DIMS?

Applying the "does it make sense" question to your flying

August 1, 2006

I was never the kind of student who aced everything, especially anything to do with math. Ironic, given that my grandfather was a mathematician who worked for NASA at one time. I could work baseball statistics with no problem, but most everything else left me frustrated and angry. The proof in the pudding came when I took a high school physics test and used the wrong formula to figure out the velocity at which a pingpong ball would fall if dropped from a building. I came up with something like 150 mph. I didn't pay any attention to the units of measurement because, after all, I had used the right formula. Who could go wrong with that?

The test had been given about a week before scheduled parent-teacher conferences, and so the teacher held on to the test to show the parents. When my parents got home from the conference, my mother told me about the answer I had come up with, and she said the teacher had told her that I needed to develop the habit after answering a question on a test of looking at the original question and asking myself if the answer made sense.

That was some of the best advice I had ever received, and from that day to the present, I apply it not just to physics, but also to just about every problem-solving venture I've encountered since. How easy it is! Does this answer (or course of action) make sense? Why didn't anyone give me that wisdom before it was too late to try to get into Harvard? Ugh!

As a pilot, I use that question all the time, and it's something that can be applied to all aspects of aviation, from being a student to being an airline transport pilot.

What follows are some common areas when does it make sense (DIMS) can save your bacon. Hopefully, DIMS will catch your dimly lit light bulb and get it shining again.

Clearances. If you are on the ground, time is on your side. In flight, it isn't. But in either case, the time will come when the good folks in the tower will throw you a curve ball. On the ground, you may request a clearance on your IFR flight plan and get something that has you flying willy-nilly all over the sky — or so it seems. Sometimes it's as simple as the controller reading you the clearance for another airplane, and sometimes it's possible that you made an error when you filed your flight plan on DUATS. For example, you want to fly from Knoxville, Tennessee, to Louisville, Kentucky, which has the airport identifier of SDF. Transpose the identifier for Louisville to FSD and you are going to get a clearance to Sioux Falls, South Dakota. The mistake may not be one so obvious, so if your clearance deviates from your original flight plan, look at a chart and ask yourself, DIMS?

Taxi instructions. It's hard to imagine anyone not being aware of the FAA's recent war on runway incursions. The potential danger of a runway incursion cannot be overstated. The problem is that it is so easy to do, and it isn't limited to one segment of aviation. General aviation, the airlines, airplanes large and small are all contributing to the problem. It's often compounded by being on new airports (to you) or large airports, being in a hurry, or having to use taxi diagrams that are difficult to read.

Take the time when given a taxi clearance to look at the chart. If that means being stationary for a moment or so, don't sweat it. Even the most frazzled controller would rather you be later and straighter than have you risk an accident. So would the pilot on short final to the runway you might incur. If you can't find the taxi route on your diagram, or if the controller appears to have left something out, ask, don't guess. Never just acknowledge a clearance without a DIMS check and chart verification. This goes double when taxiing on an airport in the midst of any sort of construction.

Weight and balance. This sounds relatively simple. You add some numbers, do a bit of division, and presto, you have the center of gravity (CG). But think about how easy it is to make a math error, especially when you're in a hurry. That's why we need a pencil to keep the balance in our checkbook. The hidden mistakes are most often the most obvious: transposing numbers, not carrying the 1 in addition, putting the decimal in the wrong place, or using the wrong arm for one of the seats or fuel. A student of mine once got a CG of 8.9 when it should have been 89. It's easy to assume that the decimal was in the wrong place, but in fact a couple of major errors were in the math. DIMS to the rescue. It's best in such a situation to just start over on a fresh sheet of paper.

This becomes more important as you move into bigger equipment capable of carrying four or more passengers. Close attention must be paid to who sits where, and it may be necessary to calculate the weight and/or CG not just for the takeoff, but for the landing as well to accommodate the change in fuel burn. Double DIMS.

Basic instructions. On a recent trip I departed Runway 27 in Cincinnati, heading to Burlington, Vermont. Departure control gave us instructions to join the departure that heads to points west, such as Omaha and Minneapolis, with a climb to 13,000 feet. This would not be the first time that a crew forgot where it was going or even went so far as to program the flight management system (FMS) for the wrong city. A quick check of our paperwork confirmed our destination, and the controller acknowledged the mistake.

Sometimes, though, the disparity isn't so obvious, and you need to make sure that you and the controller are on the same page. This is especially true if you are unsure of major heading changes or altitude changes.

Weather evaluation. One of the challenges of flying is being able to create a mental picture of what the weather is doing around you. When controllers broadcast airmets and sigmets, you should be able to picture what the weather is doing in the described geographical area. When you are evaluating weather, the most critical skill is to be able to understand why the weather is doing what it is doing, both in the macro sense and the micro sense. If you can't answer the DIMS questions about the weather, then you need to spend some time brushing up on it; weather is everything to a pilot.

Asking the simple question of whether something makes sense sounds so elementary that it seems almost trivial. But if you take the time to ask, no matter how obvious the situation may seem, you can save yourself from unwanted attention and embarrassment. If the information is not complete or in a form you can understand, then take the time to fill in the blanks.


Chip Wright, of Hebron, Kentucky, is a Canadair Regional Jet captain for a major airline.