Qs about high altitude flight in high performance aircraft

[Richard]

RE: AC 61-107A

In reading the AC these Qs occured to me. They aren't suggested in the AC.

1) In flight in the flight levels would the flight crew know, other than radar altimeter, of changes in true altitude when flying through areas of differing pressures?

2) WRT drag divergence, when flying through turbulent air is there a minimum indicated airspeed (decrease from intended cruise speed) at which the flight crew must take action to prevent further decay of airspeed?

My 2nd question is due to the AC saying turbulence may increase Coefficient of drag.

http://rgl.faa.gov/Regulatory_and_G...732ba93fd86256caa005ca97e/$FILE/AC61-107A.pdf

 

[Greg Bockelman]

1) In flight in the flight levels would the flight crew know, other than radar altimeter, of changes in true altitude when flying through areas of differing pressures?

No. Radar altimeter does not indicate above about 2,500 feet anyway so that isn't even an option. Why does it matter?

2) WRT drag divergence, when flying through turbulent air is there a minimum indicated airspeed (decrease from intended cruise speed) at which the flight crew must take action to prevent further decay of airspeed?

Why wouldn't one add thrust to compensate as soon as a decay is noted?

 

[poadeleted20]

On question 2, we just kept an eye on AOA -- alpha goes up, so does power (and, in the 'Vark, wings go forward).

 

[Dave Siciliano]

Don't know where they're going on question one, but one could know if they have a GPS on board. It would certainly show altitude changes; they may not match what the altimeter would show, but could show relative up or down movement.

Best,

Dave

 

[Teller1900]

RE: AC 61-107A

In reading the AC these Qs occured to me. They aren't suggested in the AC.

1) In flight in the flight levels would the flight crew know, other than radar altimeter, of changes in true altitude when flying through areas of differing pressures?

Like Greg said, RadAlt shuts off at 2,500 feet, so no help there. Otherwise you'd have to do a lot of math, I suppose.

2) WRT drag divergence, when flying through turbulent air is there a minimum indicated airspeed (decrease from intended cruise speed) at which the flight crew must take action to prevent further decay of airspeed?

In turbulent air we're more concerned about maximum airspeed, but again like Greg said, if you notice a decay (or increase, for that matter) just correct it when you see it. I suppose the only absolute minimum we have is stall speed (or 180kts in icing conditions).

 

[Everskyward]

1) In flight in the flight levels would the flight crew know, other than radar altimeter, of changes in true altitude when flying through areas of differing pressures?

The only time I cared about my true altitude when flying in the flight levels was when I was doing mapping and we were supposed to be a certain height AGL. As others have said, radar altimeters don't work that high and besides, we didn't have one. It seemed to me like true altitude was affected as much by temperatures aloft as pressure (hot to cold look out below and all that). We got the OAT, and the temperature and pressure of a ground station and used a whiz wheel to compute true altitude. We then subtracted the ground elevation to get AGL. Whatever number we came up with was close enough for our purposes.

2) WRT drag divergence, when flying through turbulent air is there a minimum indicated airspeed (decrease from intended cruise speed) at which the flight crew must take action to prevent further decay of airspeed?

My 2nd question is due to the AC saying turbulence may increase Coefficient of drag.

I'm not sure quite where you're going with that question. Do you mean the airspeed where you might transition from the front side of the power curve to the back? In any case, whatever it is, you wouldn't slow down that far in cruise without taking some action beforehand. I guess that's the gist of the previous answers too.

 

[AWACSEng]

Doesn't really matter if your true alt. changes in the flight levels. Everyone is flying on 29.92 anyway. If you hapen to be in the lower flight levels (i.e. right at FL180) if the pressure changes so much to be a factor, ATC will inform you that flight level is no longer usable and give you a different altitude. Either way, nice to know, but not really a huge concern.

 

[MauleSkinner]

Low speed can combine with turbulence in some airplanes at higher flight levels to cause problems...there's a phenomenon called "low speed mach buffet" that's caused by the airflow over the top of the wing increasing speed dramatically due to high angles of attack (the "low speed" part). There comes a point where this airflow actually goes supersonic, shock waves are created, and drag increases dramatically and disrupting airflow.

In a Falcon 10, at FL390, it happens at about .68M with just a little more than a 1g load on the airplane. In a Beechjet it can happen at about .59M, but it takes a little more load on the airplane to do it (discovered that with an autopilot failure once). In both cases, the IAS was well above (close to double) the 1-g stall speed. In the case of the Falcon, we actually had to point the nose down and lose about 300 feet to accelerate out of it because we simply didn't have the power to push our way through it.

Most jets have a chart in the AFM that gives data for "low speed buffet boundary" at 1.5-g's for this situation. I think it's more pronounced in a "faster" wing...the smaller the leading edge, the sharper the bend at high angles of attack, and the more airflow acceleration. Hawkers have a nice, fat airfoil, and I've never encountered it in a Hawker.

Fly safe!

David

 

[Richard]

No. Radar altimeter does not indicate above about 2,500 feet anyway so that isn't even an option. Why does it matter?


Why wouldn't one add thrust to compensate as soon as a decay is noted?

I'm eternally curious. The question shouldn't imply I am going anywhere (have a conclusion to reach, a point to make).

However, my understanding is that the drag increase is significant such that adding power is insufficient. So I was thinking what other action could the crew take? David mentions lowering the nose as that action. My reading is lowering the nose is not enough, that you have to really push the nose down to offset the drag increase due to penetration of turbulent air.

EDIT: Greg said thrust, I said power. Ooops. Would pushing over constitute "adding thrust" when excess power is unavaliable? Where am I going with that, I don't know. But now I'm thinking of David's comment about low speed mach buffet and how, if it's possible, to reconcile the two.

 

[Greg Bockelman]

However, my understanding is that the drag increase is significant such that adding power is insufficient.

Given the fact that I have never heard of this phenomenon, and have never experienced said phenomenon in the 19 years that I have been doing high altitude high performance flying, I would have to conclude that it isn't a serious issue.

 

[Areeda]

As far as #1, if you get permission to turn on a WAAS hand held like a 496 on an airliner you can see the changes in the constant pressure altitude as you cross weather systems. Kinda interesting, but as stated not very useful.

Joe

 

[Everskyward]

EDIT: Greg said thrust, I said power. Ooops. Would pushing over constitute "adding thrust" when excess power is unavaliable?

"Adding thrust" means pushing forward on the go-fast levers. I think you may be confused in that jet engines produce thrust rather than power. However, some people use the two terms interchangeably.

 

[KennyFlys]

However, my understanding is that the drag increase is significant such that adding power is insufficient.

Given the fact that I have never heard of this phenomenon, and have never experienced said phenomenon in the 19 years that I have been doing high altitude high performance flying, I would have to conclude that it isn't a serious issue.

Richard, could you be referring to "induced drag"?

 

[Richard]

Richard, could you be referring to "induced drag"?

I'm speaking of Total Drag, the product of dynamic pressure, surface area, and drag coefficient. CD here is expressed as the ratio of drag pressures to dynamic pressures.

However, specific to your question, the book (Aerodynamics for Naval Aviators) speaks of skin friction causing the preponderance of drag in streamlined forms.

 

[Richard]

"Adding thrust" means pushing forward on the go-fast levers. I think you may be confused in that jet engines produce thrust rather than power. However, some people use the two terms interchangeably.

I was thinking of excess power when I pounded out my reply which contained Greg's quote. The reason I was thinking of power was two fold;

One, the AC mentions drag divergence, "a phenomena when an airfoil's drag increases sharply and requires substantial increases in power (thrust) to produce further increases in speed." *

To me this strongly implies there may not be available excess power, or if there is, that it would be ineffective given the sharp increase in drag.

Two, David commented that the best way to 'power out' of a low speed buffet would be to push over. At least that is how I understood what he said.

Again referring to the AC, "Turbulent air may produce a resultant increase in the coefficient of drag.". To this I think of the scenario where you are in cruise fat dumb and happy, then encounter turbulent air which not only upsets your passengers but puts you into a stalled condition even though nothing else has changed. What happens will you come out the other side into smooth air? Would the addition of power now be the wrong response?

*The quote is taken from the AC. It is interesting to note they interchange the terms power and thrust.

 

[Richard]

Given the fact that I have never heard of this phenomenon, and have never experienced said phenomenon in the 19 years that I have been doing high altitude high performance flying, I would have to conclude that it isn't a serious issue.

The NTSB and FAA thought it serious enough that an addition to Part 61 was made to require high altitude training. This was a response to several accidents which were believed to have occured due to aircrew lack of knowledge and proficiency in high altitude flight.

 

[Everskyward]

I was thinking of excess power when I pounded out my reply which contained Greg's quote. The reason I was thinking of power was two fold;

One, the AC mentions drag divergence, "a phenomena when an airfoil's drag increases sharply and requires substantial increases in power (thrust) to produce further increases in speed." *

To me this strongly implies there may not be available excess power, or if there is, that it would be ineffective given the sharp increase in drag.

For one thing, you don't want to increase your speed in excess of the drag divergence mach number, at least not in subsonic civilian airplanes. Lots of bad things can happen if you go too fast.

Maybe this site will help make this clear to you.

[FONT=Trebuchet MS, Arial, MS Sans Serif]The free-stream Mach number at which the drag of the airplane increases markedly is called the drag-divergence Mach number. Large increases in thrust are required to produce any further increases in airplane speed. If an airplane has an engine of insufficient thrust, its speed will be limited by the drag-divergence Mach number. The prototype Convair F- 102A was originally designed as a supersonic interceptor but early flight tests indicated that because of high drag, it would never achieve this goal. It later achieved its goal through a redesign.[/FONT]

Two, David commented that the best way to 'power out' of a low speed buffet would be to push over. At least that is how I understood what he said.

David was addressing the low speed buffet which occurs when you are going too slow or at too high an angle of attack. Here's an explanation of low speed mach buffet.

The most likely situations that could cause the low
speed buffet would be when an airplane is flown at too
slow a speed for its weight and altitude causing a high
angle of attack. This very high angle of attack would
have the same effect of increasing airflow over the
upper surface of the wing to the point that all of the
same effects of the shock waves and buffet would
occur as in the high speed buffet situation.
The angle of attack of the wing has the greatest effect
on inducing the Mach buffet at either the high or low
speed boundaries for the airplane. The conditions that
increase the angle of attack, hence the speed of the
airflow over the wing and chances of Mach buffet are:
• High altitudes—The higher the airplane flies,
the thinner the air and the greater the angle of
attack required to produce the lift needed to
maintain level flight.
• Heavy weights—The heavier the airplane, the
greater the lift required of the wing, and all other
things being equal, the greater the angle of
attack.
• “G” loading—An increase in the “G” loading of
the wing results in the same situation as
increasing the weight of the airplane. It makes
no difference whether the increase in “G” forces
is caused by a turn, rough control usage, or turbulence.
The effect of increasing the wing’s
angle of attack is the same.

Again referring to the AC, "Turbulent air may produce a resultant increase in the coefficient of drag.". To this I think of the scenario where you are in cruise fat dumb and happy, then encounter turbulent air which not only upsets your passengers but puts you into a stalled condition even though nothing else has changed.

How is that different than what happens to all airplanes given enough turbulence? That is if the wings don't break off first. If you put a load on the wings it momentarily increases the angle of attack and the induced drag. The thing is that if it's that turbulent you want to be flying at the best speed for turbulence penetration which gives you the largest margin possible between the low and high speed buffets.

What happens will you come out the other side into smooth air? Would the addition of power now be the wrong response?

I don't really understand this question. What happens when you come out the other side in smooth air in a Cherokee? Not a whole lot. I think you may be thinking that the whole 'drag produced by turbulence' is more significant than it is.

 

[MauleSkinner]

I was thinking of excess power when I pounded out my reply which contained Greg's quote. The reason I was thinking of power was two fold;

One, the AC mentions drag divergence, "a phenomena when an airfoil's drag increases sharply and requires substantial increases in power (thrust) to produce further increases in speed." *

To me this strongly implies there may not be available excess power, or if there is, that it would be ineffective given the sharp increase in drag.

Two, David commented that the best way to 'power out' of a low speed buffet would be to push over. At least that is how I understood what he said.

Again referring to the AC, "Turbulent air may produce a resultant increase in the coefficient of drag.". To this I think of the scenario where you are in cruise fat dumb and happy, then encounter turbulent air which not only upsets your passengers but puts you into a stalled condition even though nothing else has changed. What happens will you come out the other side into smooth air? Would the addition of power now be the wrong response?

*The quote is taken from the AC. It is interesting to note they interchange the terms power and thrust.

It's entirely possible in the extremes of altitude capability for a particular airplane to require maximum thrust simply to maintain cruise airspeed...if this speed is close enough to the low speed mach buffet onset with some turbulence or hamfisted flying, the drag increase due to the buffet onset is enough to slow you down further, and since the thrust levers are all full forward already, there's no more thrust available to "power out" of the buffet. Even when the turbulence or hamfistedness stops, you're slow enough to keep the buffet and drag divergence, so the airplane can't accelerate out of the condition in level flight. At this point, a descent to gain airspeed is the only solution. This is what we ran into with the Falcon 10...a crew was trying to outfly the capability of the airplane in order to top some weather, and "discovered" low speed mach buffet. Fortunately it didn't require radical pitch changes...just an easy 300-ft descent to accelerate, and once the airplane accelerated past the drag divergence, it could climb back to the original altitude.

If the condition occurs when you don't have the thrust levers full forward, there may be enough increase in thrust available to "power out" of the low speed mach buffet condition.

In the case I had with a Beechjet autopilot malfunction, once we disengaged the autopilot and stopped the pitch oscillations, we flew out of the buffet because it was a "transient" condition, and we had the excess performance available. The same could be said for turbulent conditions...if the turbulence goes away, it's possible that the airplane could accelerate out of the buffet because it DOES have the excess performance available.

BTW...low speed mach buffet is entirely different than a "stalled condition"...as I mentioned in my previous post, my experiences with it have been at nearly double the 1-g stall IAS, and nowhere near critical AOA, as referenced by the AOA indicator.

It simply depends upon the airplane and how close you are to its performance limit. Stay away from the performance limit, and it won't be an issue. That's why the airplanes have charts for that stuff.

Fly safe!

David

 

[gismo]

"Adding thrust" means pushing forward on the go-fast levers. I think you may be confused in that jet engines produce thrust rather than power. However, some people use the two terms interchangeably.

Jet engines produce power as well as thrust, they're just rated in thrust instead of power. Theres a fixed relationship between power and thrust that's dependent on speed:

Power = Thurst * Velocity.