I owe my life to my CFI.

[Let'sgoflying!]

Here is something I read on one CFIs personal training website:
(seems the prop makes a difference!)

"What is effect of density altitude on Vx and Vy indicated airspeeds?
For a fixed pitch propeller airplane, as density altitude increases, Vx remains constant and Vy decreases.
For a constant-speed propeller airplane, same conditions, Vx increases and Vy decreases."

he also states that the same ias is used for takeoff regardless of DA.
http://whitts.alioth.net/Pageb11About Aircraft Speeds.htm#anchor951682

 

[Ed Guthrie]

That is what I have been taught by a million people and in all the readings available to me.....and its also what I mentioned earlier in the thread. Maybe I am seeing it wrong but it seems like some here are saying you need to adjust the IAS you use for each DA. Or maybe the difference is truly beyond what an average pilot is capable ie 5kts for most a/c, most DA differences -- and we are down to splitting gnats whiskers. I eagerly await the responses.

I believe (hope?) the DA influenced IAS adjustment suggestions in this thread are in reference to Vx or Vy on takeoff. IOW, on takeoff you would use as an IAS either Vx or Vy appropriately computed for the DA and the aircraft configuration (gear down or gear up); when operating at elevated DA you would not further adjust IAS after computing the appropriate book Vx or Vy IAS for that DA.

 

[bbchien]

You believe correctly, at least in my case.

 

[dogman]

Dean We should fly out together and get a mountain flying checkout. I live North of KC MO.

THIS IS SOMETHING I HAVE BEEN WANTING TO DO!

 

[SkyHog]

Here is something I read on one CFIs personal training website:
(seems the prop makes a difference!)

"What is effect of density altitude on Vx and Vy indicated airspeeds?
For a fixed pitch propeller airplane, as density altitude increases, Vx remains constant and Vy decreases.
For a constant-speed propeller airplane, same conditions, Vx increases and Vy decreases."

he also states that the same ias is used for takeoff regardless of DA.
http://whitts.alioth.net/Pageb11About Aircraft Speeds.htm#anchor951682

Hmm...that doesn't sound right to me. As far as I've always known - Vx and Vy meet each other with altitude increase. This was taught to me in a fixed pitch prop airplane (C172). Vx comes up and Vy goes down. I've never heard (nor does the POH indicate) that Vx stays constant, and Vy decreases to meet it.

I could be reading the POH incorrectly...

 

[Dean]

Dean We should fly out together and get a mountain flying checkout. I live North of KC MO.

THIS IS SOMETHING I HAVE BEEN WANTING TO DO!

Sounds good to me, lets set a date, October is a good month for me.

 

[dogman]

I have e-mailed 3 instructors in the Denver, Castle Rock CO area that do the High DA training we will see whats what.

 

[judypilot]

Ron,

I'm a bit puzzled by your comments about pitch angle. As someone who has done 95% of her flying at high DAs (>5000', and often much higher), I know that on the rare occasions I get to take off at DAs around sea level, my airplane sure SEEMS to climb at a higher pitch angle at any given airspeed. Maybe it's an illusion because I'm pulling away from the ground so much faster, but I don't think so. I have also found that my rotation speed should be (and when I say should be, I mean this is where I get the best performance) about 60 mph regardless of DA.

Judy

 

[judypilot]

Dean,

We all owe our lives to our CFIs. But actually, you can't teach what someone doesn't want to learn. You wanted to learn to be safe, and you learned what you needed to do. So you owe your life to yourself.

One other query, and please don't take this wrong. Do you use your and your wife's ACTUAL weights? One of the biggest concerns I have nowadays is all the people who estimate, and I'm sure they're estimating low. I know guys who weigh upwards of 240 lbs, to look at them, who swear they're only 200 or so. And women's clothing sizes are getting bigger (I used to be a 14 and now I wear 12s or even 10s, but my weight is the same or sometimes more) so that they can kid themselves into thinking they haven't gained weight. I'm very tough on prospective pax; I want to know what they actually weigh with clothes and I've been known to add some pounds if I even suspect they're not being forthcoming. I even do something I won't do in any other way: I try to scare them a little bit into telling me the truth (I do it in a nice way, but do it nevertheless). A C150 doesn't carry very much. Even at my lightest weight, and even though my instructor was a slender young guy, I know that with full fuel, that was about it.

Judy

 

[bbchien]

Ron,

I'm a bit puzzled by your comments about pitch angle. As someone who has done 95% of her flying at high DAs (>5000', and often much higher), I know that on the rare occasions I get to take off at DAs around sea level, my airplane sure SEEMS to climb at a higher pitch angle at any given airspeed. Maybe it's an illusion because I'm pulling away from the ground so much faster, but I don't think so. I have also found that my rotation speed should be (and when I say should be, I mean this is where I get the best performance) about 60 mph regardless of DA.

Judy

Judy, the problem is that what we're looking for is the best angle of attack (for which Vx at full gross weight is a but proxy). If you're climbing, the incident airflow comes a bit from ahead up to behind and low. If you don't have enough power to climb, the percieved same angle of attack is actually greater as the incident airstream is actually coming at you level, with nose in the up position.

I miss AOA indicators. That's why military guys simply choose best AOA and all power available....and say a short prayer as they try to remember the ejection procedure.

 

[judypilot]

Judy, the problem is that what we're looking for is the best angle of attack (for which Vx at full gross weight is a but proxy). If you're climbing, the incident airflow comes a bit from ahead up to behind and low. If you don't have enough power to climb, the percieved same angle of attack is actually greater as the incident airstream is actually coming at you level, with nose in the up position.

I miss AOA indicators. That's why military guys simply choose best AOA and all power available....and say a short prayer as they try to remember the ejection procedure.

OK, I think I understand where you guys are coming from on this, if I think about what's happening at, say, 13,000'. I hadn't really looked at it quite that way. I guess my problem is that I fly at low DAs so seldom that I really don't have a good perceptual comparison; my perceptual comparison is between 5000' and 13,000'!

I actually have a kind of AOA indicator, and I've noticed that on climbout it's right where it should be if I use the airspeed that I use. I happen to be at sea level now, so I'm going to pay attention when I take off tomorrow. Well, that's not a fair comparison. I have to stay below 1500' at takeoff tomorrow, and I already know from experience that I have to reduce my climb power very quickly after takeoff at this airport or I'll bust into the airspace I want to avoid, so I won't really have time to do the test-pilot observation. My airplane is a rocket at sea level. B)

Judy

 

[Bob Bement]

If you two can get out here to Vale, Or. We could go over into the Idaho Back country. It is possible to get into some high DA in some of those strips. For instance Big Creek and Sulfur Creek. Both over 5700' tall. Mountains run up to 10,000. I would be glad to show you around.

 

[Ed Guthrie]

Judy, the problem is that what we're looking for is the best angle of attack (for which Vx at full gross weight is a but proxy). If you're climbing, the incident airflow comes a bit from ahead up to behind and low. If you don't have enough power to climb, the percieved same angle of attack is actually greater as the incident airstream is actually coming at you level, with nose in the up position.

You are correct that the AOA is identical in both cases, which means that actually it is the other way around--the pitch angle is greater at SL than at high DA. At SL the climb rate inclines the angle of the relative wind. IOW, the angle of the relative wind is increased by arctan(climb rate(fpm)/climb speed(fpm)); the relative wind inclination angle increases as the climb rate increases.

 

[bbchien]

You are correct that the AOA is identical in both cases, which means that actually it is the other way around--the pitch angle is greater at SL than at high DA. At SL the climb rate inclines the angle of the relative wind. IOW, the angle of the relative wind is increased by arctan(climb rate(fpm)/climb speed(fpm)); the relative wind inclination angle increases as the climb rate increases.

Hmmmn. I read my post that way. Oh well. If you use the same eyeball AOA during conditions when you don't have enough power to climb, you are actually using a steeper AOA than optimal.

 

[Dean]

Dean,

We all owe our lives to our CFIs. But actually, you can't teach what someone doesn't want to learn. You wanted to learn to be safe, and you learned what you needed to do. So you owe your life to yourself.

One other query, and please don't take this wrong. Do you use your and your wife's ACTUAL weights? One of the biggest concerns I have nowadays is all the people who estimate, and I'm sure they're estimating low. I know guys who weigh upwards of 240 lbs, to look at them, who swear they're only 200 or so. And women's clothing sizes are getting bigger (I used to be a 14 and now I wear 12s or even 10s, but my weight is the same or sometimes more) so that they can kid themselves into thinking they haven't gained weight. I'm very tough on prospective pax; I want to know what they actually weigh with clothes and I've been known to add some pounds if I even suspect they're not being forthcoming. I even do something I won't do in any other way: I try to scare them a little bit into telling me the truth (I do it in a nice way, but do it nevertheless). A C150 doesn't carry very much. Even at my lightest weight, and even though my instructor was a slender young guy, I know that with full fuel, that was about it.

Judy

I make my wife step on the scales and I take the reading myself, I know she would not lie to me about her weight( plane that is) HA HA! She knows the risk of being over weight, so she has no problem with me looking. Her biggest obstacle is packing light, she wants to take the master bath with her when she travels.

 

[Ed Guthrie]

Hmmmn. I read my post that way.

I reread your previous post and I see that you did indeed.

Never mind...

 

[poadeleted20]

Ron, you should remove your blanket advice above and stick to advising folks to read and follow the POH. For the record, the 1979 C152 & 172N POHs disagree in several areas wth your advice in this thread (well, not the "read the POH" advice), as does the Piper PA28-161 Cherokee Warrior information manual, as does the 1977 Mooney M20J POH, as does I will speculate a goodly number of other POHs.

Ed's right for aircraft that call for different configurations (e.g., more flap) for the short-field takeoff. On most planes, use of more flaps reduces the rotation and liftoff speeds.

 

[poadeleted20]

Here is something I read on one CFIs personal training website:
"What is effect of density altitude on Vx and Vy indicated airspeeds?
For a fixed pitch propeller airplane, as density altitude increases, Vx remains constant and Vy decreases."

That's not true, starting with the Tiger, whose Vx goes from 70 KIAS at SL to 73 KIAS at absolute ceiling.

"For a constant-speed propeller airplane, same conditions, Vx increases and Vy decreases."

That, I'll buy.

he also states that the same ias is used for takeoff regardless of DA.

Again, he's wrong for max performance takeoffs, since Vx and Vy both change with DA.

 

[judypilot]

I make my wife step on the scales and I take the reading myself, I know she would not lie to me about her weight( plane that is) HA HA! She knows the risk of being over weight, so she has no problem with me looking. Her biggest obstacle is packing light, she wants to take the master bath with her when she travels.

And what about you?

Just kidding. Good for you.

Judy

 

[judypilot]

I reread your previous post and I see that you did indeed.

Never mind...

I read it the same way you did, Ed. So my perception may not be wrong after all.

Judy

 

[bbchien]

I read it the same way you did, Ed. So my perception may not be wrong after all.

Judy

I think your perception is CORRECT, Judy. We can't compenate for relative wind with our eyeballs. I think that is what gets aviators at high DAlt in trouble....and pilots carrying heavy loads (same effect).

 

[Ed Guthrie]

Ed's right for aircraft that call for different configurations (e.g., more flap) for the short-field takeoff. On most planes, use of more flaps reduces the rotation and liftoff speeds.

Ron, the discrepancies between your blanket advice and the reality are not limited to aircraft which change flap configuration between normal takeoff and short field/obstructed takeoff. For example, using identical flap configuration for either takeoff (15 degrees) the '77 Mooney M20J recommends identical rotation speed (62 kts) for both normal and short field/obstructed takeoff. This is in direct contradiction with your statement that all aircraft recommend an increased rotation speed for short field/obstructed takeoffs. Another discrepancy exists with your advice that short field/obstructed takeoff procedures will recommend zero AOA until rotation. The 1979 Cessna 152 and 172N POHs both recommend "Elevator control -- SLIGHTLY TAIL LOW" for short field/obstructed takeoff versus the Cessna recommendation for a normal takeoff which is a specified rotation speed. For example, the C172N POH normal takeoff procedure states: "Elevator Control - LIFT NOSE WHEEL (at 55 KIAS)". Clearly the POH recommendation "SLIGHTLY TAIL LOW" contradicts your blanket recommendation that a pilot should maintain no up elevator until rotation speed for a short field/obstructed takeoff.

As I wrote previously, I believe you should remove your blanket recommendations and stick to your previous advice--read the POH.

 

[gkainz]

Dean We should fly out together and get a mountain flying checkout. I live North of KC MO.

THIS IS SOMETHING I HAVE BEEN WANTING TO DO!

Mountain flying training is offered by the Colorado Pilots Association at http://www.coloradopilots.org/ and click on Mountain Flying
also check out Specialty Flight Training at Erie at http://www.specialtyflight.com or http://www.specialtyflight.com/mtn.htm

Lonnie - http://www.specialtyflight.com/contact.htm#CFIs (scroll to bottom) is an excellent mountain flying instructor.

 

[Let'sgoflying!]

That's not true, starting with the Tiger, whose Vx goes from 70 KIAS at SL to 73 KIAS at absolute ceiling.

Ahh so maybe we are splitting mosquito nose-hairs... three knots.
I bet thats why the rule of thumb has been; "just fly it as normal, ya got enough to worry about" (for most light singles).
I won't say three knots is a theoretical difference but for the average pilot it may well be outside the realm of practical, especially if we are talking about a 4-5000' change in DA which may mean one knot)
Maybe other a/c have a greater range of Vx?

 

[Dave Krall CFII]

If you two can get out here to Vale, Or. We could go over into the Idaho Back country. It is possible to get into some high DA in some of those strips. For instance Big Creek and Sulfur Creek. Both over 5700' tall. Mountains run up to 10,000. I would be glad to show you around.

Been going into Big Creek for many years because it is so perfect for most GA planes; off load camping gear and/or PAX and go lightly loaded to the others nearby that are more demanding in all ways.

After all those years flying in and out of Big Creek with nothing really remarkable noted, I just recently went around TWICE at this long, 3000 foot strip that I never considered much of a challenge (Big Creek, ID elv. ~5000+FT) and both go arounds were after slipping with full rudder & nose down, even after we were on a stabilized, 60 knot short final approach !

Turns out there was quite a down draft at about that last 1/4 mile from threshold. Even the light, powerful Super Cubs along with our flight sat up and took notice on their LDGs, so I didn't question in hindsight my own safe decision to abort the forward slips and go around before there was no room left to maneuver in the high mountain vally with not only high DA, but only an anemic, fully loaded SkyHawk to battle it with !

My student & I learned later at a freshly grilled dinner at the historic, antique filled lodge that the same downdraft was instrumental in the recent death of the previous lodge owner. I dare say he had a fair amount of experience with Big Creek before he took off into that last downdraft.

We waited until just before dark when the variable (often a light tailwind) winds calmed down some and took off without incident in the heavy SkyHawk but, were prepared to camp the night awaiting the usually calm morning air if the winds had not improved.

I'm wondering if the recent burn-offs of forest greenery in big areas under the approach path to Big Creek have somehow changed the thermodynamics of the area to increase the downdraft.

Sulphur Creek looks relatively unobstructed, and so it is but, a recent takeoff saw the pilot unable to turn his aircraft in the fairly wide valley to gain altitude out of it.

 

[gismo]

More clearance is usually better but, 200 feet's not too bad usually.

A lot of times if I've got my critical airspeed by 1/2 down the runway then I pull AoA to fly as close to the obstacle as is safe and what ever airspeed/energy there may be in excess, is still available for downdrafts. If there is no excess airspeed/energy going over the obstacle, well then that would have been a close one.

I believe this actually reduces your safety margins if you decrease the AoA below what's necessary for Vy. Once you become airborne, Vy converts the greatest percent of your available power to altitude (potential energy). Flying any faster exports more of that power in to the kenetic energy of heat. Vx is a speed that also sacrifices some energy in return for a steeper climb and as you have surmised, you are better off going for Vy as soon as obstacle clearance is assured.

 

[Dave Krall CFII]

I believe this actually reduces your safety margins if you decrease the AoA below what's necessary for Vy. Once you become airborne, Vy converts the greatest percent of your available power to altitude (potential energy). Flying any faster exports more of that power in to the kenetic energy of heat. Vx is a speed that also sacrifices some energy in return for a steeper climb and as you have surmised, you are better off going for Vy as soon as obstacle clearance is assured.

You're probably right on that.

Now I'm very curious and I'd like to know about how many ergs would be lost to the kilocalories generated by increasing say, a Cessna 206's speed by say, 20 knots ?

 

[gismo]

That is what I have been taught by a million people and in all the readings available to me.....and its also what I mentioned earlier in the thread. Maybe I am seeing it wrong but it seems like some here are saying you need to adjust the IAS you use for each DA. Or maybe the difference is truly beyond what an average pilot is capable ie 5kts for most a/c, most DA differences -- and we are down to splitting gnats whiskers. I eagerly await the responses.

IAS itself doesn't need to change for DA when looking for the same AoA and aerodynamic performance of the wing(s). The issue with Vy and Vx vs DA is that both of these result from a combination of the aerodynamics of the wing and the thrust from the engine/propeller and therefore require that the AoA itself be adjusted for DA. OTOH, as somebody pointed out, the change in Vx with DA is usually small in airplanes with fixed pitch props.

 

[gismo]

Because the pitch attitude when climbing out at 800 fpm will be significantly steeper than climbing out at 250 fpm (both at the same IAS, the shallow climb of course being at high DA).

Jeff

I gotta go with Jeff on this one, even if the AoA is the same, the pitch attitude goes down with the climb angle and climb angle suffers dramatically at high DA due to both a loss of HP and a higher TAS for the same IAS.

 

[gismo]

Thanks to all that pointed out things that I did wrong, and those that pointed out what I did right. There is nothing better for a low time pilot like myself than hearing from the experienced ones here at POA. Your words of advise are helpful and taken to heart.

As evidenced by your being alive to make the post, you indeed did well (you passed the test). Every pilot makes mistakes, the good ones maintain their cool and apply the fundamentals (push the nose down when too slow no matter what) like you did. The best ones also learn from those mistakes. Guess which category you're in.

 

[gismo]

I make my wife step on the scales and I take the reading myself, I know she would not lie to me about her weight( plane that is) HA HA! She knows the risk of being over weight, so she has no problem with me looking. Her biggest obstacle is packing light, she wants to take the master bath with her when she travels.

Even when you "know" all the occupant's weights, there are often several items that get overlooked. If you add up the combined weight of the couple quarts of oil, tiedowns, towbar, POH (even the W&B list), flashlight(s), portable GPS, windshield cleaning supplies, fuel sampler, misc. tools and spare parts, and the lunch/snacks + bottles of water, you may find you're carrying an extra 10-20 pounds that never seem to get included in the W&B calculation. Then there's the extra weight the plane itself has gained over the years unless it was recently weighed. As Bruce and others posted, more power is the only answer if you must carry the load, but regardless of how much power you have, high DA (like short field length) is an environment that demands you operate well below the published max gross weight if you want decent safety margins.

 

[gismo]

You're probably right on that.

Now I'm very curious and I'd like to know about how many ergs would be lost to the kilocalories generated by increasing say, a Cessna 206's speed by say, 20 knots ?

More than I could afford if the 206 was heavy and the trees were tall?

BTW, in retrospect, I should have added that once you do encounter a downdraft you may well do better to fly at a speed above Vy as that should get you through the descending air sooner. Nothing's ever easy.

 

[waldo]

I make my wife step on the scales and I take the reading myself, I know she would not lie to me about her weight( plane that is) HA HA! She knows the risk of being over weight, so she has no problem with me looking. Her biggest obstacle is packing light, she wants to take the master bath with her when she travels.

I know the feeling. My wife and I went through the same thing. The Cessna 150 is such a sweet little plane. It was VERY cheap to operate. I had the '76 with a new engine and IFR rated. I had the jump seat in the back for my 8 year old. It was either me and the wife OR me and my 12 and 8 y/o. We were always near weight limits. Even with the new engine she would only climb out 200-250 fpm here in South Louisiana during the summer. I had a few "not so comfortable" calls myself.

My wife was much like yours. . . . Get a larger plane before you get us killed It wasn't quite THAT bad but was getting close. I went with the Piper Cherokee 180. It is relatively inexpensive and you get enough room for 4 normal sized adults or a family or 4 with baggage. We have made several week long trips with baggage and full of fuel. Even within 25 lbs of maximum it performed well all the way up to 9500'.

Thanks for the story

 

[Dave Krall CFII]

More than I could afford if the 206 was heavy and the trees were tall?

BTW, in retrospect, I should have added that once you do encounter a downdraft you may well do better to fly at a speed above Vy as that should get you through the descending air sooner. Nothing's ever easy.

Well then, tell us what you can afford... ;>)

That's the realm we're often flying in for these high DA ops. With the heavy C206 above, or most anything else in airplanes, AND the tall trees, accelerating beyond Vy isn't an option, because of terrible climb rates in general and needing to clear the obstacle.

Lot's of times in high DA the pilot is fortunate to have the option of any additional speed at all above Vx.

 

[gismo]

That's the realm we're often flying in for these high DA ops. With the heavy C206 above, or most anything else in airplanes, AND the tall trees, accelerating beyond Vy isn't an option, because of terrible climb rates in general and needing to clear the obstacle.

Lot's of times in high DA the pilot is fortunate to have the option of any additional speed at all above Vx.

Understood, but remember if your goal is to stay aloft (vs outclimb terrain or obstacles) Vy (corrected for DA etc) is the speed to fly, not Vx. If you have the room to descend a bit, you will soon outclimb an airplane that remains at Vx if you pitch down to reach Vy and then hold that. Going faster than Vy to escape a downdraft is better than holding Vy under a very narrow range of situations if you are near the ground, but always wins if you are already well clear of terrain.

 

[poadeleted20]

Keep in mind that lowering the nose to pick up speed above Vy while maintaining a flight path that clears the obstacle actually increases your total energy budget with a fixed pitch prop. Remember that total energy equals potential (weight times height) plus kinetic (1/2 mass times velocity squared). Extra speed lowers prop drag, allowing the engine to spin up to a higher RPM and generate more power. Even with a CS prop, the increased airspeed at full power allows the prop to take a deeper bite of the air at the same RPM.

True, you won't clear the obstacle by as much as if you flew Vy all the way, but your TOTAL energy (kinetic plus potential) will be higher when you get to that point over the ground, leaving you more options if the engine quits at that point, the extra kinetic energy being convertible into more potential energy by zooming for altitude.

 

[Dave Krall CFII]

Keep in mind that lowering the nose to pick up speed above Vy while maintaining a flight path that clears the obstacle actually increases your total energy budget with a fixed pitch prop. Remember that total energy equals potential (weight times height) plus kinetic (1/2 mass times velocity squared). Extra speed lowers prop drag, allowing the engine to spin up to a higher RPM and generate more power. Even with a CS prop, the increased airspeed at full power allows the prop to take a deeper bite of the air at the same RPM.

True, you won't clear the obstacle by as much as if you flew Vy all the way, but your TOTAL energy (kinetic plus potential) will be higher when you get to that point over the ground, leaving you more options if the engine quits at that point, the extra kinetic energy being convertible into more potential energy by zooming for altitude.

Another preflight task so routinely basic to high DA takeoffs in really critical performance realms that I forgot to mention it, is adjusting CG fore/aft within the approved envelope so that minimal control inputs are required to achieve the desired AoA, as well as adjusting CG right/left (ie: balance/unbalance fuel & PAX/luggage) so that aerodynamic energy is not wasted correcting with control surface's drag for any significantly undesireable CG, no matter how small.

 

[Richard]

Keep in mind that lowering the nose to pick up speed above Vy while maintaining a flight path that clears the obstacle actually increases your total energy budget with a fixed pitch prop. Remember that total energy equals potential (weight times height) plus kinetic (1/2 mass times velocity squared). Extra speed lowers prop drag, allowing the engine to spin up to a higher RPM and generate more power. Even with a CS prop, the increased airspeed at full power allows the prop to take a deeper bite of the air at the same RPM.

True, you won't clear the obstacle by as much as if you flew Vy all the way, but your TOTAL energy (kinetic plus potential) will be higher when you get to that point over the ground, leaving you more options if the engine quits at that point, the extra kinetic energy being convertible into more potential energy by zooming for altitude.

That would explain why you often saw underpowered planes fly straight for the trees, pull up, and drop back down to disappear, only to reappear above the next row. Repeat as required until adequate climb speed can be maintained. "Hedge hopping", I think they called it.

 

[Ed Guthrie]

That would explain why you often saw underpowered planes fly straight for the trees, pull up, and drop back down to disappear, only to reappear above the next row. Repeat as required until adequate climb speed can be maintained. "Hedge hopping", I think they called it.

I had a failry savvy sailplane pilot tell the that when clearance of an obstacle is in doubt I should aim at the top of the obstacle. If while doing so the airspeed climbs above Vx (or Vy) I have beau coup excess energy available. OTOH, if the airspeed bleeds below Vx the aircraft will not clear the obstacle, no how, no way, and it is time to make another plan--quickly.

 

[Dave Krall CFII]

I had a failry savvy sailplane pilot tell the that when clearance of an obstacle is in doubt I should aim at the top of the obstacle. If while doing so the airspeed climbs above Vx (or Vy) I have beau coup excess energy available. OTOH, if the airspeed bleeds below Vx the aircraft will not clear the obstacle, no how, no way, and it is time to make another plan--quickly.

For sailplanes, gliders, ie: engine failure, dropping below Vx while heading for obstacle top might mean make another plan but, not necessarily so with engine making power.