Mountain Wave Question

[VH-FTS]

I'm hoping someone can answer the following question. If you are caught in the downdraft of a mountain wave, are you better off using your best rate of climb speed or best angle of climb?

Best rate - most height in given time
Best angle - most height in given distance

I just can't put two and two together with this one.

Thanks guys.:ok:

[cficare]

Most aircraft in the GA community dont have the performance to beat a mountain wave downdraft.
The best thing is to work out how to get out of the downdraft.

[Monopole]

Most aircraft in the GA community dont have the performance to beat a mountain wave downdraft.

Depends of course on the serverity of the downdraft.

Best rate - most height in given time
Best angle - most height in given distance
It's a similar argument to blue or red line speed in a twin with an engine failure. My personal opinion is that if you have no other choice but to try and out climb it, and you are going down, use what ever attitude will give you the better RoC.

Pegging a speed that is not giving you the performance you are requiring is just plain silly in my opinion.

[karrank]

My personal opinion is that if you have no other choice but to try and out climb it, and you are going down, use what ever attitude will give you the better RoC. This makes as much sense to me as experimenting with different airspeeds in an engine failure coz you'd have to be completely anal to do what you have been taught and practiced in a REAL failure!

Sparky's 10 commandments (http://www.mountainflying.com/Errata/002%2010cmdmts%20Shirt%20Pocket.pdf) Say that if terrain is a problem you should not put yourself in the situation where the difference is vital. If you start going down turn away.

Elsewhere it says:If you encounter unexpected downdrafts, diving–away from the visualized downdraft–to maintain airspeed will generally lessen the total displacement effect of the downdraft (altitude loss). Although the rate of descent is greater at the higher airspeed, you will be under the influence of the sink for a shorter period of time. (as CFICARE suggests).

So to answer your question I would suggest that:
:uhoh: Don't put yourself in the situation near terrain, or leave an escape route OTHER than climbing through it.
:uhoh: If you ignored Sparky and there is a hill in front BAOC.
:uhoh: If there is nothing in the way except an unwanted trend of down-ness, do whatever.

'Course this is just stuff I googled, I have no mountain flying qualifications to spruik here...

[RENURPP]

It's a similar argument to blue or red line speed in a twin with an engine failure.
I am not familiar with this argument, would you mind giving me a brief outline on why one would fly at red line after an engine failure.

Thanks

[Monopole]

Ok.I thought I might open a can of worms here, and for that I am sorry :{ .
I'll see if I can explain better what I was trying to get at.

Best rate and best angle are determined by excess power and excess thrust. It is not a set speed (although the flight manual may, and usually will in lighter aircraft, stipulate a speed). They are also effected by aircraft weight and enviromental factors (temp, humidity ect).
The writting is on the wall as far as i'm concerned. When we are heavy we will climb out at a lesser RoC then when we are lighter (no-one should disagree with this bit at least) because we do not have as much excess power.
So considering 'blue line' is a best rate speed, and 'red line' is best angle speed, why must you climb at those speeds if you are lighter. Larger a/c reduce their V1, Rotate and V2 speeds depending on a/c weight. Why should a little C310 driver accept a negitive rate of climb just because he's a guru pilot and has pegged 100 kts (or whatever it is for them) when 95 kts for example may give him a positive RoC.

As for Red Line, people are scared of it because it is often close to Vmca. Well, if I had an engine failure on TKOF and there was an obsticle in my way and turning was out of the question, well then yes, as best angle RoC I will fly it.
So Karrank you'd have to be completely anal to do what you have been taught and practiced in a REAL failure I agree. What you get taught is not always the best way to do it in real life (although I dont think Karrank was agreeing with me)

As for the mountain waves. See above. If you are bashing along in a C172 and Best Rate is say 68 kts, but 65 gave you an extra 100 fpm RoC, you would be a fool in my opinion to 'PEG' the 68 kts.

[RENURPP]

Sorry to go off subject a little,

Monopole
One of us needs to rethink this.
So considering 'blue line' is a best rate speed, and 'red line' is best angle speed,
As for Red Line, people are scared of it because it is often close to Vmca
So close??

Aircraft receiving type certification under FAR Part 23 after November 1965
Vmca = Air minimum control speed

Takeoff or maximum available "power"on operating engine, critical engine windmilling (or feathered if autofeather device installed) landing gear retracted, flaps in takeoff position and CoG at aft limit. = RED REDIAL LINE = Obviously changes slightly

Vxse = Best Angle of Climb speed (Single Engine)
Speed which produces most altitude gain over a given distance with one engine inoperative.
Is not indicated on the ASI it is published in the flight manual.
Vxse is normally higher than Vx
Climbing at RED LINE is fraught with danger.


Larger a/c reduce their V1, Rotate and V2 speeds depending on a/c weight
Not true either.
To maintain a balance field length, the FMC computed V1, Vr and V2 for a lower weight on the same runway is often higher. There are all sorts of variables including weight, flap setting, thrust setting, wet/dry rwy for starters that effect V1 etc.
In some aircraft under certain circumstances your comment maybe true, in medium weight twins like Dash 8, that is probably only because they have a simplified method of calculating speeds. Normally some tabulated form rather than a computer. The computed speed for lower wights may well increase the V1, Vr and V2.

[Utradar]

There's absolutely no room for error maintaining red line. In a Baron, the red line is 78Kts also pretty damn close to stall speed as well at max weight in some aircraft. Your walking on a tight rope. People SHOULD be scared flying at red line as doing this whilst during a turn would also increase the stall speed. Nothing but dangerous! Speeds in an AFM are there for a good reason to keep you alive. :eek:

[OZBUSDRIVER]

Speaking from experience. Standing wave will exceed the climb performance of a C172 by a considerable margin. I have witnessed standing wave off Cunninghams Gap peg the VSI in a C172 in both climb AND descent. It is a wierd sensation to see the VSI showing 3000fpm climb and the altimeter winding up like you are in a fighter then a short while later have the VSI do exactly the opposite. Do not do what my instructor made me do and turn parrallel to the ridge. Imagine trying to descend and keep below the yellow arc and still be going up at 2000fpm:ooh: Either maintain best angle if enough height to clear the ridge by a large margin and avoid the rotor or do a 180 away from the ridge.

To answer original question= Best Angle. The reason being that you want to get maximum performance for whatever distance you are traveling in downdraft.

[Cloud Cutter]

As for the mountain waves. See above. If you are bashing along in a C172 and Best Rate is say 68 kts, but 65 gave you an extra 100 fpm RoC, you would be a fool in my opinion to 'PEG' the 68 kts.

Monopole, that doesn't make any sense (much like the red line comment). If you have a best rate speed of 68 knots (for given conditions/weight) you will not get extra ROC an ANY other speed. The problem you're eluding to, is not knowing exactly what your Vy is, and yes, you could work it out by experiment, but you should have a good idea given the Vy at MTOW, and the current aircraft weight. In the example you've given, 65 kts IS the Vy speed for your 172 given the conditions - by deffinition (even though 68 kts may be the MTOW Vy given in the flight manual).

Pilots should know the approx speeds to fly at different weights. If you ask me, it is stupid NOT to follow what you have been taught, and practiced in an engine failure. The stress of an urgent situation is no time to become a test pilot. By all means, do it under simulated conditions, that is good airmanship.

Close to the ground, you would normally be best to use Vxse (normally 5-10% below blue line) after an engine failure in a light twin, that is basically the proceedure in larger aircraft. The danger is, as you reduce below Vxse, induced drag increases rapidly, and approaching red line, you start to run out of options.

[Monopole]

Ok, while I still believe I am correct in what it is I am trying to say, I think I do see the error of my way in trying to explain it :\
I was incorrectly taking it as
Vyse = Blue line
Vyxe = Red line


Note to the Newbies.. Do not take anything you read on Pprune and esp. anything that I say as gospel :}

RENURPP
I agree with most of what you have said except for
V2 for a lower weight on the same runway is often higher.
Isn't V2 a calculation off the stall speed, with the stall speed reducing with weight (genuine question)? :confused:

Spoilher
There's absolutely no room for error maintaining red line
Agreed. Still does not mean you can't go there if it is going to produce the performance required (not performance desired). I am only talking about if you have done all you can and you are still going down and you are in dire situation straying two or three kts from the Vyse or Vyxe could make all the difference on the day.

[RENURPP]

Monoploe,
V2 for a lower weight on the same runway is often higher

The FMC generally calculates the V speeds.
In any case as the weight reduces the aircraft does accelerate faster, and less brake energy to slow down again. Keeping that in mind the aircraft even at the same flap setting can accelerate to a higher V1 and stop in the same distance as a heavier aircraft, thats the speed the FMC produces.
At lower weights, generally you would use a lower flap setting, of which the options are unlimited, i.e. 5.6 degrees 5.7 degrees and so on. lower flap settings = higher V2


I agree that people shouldn't beleive anything they read on here without supporting evidence. We all make mistakes and we don't know what we don't know.

[empacher48]

From experience flying a lot in these conditions - you will be caught in a downdraught (its a fact of flying in wave). This is caused by the upflowing air from the mountiain range being deflected upwards, the net affect is a few miles downwind the air descends back to its original level, then a while later the upflow commences again, but a reduced amount, descends, rises and descends. The big thing about wave is that it is smooth and powerful movements of air.

Take a mountain range - Mount Cook National Park in NZ (not only my stomping ground, but here you have a substantial Mountain range), with a 40 knot wind from 330 degrees magnetic (hits the mountains, ranging from 8000' to 12000' AMSL, at a 90 degree angle). Fly your aircraft downwind above 10500' AMSL (North West to South East) you'll get an initial altitude rise (hang on there 3000 to 4000 feet per minute in a loaded C206 is extremely easy) you'll level off nice and high (careful you could be close or even over 13000' AMSL) then you 'll have a nice smooth descent - again 3000 to 4000 fpm is easy) you'll drop back below 10,500' AMSL level off then rise again probably a bit less the further you travel downwind. Travelling downwind just accept the ups and downs especially if you're maintaining height. If you want to descend, and are confident and experienced in wave flying, then you can anticipate the "downs" and pole it forward, while turning through 90 degrees, try and maintain a line surfing the "down" to loose height - but CAREFUL too far downwind and you enter Rotor, or probably the most violent ride you'll have.

If you are trying to climb apply the opposite - in the Cook area, you'll get a few bumps from 8500' AMSL to 9500' AMSL as you climb through a rotor zone, try and climb at best rate when the VSI shows a climb, when the VSI shows the climb slowing off push forward to punch through the "down flowing air" as long as terrain clearance is suitable. Once you are through to about 9500' you'll feel like you're on an elevator, large smooth climbs are easy. If you are flying South East to North West as mentioned above (ie into wind) you'll want to take all the lift you can find, by watching the VSI, turning to parallel the track to maintain the climb (at best rate). If you get into the "Down" then push forward, build airspeed to punch through the down flowing air to get once again back in the "Up" But only do this if height loss is acceptable - any doubt about oncoming terrain, a 180 degree turn is the only option - remember turning to parallel the mountain range will mean you will still be in down flowing air, loosing more height. The up flowing air is either behind you or in front of you. Just like waves on a beach

As a side piece - in these conditions I have had a C172 at 11,000' 20Kg under MAUW, pulled the throttle closed and surfed the wave to 13,000' without engine power - good fun. Mind you, I don't know how it all works, I just use it most days of the year to fly punters around the place in nice smooth air, in comfort and safety.

[sailing]

Had an interesting flight in a glider at Omarama a few years ago, with a local instructor. We got into the wave lift at about 7.000', then headed for Mt Cook doing 100 knots, with the VSI on 1,000 fpm up all the`way. Arrived over Mt Cook 12 mins later at 19,500', smooth air all the`way! Had to use the dive brakes to stop the climb. Great fun!
To answer the original question, the best speed to fly for max climb in the downdraft if you insist on staying in it is obviously best ROC speed. Much cleverer to fly out of it, the faster the better (beware possible turbulence!).

[currawong]

If you run into a real mountain wave (ie with rotors) your best bet is to remove yourself.

At a speed not too close to the yellow line on your ASI.

In a GA aircraft you will not outclimb the down, and possibly not outdive the up.

The ride out can be rough, off the clock climb followed by the mess in the cockpit stuck to the roof.

So the answer to the original question is neither.

[Lord Snot]

Is that a poem?? :p

The average GA piston has been flogged so hard there isn't much difference in performance between Vx and Vy.

Anyway, time is not the factor in a terrain situation but geographical position is. That means Vx is your speed.
As for Red Line, people are scared of it because it is often close to Vmca.
Red radial IS Vmca. People should be well-scared of the red radial because it represents the manufacturer-determined Vmca. Why the fcuk would you climb at this airspeed???
Well, if I had an engine failure on TKOF and there was an obsticle in my way and turning was out of the question, well then yes, as best angle RoC I will fly it."....at the first indication of an engine failure during climbout, or while on approach, establish Vyse or Vxse, whichever is appropriate. (Consult your Handbook or Flight Manual for specifics)."

And...

"Vyse (Blue Radial) -- always fly this airspeed during a single engine emergency during climbout (except when necessary to clear an obstacle after takeoff) and on final approach until committed for landing."

And...

"Vxse - Fly Vxse to clear obstacles, then accelerate to Vyse."

[EC Does It]

In a GA aircraft you will not outclimb the down, and possibly not outdive the up.

I was sat in the back of a PA 28 recently, watching a really good instructor demonstrating stalls to a new student, and as he said "you will notice that one of the characteristics of the stall is a high rate of descent" we were going up like a rat in a drainpipe. Nearly p1$$ed myself laughing, but a salutary reminder of how strong the ups and downs can be in wave.

Fantastic for gliding though.....

Safe and happy flying, ECDI.

[cirrus driver]

Head directly downwind and you shall escape the down side of the wave.
It is a standard drill for glider pilots flying in wave.
This is much more important than power settings.

The same goes for a glider tug!:ok:

cirrus driver

[Monopole]

VH-FTS,
Sorry to have hijacked your post like that, but hopfully through the errors in my original post your question has in facted been answered.

Renurpp, Cloud Cutter and Lord Snot
After a good nights sleep I am almost embarrassed to have read what I was saying :\ :\ :\ . I can't even hide behind the excuss of having consumed a few ales :E

I had mistaken the little 'red line' on the IAS as Vyxe. I fully agree that you do not want to fly at Vymca.
Cloud Clutter
This is excactly what I was trying to elucidate.
If you have a best rate speed of 68 knots (for given conditions/weight) you will not get extra ROC an ANY other speed. The problem you're eluding to, is not knowing exactly what your Vy is, and yes, you could work it out by experiment, but you should have a good idea given the Vy at MTOW, and the current aircraft weight. In the example you've given, 65 kts IS the Vy speed for your 172 given the conditions - by deffinition (even though 68 kts may be the MTOW Vy given in the flight manual).

I was trying to point out that most flying schools will have only 1 speed for Vy Vx on there A/C type notes they hand out to their students. As a consequence, new pilots tend to become a 'puppet to the numbers' without understanding where the performance figures had really come from.

Quote:
Originally Posted by monopole
Well, if I had an engine failure on TKOF and there was an obsticle in my way and turning was out of the question, well then yes, as best angle RoC I will fly it.

"....at the first indication of an engine failure during climbout, or while on approach, establish Vyse or Vxse, whichever is appropriate. (Consult your Handbook or Flight Manual for specifics)."


And hopefully with that I can redeem myself as you can see that while I was incorrectly saying 'RED LINE', I was in fact thinking Vyxe :D

Thanks for pulling me back into line
(as I put my head down and quietly sit in the corner) :ok:

[VH-FTS]

I'm still not 100% sure what the answer is - best angle of climb?

The real life scenario I faced was a couple of years ago flying in a Warriror from Brisbane to Coffs Harbour via the Gold Coast. Mountain waves were forecast, so we avoided the ranges and we flew about 10nm east of the ranges (a 35kt westerly was blowing) at 7,500 feet. This had us in controlled airspace and well clear of terrain.

We hit the wave, attempted to climb at best angle of climb but still lost significant height - over 500 feet from memory. ATC cleared us to a lower level (still well clear of terrain though). Until you get caught in one you never appreciate how bad they could be.

And with the SE QLD westerly winds just around the corner I thought I'd ask the question. Remember, I only fly small singles, so no blue line or red line answers for me. I'm a bit slow on the uptake.

[empacher48]

We use best rate of climb for flying in up going wave, particularly when trying to gain height as I mentioned previously. Hitting the up flowing air and going to best rate. Mainly because of your ground speed. But also gives protection against local effects of airflow over the wing - causing a stall.

If you are flying into a 35 knot wind, best rate of a generic aircraft (no particular type) is 70kts IAS at 10,000' AMSL. Flying into your headwind at 70kts IAS (for arguments sake about 80kts TAS) with a 35kt wind is going to be around 45kts!!! Where as say Best Angle is 65kts IAS, (75kts TAS) which will give a Ground Speed of 40 kts. You aren't going to go anywhere very fast at either these speeds, and as I mentioned climbing at a 45kt groundspeed in downflowing air will mean you are experiencing this for longer, where as in downflowing air - lowering the nose, increase your Groundspeed, your rate of descent will increase, but you'll be out of the down quicker! Sometimes better than sitting there with a high nose attitude, high rate of descent, mountains/terrain approaching from underneath. You start running out of time and airspeed to do anything but hope you'll cross the saddle, or you'll smack into it.

By lowering the nose and gaining airspeed, you can see better the terrain you are approaching, you can judge the closure rates of height/speed/terrain and make a call whether you can cross the saddle easily, if not - then you have the airflow over the wings to make that Steep to Max Rate turn to get out of there.

I would never ever ever ever ever ever be in down flowing air in a C206 below 80kts - if I need to turn quickly below 80kts its almost a sure thing that at 10,000' it'll stall - and that turns a bad situation into a worse one!!!! But would not be more than a few knots over Va (121kts) - coz if you fall out of the wave and into rotor you don't want to be overstress the airframe either!

In wave if you're in downflowing air, there will be upflowing air within a mile upwind or downwind of where you are (it depends on mountain shape and form, windspeed and direction on the wavelengths, if its moist the lenticular clouds and rotor clouds will show when air is flowing up and down and areas of turbulence).

[karrank]

Best angle of climb will give you the best chance of avoiding obstacles, but there weren't any.

Best rate of climb will minimise your height loss in your situation & minimise any intereference with other airspace users:) .

I flew through a big thermal as I chopped the power on base on my second solo. Speed reducing nicely, attitude normal, two stages of flap out, a bit bouncy, glance at VSI & shows 2000 fpm climb - WHAT! Master overpowering urge to stuff the nose down but feeling a bit confused, recheck the nose is where it should be & AH agrees, speed still reducing to my target of 80. Decide it isn't me but I'm obviously high & not about to stall so reduced power and lowered the nose a bit. A few seconds later I'm descending normally. Probably old hat to more experienced pilots but it felt to me as if Emperor Ming the Merciless had taken control of my aircraft