots at the END of the low pass, not the start!)

On 5/7/2014 2:55 AM, Bruce Hoult wrote:
On Wednesday, May 7, 2014 1:20:44 PM UTC+12, Bill D wrote:
I can assure you that the higher a glider's performance, the safer
it is. It's the old, low L/D gliders that can run out of altitude
before getting lined up with the runway.
I agree with you. I'm shaking my head every time I read this thread.

In a modern glass glider (such as the DG1000's I instruct in) with a
40 knot stall speed and being towed at 70 knots you should be able to
execute a safe 180º turn with*zero* loss of height.

The comparison isn't quite as simple as just looking at L/D. Turn
radius also has a lot to do with your chances of making it back to the
field, and turn radius is proportional to the SQUARE of airspeed.

Compare your example (40 knot stall) with a (horrors) 2-33. The highest
stall listed for a 2-33 is around 30 knots. If you do the math, you
will find that your DG1000 has nearly double the turn radius of the
slower glider.

Vaughn

....now redo your calculations while flying through 8 knots of sink.

Mike

On Wednesday, May 7, 2014 7:20:33 AM UTC-6, Vaughn wrote:
On 5/7/2014 2:55 AM, Bruce Hoult wrote:

On Wednesday, May 7, 2014 1:20:44 PM UTC+12, Bill D wrote:

I can assure you that the higher a glider's performance, the safer

it is. It's the old, low L/D gliders that can run out of altitude

before getting lined up with the runway.

I agree with you. I'm shaking my head every time I read this thread.



In a modern glass glider (such as the DG1000's I instruct in) with a

40 knot stall speed and being towed at 70 knots you should be able to

execute a safe 180� turn with*zero* loss of height.



The comparison isn't quite as simple as just looking at L/D. Turn

radius also has a lot to do with your chances of making it back to the

field, and turn radius is proportional to the SQUARE of airspeed.



Compare your example (40 knot stall) with a (horrors) 2-33. The highest

stall listed for a 2-33 is around 30 knots. If you do the math, you

will find that your DG1000 has nearly double the turn radius of the

slower glider.



Vaughn

You're exaggerating the stall speed differences. Regardless of what the 2-33 "manual" says, no 2-33 ever got as slow as 30 knots. 35 knots is a practical minimum speed. The flight test stall speed for a DG 1000 is 37 knots..

However, no one should consider a turn back at stall speed. Virtually all gliders will be at 50 - 55 knots so the turn radius will be essentially the same.

8 knots sink? That's a straw man argument. While extreme air movement is always possible, most PT3 incidents are in relatively benign conditions. In extreme conditions a turn back is probably moot anyway.

The point is when conditions allow, a pilot should know how to turn back safely.

On Thursday, May 8, 2014 1:20:33 AM UTC+12, Vaughn wrote:
The comparison isn't quite as simple as just looking at L/D. Turn
radius also has a lot to do with your chances of making it back to the
field, and turn radius is proportional to the SQUARE of airspeed.

Compare your example (40 knot stall) with a (horrors) 2-33. The highest
stall listed for a 2-33 is around 30 knots. If you do the math, you
will find that your DG1000 has nearly double the turn radius of the
slower glider.

I've done the math :-) With a 30 knot stall instead of 40, you'll want 41 knots for that 45º banked turn instead of 55 knots. And, yes, the radius will be about 56% as large, or about 65m instead of 115.

Whether that 100m difference laterally after the turn makes a difference is highly airfield dependent. On a wide field you can land straight in in either case. Somewhere else, it might put you on the wrong side of a row of trees.

In a typical place, I'd much rather be twice as far off the center line with twice the L/D :-)

On Tuesday, May 6, 2014 6:20:44 PM UTC-7, Bill D wrote:
On Tuesday, May 6, 2014 5:56:44 PM UTC-6, Don Johnstone wrote:

At 22:22 06 May 2014, Bill D wrote:



On Tuesday, May 6, 2014 2:41:23 PM UTC-6, Don Johnstone wrote:







Frankly I would be horrified to be required to conduct a turn back at



=20



200ft, I would suggest that this is one of those occasions where the



dang=



er



=20



of practice is to great to justify.







If you should check out in the USA, you'll be required to demonstrate



compe=



tence in this maneuver. Every pre-solo student is required to do so and



mo=



re than a half century of safety records do not suggest a problem. In



fact=



, even with low performance gliders, there's quite a large safety margin.



T=



he most likely outcome is a pilot will find the glider uncomfortably high



f=



or a downwind landing requiring full spoilers and a slip.







The logic is simple - it's better to have pilots trained for the option.



N=



o one says a pilot is required to turn back or that 200' is always



adequate=



to do so. What is illogical is to suggest a pilot be required to crash



in=



unlandable terrain when a safe option exists to land on the departure



runw=



ay.







What are you trying to save? The pilot or the aircraft? The priority should



be survival of the soft bit, that is you and me.



As an instructor with nearly 50 years experience I know that when I



initiate an emergency procedure I do so allowing a margin to ensure my



survival if it does not work out, I have been bold but never certifiable.



Most living instructors have the same survival instinct. That is why I have



lived long enough to do 10,000 launches, and of course landings. It has



already been hinted that the practice you describe involves modifying what



you normally do, in my view that probably makes it pretty useless and not



real preparation for the event. If you did carry out the training in



exactly the same way as the possible real event you might find that the



results were very different, not to mention painful. I will stick with my



300ft thank you, I know it works. Low turns, below that height may have



been acceptable in old wooden gliders, the minimum height in T31 and T21



gliders was 150ft, but for modern glass gliders it is just far too low, you



only have to look at the accident statistics to see that low final turns



figure to a large degree in accidents so why plan for it?



I repeat a controlled descent with wings level is far more likely to have a



better result than hitting the ground in a turn or even worse spinning in



trying to avoid it.







PS Despite all that there have been times when I have initiated a practice



emergency and very quickly wished I had not, no plan survives first



contact.



So, you're saying the pilot will be safer if they don't learn to perform the return to runway maneuver when it's safe to do so?



I can assure you that the higher a glider's performance, the safer it is. It's the old, low L/D gliders that can run out of altitude before getting lined up with the runway.

The discussion seems to focus exclusively on the decision height for a turn around. I think that is only one factor in making this decision. As reported by Bob T there was heavy sinking air on the departure end of the runway. Returning to the airport would have required transiting thru this air a 2nd time, which strikes me as inadvisable without much more altitude than Knauff had.

The other issue is that a tow rope break requires immediate lowering of the nose. This is done routinely at altitude, but at low altitude this means pointing the glider's nose uncomfortably down at the ground while executing a steep banked turn. If the ground is rising, as it is at Sampley, the picture seen by the pilot is even more disturbing. All that it takes is a momentary hesitation in this reflex and the outcome can be fatal.

As an aside, I once did a wind mill start in my DG400 below 1000' (over a runway). This maneuver requires achieving in excess of 90 kt airspeed. Because the engine & prop act like dive brakes, you feel like you are standing on your rudder pedals when you do this close to the ground. I got to this airspeed and the prop still didn't rotate. This meant that I had to steepen the descent even more. All of my instincts said no, but my brain said yes, which is what I did. The engine started, but I decided that this maneuver really needs to be started at a higher altitude.

On Thursday, May 8, 2014 6:39:06 PM UTC-6, 2G wrote:

The other issue is that a tow rope break requires immediate lowering of the nose.

Why would anyone lower the nose? The glider is presumably at aero tow speed - 65 - 70 knots which is way above the pattern speed. A better plan is to use the excess airspeed to maintain height while turning until airspeed drops to pattern speed.


This is done routinely at altitude, but at low altitude this means pointing the glider's nose uncomfortably down at the ground while executing a steep banked turn. If the ground is rising, as it is at Sampley, the picture seen by the pilot is even more disturbing. All that it takes is a momentary hesitation in this reflex and the outcome can be fatal.

The nose is not "uncomfortably down". The turn back is a normal turn.



As an aside, I once did a wind mill start in my DG400 below 1000' (over a runway). This maneuver requires achieving in excess of 90 kt airspeed. Because the engine & prop act like dive brakes, you feel like you are standing on your rudder pedals when you do this close to the ground. I got to this airspeed and the prop still didn't rotate. This meant that I had to steepen the descent even more. All of my instincts said no, but my brain said yes, which is what I did. The engine started, but I decided that this maneuver really needs to be started at a higher altitude.

Aha! You're really a motor glider pilot, not an aero tow pilot which explains your misconceptions.

On Thursday, May 8, 2014 8:55:27 PM UTC-4, Bill D wrote:

Why would anyone lower the nose? The glider is presumably at aero tow speed - 65 - 70 knots which is way above the pattern speed.

Perhaps I have a fundamental misunderstanding...
but I thought that the AofA at 65 knots on aerotow is steeper than the AofA at 65 knots in free flight. So if you don't reduce the AofA (aka drop the nose) after the rope breaks, the glider will slow down. If you start the turn before reducing the AofA, you may find yourself going too slow for the turn.

It is true that you can use the speed coming off aerotow or PTOT to gain a bit of altitude, but that just means lowering the nose gradually as you bleed off the speed. In both cases the AofA needs to be adjusted to match the desired free flight speed.

I thought that we practiced this 'gain altitude and slowly drop the nose' every time we release from aerotow?

On Thursday, May 8, 2014 5:55:27 PM UTC-7, Bill D wrote:
On Thursday, May 8, 2014 6:39:06 PM UTC-6, 2G wrote:



The other issue is that a tow rope break requires immediate lowering of the nose.



Why would anyone lower the nose? The glider is presumably at aero tow speed - 65 - 70 knots which is way above the pattern speed. A better plan is to use the excess airspeed to maintain height while turning until airspeed drops to pattern speed.





This is done routinely at altitude, but at low altitude this means pointing the glider's nose uncomfortably down at the ground while executing a steep banked turn. If the ground is rising, as it is at Sampley, the picture seen by the pilot is even more disturbing. All that it takes is a momentary hesitation in this reflex and the outcome can be fatal.



The nose is not "uncomfortably down". The turn back is a normal turn.







As an aside, I once did a wind mill start in my DG400 below 1000' (over a runway). This maneuver requires achieving in excess of 90 kt airspeed. Because the engine & prop act like dive brakes, you feel like you are standing on your rudder pedals when you do this close to the ground. I got to this airspeed and the prop still didn't rotate. This meant that I had to steepen the descent even more. All of my instincts said no, but my brain said yes, which is what I did. The engine started, but I decided that this maneuver really needs to be started at a higher altitude.



Aha! You're really a motor glider pilot, not an aero tow pilot which explains your misconceptions.

PLEASE stop with the sanctimonious crap! I have done hundreds of aerotows. You need to lower the nose because you are in a climb attitude and need to transition to a glide attitude. On your next tow note where the horizon is on the canopy and compare it to where it is after release at the same airspeed.

Tom