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.

Thursday, January 31, 2013

Flying Blind

My last post, about the ASI and static port blockages, was inspired by a practice problem in my under-development Commercial Pilot License: Written Exam Preparation. This post is also inspired by a product under development. This time it's a far-less-developed book about flight training exercises and scenario-based training.

When I was reviewing the current (very early!) draft of this book, the exercise that caught my eye was related to instrument flying. We fly by instruments when we lose visual reference because our bodies can't stay oriented in space and as a result we can't stay upright. This is why continued VFR flight into instrument meteorological conditions is one of the biggest killers in general aviation—loss of control in cloud is often fatal.

To help combat this source of accidents we expend great effort in teaching student pilots to avoid the conditions leading to spatial disorientation. To provide an extra last-ditch-hail-mary backup, we also provide PPL students with 5 hours of basic instrument flying training. To support this training, we often refer to the article "178 Seconds". This article was written in response to a study, conducted at the University of Illinios, on pilot survivability after loss of visual reference. The results were compelling. They're summed up nicely in the second paragraph of this article from Transport Canada:
How long can a licensed VFR pilot who has little or no instrument training expect to live after he flies into bad weather and loses visual contact? In 1991 researchers at the University of Illinois did some tests and came up with some very interesting data. Twenty VFR pilot "guinea pigs" flew into simulated instrument weather, and all went into graveyard spirals or roller coasters. The outcome differed in only one respect - the time required until control was lost. The interval ranged from 480 seconds to 20 seconds. The average time was 178 seconds -- two seconds short of three minutes.
Other articles on 178 Seconds can be found here and here.

So, what about the exercise I mentioned?

When students are first introduced to instrument flying, it helps to motivate the training if they understand the concept of spatial disorientation. Spatial disorientation is pretty straightforward on the ground. We have three sources of information: (kinesthetic, vision, and vestibular). As soon as we get airborne, the kinesthetic sense no longer provides useful information since angular and linear accelerations of the aircraft can have the same effect on our bodies as gravity (this is the Equivalence Principle of physics). This leaves us with visual and vestibular senses. That's fine. But if we lose visual reference, we are left with just the vestibular sense. The vestibular sense is easily confused when it isn't cross-referenced to at least one of the other two orientation senses.

So in a cloud, for example, we lose visual reference and subsequently lose our sense of up-vs-down. This is what ultimately leads to fatal accidents such as those that killed JFK Jr., Rocky Marciano, Jim Reeves, and Patsy Cline.

The problem with all of this is that most people have never experienced spatial disorientation. If you haven't experienced it, it's very difficult to imagine or visualize. Some flight instructors compare it to the sense of vertigo you get after spinning around. But it's really not the same thing. It would be useful if we could find a way for students to experience spatial disorientation in a safe and controlled environment. This is where the exercise I was reading about comes in.

Before commencing initial instrument training (ok, it doesn't have to be before, but that seems like the most logical time), we can simulate a loss of visual reference by having our student fly with their eyes closed. That may seem like a pretty nutty idea, but eliminating visual reference—which would be difficult in VFR conditions with eyes open—is necessary for our students to become reliant on the vestibular system alone, and to see where that leads.

So where does it lead? Well, that depends. This little simulation tends to not be as effective as actually entering cloud. Secondary cues such as the sun heating one side of your face, or shining through your eyelids help to maintain orientation and therefore reduce the effectiveness of this demonstration. So it's best if we can do this on an overcast day. There are also at least three ways to approach the exercise. The student can (attempt to) report flight maneuvers flown by the instructor. The student can (attempt to) fly straight and level. Or the student can (attempt to) fly maneuvers (a simple 180° turn works well).

All three of these work to demonstrate disorientation, but the third tends to be the fastest. Personally, I like to use all three, but some might argue that that's overkill. In either case, once the student is disoriented—which you will know because they are either reporting the wrong maneuver or conducting the wrong maneuver—have them open their eyes to get re-oriented.

Students are often entertained by this exercise (which is great for motivation), but more importantly, they develop a deeper understanding and more solid belief in disorientation and the effects it can have on them. This provides them with a pretty solid basis for moving into instrument flying. And it motivates them to avoid flying in weather that is beyond their capabilities once they are license and operating without supervision. In other words, this exercise could be a life saver.

Happy Flying!

Friday, January 25, 2013

Static Port Blockage - Airspeed

I've been working pretty hard lately on my next publication, Commercial Pilot License: Written Exam Preparation. The goal is to get it published in the first half of 2013. That's a pretty aggressive goal considering how much work is required, but I'm on it!

Today I was running through the book, reviewing and developing practice questions, and I stumbled across one that I expect to cause some heartache. I can see a long list of students and instructors contacting me to tell me that either the question is wrong or the answer key is wrong. So I think I'll just preempt that and look at the question here. The question is as follows:
While cruising at 3,500 feet ASL, your aircraft experiences a complete static source blockage. Upon commencing descent, you recognize the blockage due to the combined indications of the pitot-static instruments. The aircraft is not equipped with an alternate static source, so you continue your approach to your destination with the malfunctioning instruments. At your destination aerodrome, which has an elevation of 1,500 feet ASL, you decide to abort your approach and go around for another try. During the climbout from the go-around, your ASI will read:

           a) zero.                     c) high.
           b) accurately.            d) low.
This question is actually very straightforward (despite the wordiness!). It boils down to: "How does your ASI behave after a static port blockage?".

Why do I expect complaints about this questions? It comes back a a faulty mnemonic that gets used for this problem.
After a static blockage, the ASI will under-read in a climb and over-read in a descent. WRONG!
This statement is based on a faulty assumption, and can be misleading—for example in the present question.

Let's look at how the ASI works and how a static blockage affects indications. The ASI takes total pressure (which consists of both static and dynamic pressure) from the pitot tube, subtracts static pressure read from the static port, and gets dynamic pressure (the ½ρV2 portion of the lift equation). In order for this to work, both the total pressure readings and the static pressure readings have to be accurate.

Static pressure varies with altitude—climb and static pressure drops, descend and static pressure increases. Under normal conditions, the static pressure reading from the static port cancels the static pressure component of the total pressure from the pitot tube. However, in the event of a blockage, these two pressures can become mismatched, resulting in erroneous readings.

If the static port is completely blocked, the static pressure reading will remain fixed. As long as we remain at the same altitude, this really doesn't matter. But if we change altitude, the static portion of the total pressure (sampled at the pitot tube) will change accordingly, but the static pressure sampled at the static port will not change. This mismatch between the two static pressures is what causes the problems with the ASI.

If we climb to an altitude above the blockage altitude, the static pressure portion of the total (pitot) pressure will be reduced. This means that the (fixed) static pressure in the static line will overbalance the actual static pressure and cancel out part of the dynamic pressure. The result: an under-reading ASI. The opposite happens if we descend to an altitude below the blockage altitude. The static component of the total pressure will be greater than the (fixed) static pressure in the static line. The excess static pressure in the pitot portion of the system will be interpreted by the ASI as more dynamic pressure. The result: an over-reading ASI.

Note that the over- or under-reading of the ASI is not influenced by whether we are climbing or descending. The (incorrect) statement above about climbing and descending is based on the assumption that the climb/descent starts from the altitude at which the blockage occurred. The question, however, contradicts that assumption. If we eliminate the assumption, the new rule for ASI errors after static a blockage is as follows:
After a static blockage, the ASI will under-read when above the blockage altitude and over-read when below the blockage altitude. RIGHT!
Using the first (incorrect) rule above, we can refer back to the question and note that the aircraft is climbing. Therefore we expect the answer to be (d) low. However, if we apply the second (correct) rule above, we expect the answer to be (c) high. This is indeed the correct answer.

As a final note, this misunderstanding is potentially dangerous if you ever face this situation. If we think the ASI is under-reading because we are climbing, but it is in fact over-reading because we are below the blockage altitude, any attempt to correct can quickly lead to a stall scenario.

Happy Flying!

Friday, January 18, 2013

AviationChatter - Pat Flannigan

I recently discovered another aviation blogger who has some interesting posts to read. Pat Flannigan at AviationChatter writes some great articles, including Making Sense of GPS Approach Minimums and Five Ways to Fly for Free. I should note for my Canadian readers, the first "free-flying" method is not log-able here, even if you do manage to get into that coveted right seat. But the rest can work quite well.

Monday, January 14, 2013

Slots and Slats

In learning to teach, a set of principles that's useful is the Laws of Learning, originally developed by Edward Thorndike and then developed further by subsequent researchers. The laws of learning consist of eight1 laws that help us describe what makes people learn more effectively.

Primacy is one of these laws. Quoting from Wikipedia:
"Primacy, the state of being first, often creates a strong, almost unshakable, impression. Things learned first create a strong impression in the mind that is difficult to erase."
Bottom line: Teach it right the first time! This is a mantra that new flight instructors are drilled with. Unteaching/unlearning incorrectly-learned material is very difficult and inefficient.

What does all of this have to do with slots and slats? Well, I don't know what they are. Or, more correctly, I know what they are, but they are similar devices—one fixed in place and one extendable/retractable—and I don't know which is which. This is despite the facts that I've known of their existence and function for well over 20 years, and I've discussed my not-knowing with flight instructor candidates dozens of times—each time being reminded by my students of which is which, but it just doesn't stick.

Why not? PRIMACY! I was first exposed to the function and structure of slots and slats on an Air Cadet course in CFB Borden. The instructor was very knowledgeable and capable, but he had a very strong french accent. So when we got to the material about "slots and slats", he proceeded to teach us all about "sluts and sluts". At least that's what we teenagers heard through the accent...

A "slut" is a permanent opening in the leading edge of a wing that allows airflow to be redirected at high angle of attack, resulting in a higher maximum lift coefficient and lower stalling speed. A "slut", on the other hand, is an opening near the leading edge of the wing which closes at low angle of attack (high speed) but opens at high angle of attack (low speed), resulting in a higher maximum lift coefficient and lower stalling speed.

Do you see the confusion?

So, to you instructors out there, TEACH IT RIGHT THE FIRST TIME! And to you students, LEARN IT RIGHT THE FIRST TIME! In both cases, this is sometimes easier said than done, but it's well worth the effort.

1 – This number actually varies depending on what reference you look at. The FAA's Aviation Instructor's Handbook says there are six laws. Transport Canada's Flight Instructor Guide says there are seven, including Relationship, which I've never seen anywhere else (and which seems to be redundant, but that's another post).

Tuesday, January 1, 2013

Slip Slip Slipping Away

After my last—very long ago—post about being coordinated, and more specifically, measuring coordination, I though I might follow up with a post about being uncoordinated. So today I'm going to ramble about the use of a slip in flight operations.

What is a slip?
A slip is a condition of flight where the relative airflow is crossing over the fuselage with a side-ward flow. Slips are generally considered to be undesirable. They produce drag, they produce a rolling response, and they can result in all kinds of terrible things in the vicinity of a stall. Inadvertent slips can be caused by the yaw associated with the engine, by turbulence, or by aileron drag / adverse yaw.

Occasionally, we want to slip. There are really two scenarios that lead to this: the desire to produce drag (forward slip and slipping turn), and the need to correct for a crosswind during landing (sideslip). It's important to note that, aerodynamically, the forward slip and the sideslip are exactly the same thing. They differ in application and visual cues (due to the effect of wind).

Forward Slip
The purpose of a forward slip is to produce drag so the descent rate can be increased without a corresponding increase in airspeed. This is accomplished by creating a side-on airflow to the fuselage, thus reducing the streamlining of the aircraft and increasing drag. The drag being produced is parasite drag, so the forward slip tends to be more effective at higher speeds. However, as a practical matter, forward slips are normally done during an approach, so our ability to increase our speed without de-stabilizing the approach may be limited. A notable exception to this is the emergency descent.

Slipping Turn
A slipping turn is really just a forward slip with an increased bank angle or reduced rudder deflection. The effect is that the aircraft will turn while slipping (hence the name!). This allows us, for example, to start a slip to lose altitude on the base leg of an approach, and continue the slip through the turn to final. This can be especially useful in a forced approach scenario, where we have only one shot to get it right. But, as with the straight-ahead forward slip, it can be used during normal approaches to bleed off excess energy.

Side Slip
The sideslip is used to align both the aircraft and the flight path with the runway for an intended landing. Under crosswind conditions, coordinated flight demands that our heading and track are mismatched by the wind-correction-angle. This is not acceptable for landing since it will result in side-loading the landing gear, and the production of rolling and yawing moments on touchdown (in a taildragger, the side-loading of the gear can quickly evolve into a ground-loop). In order to align the heading with the flightpath, we must have some component of slip (i.e. - airflow across the fuselage).

Entering a Slip
The entry into a slip is where one of my pet peeves shows up—and is really what this post is about. Common wisdom (for example: the Wikipedia entry, the Wikihow entry, and a Flying Magazine article) has it that to enter a slip, you lower one wing and then apply opposite rudder to prevent a turn. Aerodynamically, this will work. But in flight operations, it has questionable usefulness. This is because in practical scenarios, we normally want to change our heading on entering the slip.

In a forward slip to lose altitude, we are initially established in coordinated flight on approach. This means that we are heading and tracking toward the runway. We would like to keep tracking toward the runway while changing our heading so that we are effectively flying sideways. Lowering the wing and then preventing a turn with rudder will not accomplish this. Instead, it will put us in a slip with a heading toward the runway, but a track that takes us off the approach. This is no good. If instead we enter the slip with rudder first to yaw the aircraft, and aileron second to roll into the slip and prevent further yaw, we will find ourselves still tracking toward the runway, but pointing elsewhere (which is exactly what we want!).

Similar logic can be applied to a sideslip to correct for a crosswind. In the initial approach, we are in coordinated flight with a wind-correction-angle (AKA - crab angle). So our track is toward the runway, but our heading is angled into wind. As with the forward slip entry, we would like to maintain our present track, but change our heading. So the entry is the same—rudder first and aileron second.
- - - - - - - - - -

So really there are two way to enter a slip: rudder-then-aileron, and aileron-then-rudder. Both control actions will result in a slip. However, in most practical applications, the rudder-first method is what we actually use. Note that by "most" practical applications, I mean to leave room for the slipping turn, which can indeed be entered aileron-first in practical applications.

Happy Flying!