About the Author

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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Monday, October 25, 2010

Tubulence Penetration

Several posts ago, I wrote a 2-part piece about Maneuvering Speed—Va in V-speed notation. One of the common misunderstandings about maneuvering speed is that it is a turbulence penetration speed—or, according to some, a maximum turbulence penetration speed—not to be exceeded in gusty or turbulent conditions. As discussed in the posts, Va is used to define the minimum allowable structural strength of the control surfaces. The requirements set for Va make no reference whatsoever to turbulence.

We know, of course, that we do often fly in turbulence, and that turbulence subjects the airframe to structural loads. We also know (or can figure out) that these structural loads will vary with airspeed—they’ll be higher at higher airspeeds and lower at lower airspeeds. So it might be useful to know if there are V-speeds related to turbulence loading, what these speeds are, and what criteria is used to determine these speeds.

As it turns out, there are several speeds at which there is a (direct or indirect) required structural tolerance for turbulence (and Va is not one of them!). Two of these speeds are always published in the flight manual—and marked on the ASI—for normal and utility category aircraft: Normal Operating Speed (Vno), and Never Exceed Speed (Vne). Also related to turbulence, but not always published, are Vb (Gust Penetration Speed), Vc (Design Cruise Speed) and Vd (Design Dive Speed).

The required turbulence tolerance of the airframe is directly defined at three of these speeds, with progressively lower requirements at the progressively higher speeds of Vb, Vc, and Vd. The turbulence requirements for Vno and Vne are indirect since these speeds are defined in terms of other speeds (Vc and Vd, respectively).

Gust Penetration Speed, or Vb, is required to be determined and published for commuter category aircraft, but is usually unpublished for normal and utility category aircraft. Vb is the maximum speed at which the aircraft structure can withstand a 66 foot per second (fps) gust perpendicular to the flight path. How much is 66 fps? It’s about 39 knots, and that’s a helluva gust!

At first glance, 39 knots may seem pretty extreme, but isn’t really when you consider the nature of turbulence. In turbulence, we can encounter two opposing gusts one immediately after another. If these gusts are 20 knots each, the net effect as we transition from one to the other is a 40 knot change. So turbulence that corresponds to a 20 knot gust factor should have us slowing down to (or below) our Gust Penetration Speed, Vb.

Design Cruise Speed, or Vc, is the maximum speed at which the aircraft structure can withstand a 50 fps gust perpendicular to the flight path. At Vc, stronger gusts will overstress the airframe. Above Vc, a 50 fps gust will overstress the airframe. How much is 50 fps? About 29 knots. Using similar reasoning as above, a double gust of 15 knots each can bring us just beyond this limit. So if we’re operating in conditions where the gust factor is at, near, or above 15 knots, we should keep the airspeed below Vc.

How do we do that if Vc isn’t published? Easy peasy! Normal Operating Speed, or Vno, is based on Vc. Vno is usually equal to Vc, but under some conditions it can be lower than Vc. So using Vno as our turbulence limit will be correct, or, in some cases, will err on the side of caution. We can see Vno on the ASI: the top of the green and bottom of the yellow arcs.

It’s generally said that Vno should only be exceeded in very smooth air and even then only with caution. This is fair comment, if for no other reason than we like to have a comfortable margin of safety when it comes to structural failure. Further, even in smooth air, there can be turbulence that we don’t “see” until we’re in it. However, the certification standard does not require perfectly smooth air above Vno. As stated above, the turbulence allowed at Vno is 50 fps (almost 30 knots) perpendicular to the flight path. As we move above Vno, the airframe’s tolerance for gusts is reduced progressively until the lowest requirement (25 fps, or almost 15 knots) is met at Vd. Vd, in turn, is closely related to Vne.

Vne, as all pilots know, is the NEVER EXCEED speed. We never fly at speeds above this for fear of ripping the wings off—quite literally—and then flying much faster and in the manner of a lawn dart, followed by an unpleasant encounter with the ground. This is not hyperbole. Vne is never to be exceeded for several very good reasons (flutter, static divergence, control reversal), which will probably be the subject of a future post. For the moment, let’s have a quick look at the turbulence tolerance required at Vne.

Vne is defined as being no higher than 0.9 times Vd. Vd is where all of the nasty things noted above might happen and the airframe may rip itself apart. The 10% margin is there to allow for manufacturing tolerances and aging of the aircraft, so don’t fool yourself into thinking you can exceed Vne by 10%. The turbulence requirement is applied at Vd, and as such, we can "theoretically" take 10% more gust velocity at Vne. But why push our luck?

At Vd (and therefore at Vne for all practical purposes), the airframe must be able to withstand a gust of 25 fps perpendicular to the flight path. This is almost 15 knots (14.8 knots if you’re picky), which really isn’t very much at all when you account for the double-opposing-gust possibility discussed earlier. Flying in gusty conditions where the gust factor is over 7.5 knots is commonplace. In fact, a gust factor lower than 7.5 knots would barely be noticed in many aircraft bigger than a light 2-seater.

So if we plan on flying at speeds approaching Vne, we definitely want smooth air. This is especially true since Vne does NOT include a tolerance for momentary overspeeding due to airspeed variations in gusts (some speeds, such as the flap limit speed, Vfe, do in fact allow for such momentary overspeeding due to gusts). It’s possible (in truth, not entirely likely, but possible) that during flight at Vne, a gust can introduce catastrophic flutter and prematurely end our career. This alone is a good reason for applying a margin to Vne and avoiding turbulence when flying at very high speeds.

Ok, that’s all well and good, but what about Va? Well, as stated earlier, the standard for Va makes no reference whatsoever to turbulence or gusts. However, there is an argument that in extreme turbulence we may need to use full control authority (read “deflection”) in order to retain control of the aircraft. This is a valid point. However, I hope never to experience it first hand! The best way to deal with this problem is to avoid it. Check your weather, avoid forecast and reported severe turbulence, avoid thunderstorms, and avoid flying near ridges in strong winds. If you have stormscope or weather RADAR, use it. If there are turbulence PIREP’s, pay attention to them. Turbulence that requires regular use of full control authority likely goes beyond any turbulence defined by the certification standards—and beyond the ability of most pilots to maintain positive control for any extended period.

One final note with regard to turbulence PIREP’s. An aircraft’s sensitivity to turbulence is based almost entirely on it’s wing loading (almost because dynamic stability will also play a role). Higher wing loading results in a smoother ride. Lower wing loading results in a wilder ride. So if you’re reading (or listening to) a turbulence PIREP from a heavier aircraft with higher wing loading, interpret accordingly!

So there you have it. A few more tips on V-speeds, where they come from, and how they apply to flight operations.

Happy Flying!

Monday, October 18, 2010

How High Can You Fly?

I was recently reading a thread on an online forum (How high can You Fly? at www.aviationbanter.com).

The question asked by the original poster was basically, “If engine limitations disappear, how high could an aircraft fly, given aerodynamic and airframe limitations?”. Interesting question, I thought. Unfortunately, the thread degenerated into personal sniping and, in many cases, a demonstrated lack of understanding of the problem(s) being discussed. So I thought I’d take a run at the question here.

First of all, engine limitations will never really disappear. That is, unless someone (who will subsequently become very rich) finds a way to circumvent the Second Law of Thermodynamics. However, with enough technological advances, these limitations can become practically insignificant in comparison to other limitations we face. So let’s look at the question from the idealized position of no engine limitations other than the fact that the engine must be air breathing (for combustion or some other chemical reaction) or at least air processing (for a propeller or fan).

The fact is, for currently existing airframes, eliminating engine limitations wouldn’t significantly improve maximum altitudes. It would almost certainly improve cruise and climb performance within the existing altitude range—consider an engine that produces more power from a the same weight, or one that doesn’t lose power as altitude is gained. It might also lead to small improvements in maximum altitude. But big changes in maximum altitude would also require changes to the airframe design.

Two non-engine problems that show up at high altitudes are critical Mach (resulting in the so called "Coffin Corner"), which is due to compressibility effects, and a decrease in the damping of flutter, which is the result of reduced air density.

Critical Mach (Mcrit) is the lowest speed at which a portion of the airflow over the top of a wing travels at or above the speed of sound. As the airflow slows down again, it forms a shock wave, which causes airflow separation—leading to a loss of lift and a massive rise in drag. As a general rule, thinner wings have a higher Mcrit than thicker wings. This is one reason why high altitude aircraft tend to have thinner, more lightly cambered wings.

Mcrit becomes an issue because at higher altitudes, a given indicated airspeed corresponds to a higher true airspeed (due to reduced air density). At the same time, the speed of sound is getting lower (due to reduced air temperature). So for a given indicated airspeed, Mach number gets higher as altitude increases. Conversely, for a given Mach number, indicated airspeed gets lower as altitude increases. This means that Mcrit, which is a fixed function of wing design, corresponds to a progressively lower indicated airspeed as we climb higher.

Sooner or later as we climb, Mcrit enters into the cruise speed range. At some altitude, stall speed and Mcrit meet, and there is no speed at which controlled equilibrium flight can be maintained. No matter how good our engine is, we must remain below this altitude at all times. We must also be cognizant of the narrowing range of useable airspeeds available as we approach this altitude. As an example, the U-2 high altitude spy plane has a tiny speed margin at it’s operational (read “spying”) altitude. 5 knots too slow and it stalls, 5 knots too fast and it overspeeds! Airline pilots, flying aircraft that are designed for high altitudes, must be very aware of the reduced speed range available at higher altitudes. The problem shows up at much lower altitudes in aircraft that are not designed for high Mach number flight.

Airframe problem number 2 is the effect that altitude has on Vne. One of the defining features of flight at speeds over Vne is the presence of flutter, which is an unstable and destructive vibration in the airframe which is aggravated by high speed airflow. This vibration is damped at low altitude in the higher density air. The damping effectively increases the value of Vne. To a first approximation, we can treat Vne as a constant true airspeed v-speed.

So the bottom line here is that the indicated Vne actually drops as altitude increases—it isn’t constant, as we normally assume. Manufacturers account for this in many cases by adding a carefully determined safety factor that allows for reduced damping at the aircraft’s absolute ceiling. Some manufacturers choose instead to publish multiple Vne’s, which are lower at higher altitudes—up to the aircraft’s absolute ceiling. Either of these approaches works fine, but what happens if we strap a new engine onto the airframe and enable flight at higher altitudes? In the absence of additional manufacturer’s Vne data, flying to higher altitudes is a gamble. We could inadvertently exceed the altitude-appropriate Vne and find ourselves in a world of hurt. This problem is easy to fix if we have data on the Vne change with altitude. But we will still eventually run into problems because the margin between Vs and Vne is too narrow—assuming we don’t run into Mcrit problems first!

So all in all, it fair to say that if-and-when far superior engine technology becomes available, airframe design will have to be adapted accordingly. Simply bolting newer and better engines onto existing airframes won’t allow us to realize the full potential of these engines. Alas, this is not a problem we’ll likely need to address for many years yet!

Happy Flying!

Monday, October 11, 2010

Fall Like a Leaf, Grasshopper

Today I went on a "Mutual Proficiency Flight" in the Grob. These flights are used here to keep instructors proficient without the instructor practice being obtained at the expense of the students. It’s an excellent program for several reasons, not the least of which is the fact that we can work on improving our own flying skills without worrying about students seeing us get it wrong. This is not about pride or ego, but about the fact that students are following our example, and if they see us do something wrong, there’s always the risk that that’s the example that will be followed.

One of the exercises we did was a "Falling Leaf Stall". The Falling Leaf is a maneuver in which we enter a stall and, rather than recover immediately (as we usually do), we remain in the stall for an extended period—often losing multiple thousands of feet of altitude. This gives us an opportunity to observe the symptoms of the stall for an extended period and to (try to) keep the wings level with rudder. Most aircraft are directionally unstable in the stall, so directional and roll control can be tricky, and ailerons are usually a no-no (“Why?” You ask? This is a topic for another post!). I wanted to do the Falling Leaf because I’m still very new to the Grob, and I’ve always considered the maneuver to be an excellent skill-building exercise.

Much to my chagrin, I didn’t do a very good job. I guess I didn’t do terrible, but mediocre would be a fair assessment. My training partner, Brandon, did a much better job. So, of course, I had to try a second attempt. It was an improvement, but still not as good as Brandon’s first try. In both of my attempts, I had to recover earlier than planned in order to avoid spinning—which, of course, is one of the points of the exercise!

During normal stall training, we recover from the stall either at the "first indication" (which, in practice, means before the stall), or immediately upon entering the stall. This makes a lot of sense, since the objective is to recover with a minimum loss of altitude. Why is this minimum loss of altitude so important? Stalls in the “real world” generally occur at low altitude. So they mandate a quick and effective recovery to prevent an unpleasant encounter with the ground (which, in case you can’t tell, is a euphemism for “crash”). So it makes sense to train the way we fly. A quick recovery in training will promote a quick recovery in the real world.

The problem with this quick recovery approach is that it limits our exposure to aircraft behavior in and around the stall. Why is this bad? Well, for at least two reasons: 1) The more exposure we have to stall symptoms, the better we are at recognizing an unplanned stall, and 2) Aircraft behavior (read “control response”) in the stall is usually very different than aircraft behavior out of the stall (I say "usually" because different aircraft types have different stall characteristics).

Point (1) is important because in order to recover from an inadvertent stall with minimum loss of altitude, we first have to recognize that we are stalled (or about to stall). Failure to recognize a stalled condition is just as bad as an inability to recover.

Point (2) is important because the vast majority of aircraft types require that we not use ailerons while stalled (and like I said above, this is a whole ’nother post). This is a hard habit to break since at all other times, we control roll with ailerons. Ask a flight instructor how difficult it is to get students to not use ailerons to correct for a wing-drop in a stall and they’ll tell you all about it. But find an instructor who uses the Falling Leaf, and you’ll probably hear a different story. The difference comes down to the oft-quoted mantra of "Practice Makes Perfect" (or, more correctly, "Proper Practice Makes Perfect"). More time spent in the stall, correcting correctly, leads to better directional corrections, even during the quick stalls that we normally train for.

It’s always struck me as odd that the Falling Leaf isn’t a required exercise for the training of pilots. In fact, I’ve met instructors who have never heard of the exercise. If you fly and you’ve never tried the Falling Leaf, go up with an instructor who is familiar with the exercise and try it out. If you’re an instructor and you’ve struggled with your student’s directional control during stall training, try the Falling Leaf exercise and see how things improve.

Happy Flying!

Monday, October 4, 2010

To Be or Not To Be -- An Instructor, That Is

One of the questions that often gets asked by new Commercial Pilots or Commercial Pilot candidates nearing completion is the dreaded, "Should I get my Instructor Rating?". It shows up again and again on online aviation forums (for example: here), and practicing instructors are constantly getting the question from students.

I refer to the question as "dreaded" because there really isn’t a right answer. The answer is different for everyone. Should you or shouldn’t you? Maybe. Maybe not.

I’m inclined to say that if you have no experience beyond your own training, then you shouldn’t be instructing. But that view ignores the present reality of the industry, and I’ll leave it aside for now. Perhaps it will make an interesting future post.

The main question you need to ask yourself is, "Do you want to be a flight instructor?". This shouldn’t be confused with, "Do you want to build hours at someone else’s expense?", which , alas, is a far-too-common motivation for people who don’t actually want to instruct. There are other ways to build hours. If this is what you’re looking for, do some homework and find them. But do your homework early, the direction you choose may influence choices you make during your CPL training (e.g. – do you need float time? taildragger time? multi-engine time? mountain experience?).

So, do you want to be a flight instructor? Think before you answer.

Think about it another way: If your instructing flight hours didn’t count in your logbook, would you still be interested? If your answer is no, that’s a red flag. Perhaps it’s not a show stopper, but it’s definitely a red flag. There are in fact some charter and airline companies that don’t recognize instructing as "real" experience. Frankly, I don’t agree, but again, this is a topic for a future post. But you have to realize that these companies are out there. Of course, there are also charter and airline companies that show a (usually small) preference to ex-instructors—at least partly because they make promising future instructors (i.e. - training captains, company check pilots, and various pilot/manager positions).

The bottom line here is that if you’re just instructing to get hours and move on, it’s a bad idea. Even if you ignore ethical and professional considerations—such as the instructional commitment and quality of instruction received by your students—you will be miserable. Instructing, done right, is hard work. It’s very rewarding for those who have a genuine desire to teach, but it’s tedious and frustrating to those just building hours. Between the classroom, simulator, and administration (i.e. – paperwork), your actual flight time is less than half of your work. Again, this really isn’t a problem for those who want to teach. But if you just want to fly and don’t care so much about the development of others, this can be a painful existence.

So if you want to instruct and believe you will find it rewarding, do it! If not, move on. Get an entry level position towing banners, doing pipeline patrols, or working the dock in anticipation of flying floats (which will require you to get float experience first—do that as part of your CPL!).

Happy Flying!