Instrument Flying by the Numbers

Instrument flying – flying an airplane on an IFR clearance – consists of three skills: 1) basic instrument flying 2) radio navigation, and 3) operating within the IFR system.

Basic attitude instrument flying: the ability to achieve a given performance – airspeed and vertical speed – quickly and precisely by reference to instruments.

Here we are going to talk about the first of these, sometimes known in the literature as basic attitude instrument flying. Call it what you will, we are talking about the ability to keep the airplane right side up solely with reference to instruments when necessary. More expansively, the ability to quickly and precisely make the airplane do what we need it to do using the instruments in the panel and the power and configuration controls (flaps, gear, etc.)

The pitch vs. power chart: Vy climb / cruise climb / cruise / cruise descent / approach / approach descent. Power (rpm, manifold pressure), configuration (flaps, gear), airspeed, vertical speed defined for each of the above.

Any book on instrument flying will have, somewhere in its early chapters, a chart of a certain type. The presentation may differ, but the principal is always the same. It will be a chart listing the power settings, configurations, and pitch attitudes required to achieve a given set of performances from a given airplane. What performances? The basic airspeeds and vertical speeds required to complete an instrument flight (or any flight) from takeoff to the conclusion of an instrument approach: Initial climb, cruise climb, cruise, cruise descent, approach, and approach descent. The performance targets are usually listed in a column down the left side of the chart, the power settings, configurations, airspeeds and vertical speeds in a row along the top. The idea is two-fold: to impress upon the reader the need to learn and fly by these numbers and to provide a template for deriving these numbers for any given airplane.

We fly by the numbers because we do not have time to fly any other way.

In casual, VFR flying, without time or distance constraints, it may be possible, although clumsy, to control the speed and vertical speed of the airplane by trial and error – but such is way too tedious for instrument flight where the workload can be high and the demands on the pilot’s attention can be immediate. When we are steaming along at 4000 feet and 180 knots in an airplane with an approach speed of 120 knots and the approach controller says, “Cessna 34567, four from HICUP, turn left heading one-two-zero, maintain three thousand three hundred until established on the localizer, cleared for the ILS 9 approach, report established,” it is not the time to be wondering how to get the airplane to go down and slow down

It is possible in a simple airplane to make it go up by pointing it up and to make it go down by pointing it down, but it is a misunderstanding of aerodynamics; it is an inelegant way to fly; and under some circumstances it will get you hurt.

So, what do we do? We know the numbers – the power setting, the attitude, and the configuration that will achieve the performance we need. Instrument flying is not about the little round things in the panel, nor is it about the multi-colored, dazzling glass panel of which we are so proud. It is about a fundamental concept: power plus attitude equals performance.

Real horizon vs. artificial horizon.

When you learned to fly in a 152 and your instructor taught you that if you set the power at 1900 rpm and put the top of the cowling on the horizon you will be in level flight at 70 knots, you were having your first lesson in instrument flight. You were setting a predetermined power setting (1900 rpm) and a predetermined attitude (cowling on the horizon) to achieve a predetermined performance (level at 70 KIAS). You were simply using the really good attitude indicator, namely the horizon, instead of the crappy little attitude indicator in the panel. Back in the day, we called that crappy little attitude indicator the artificial horizon because it was in fact a poor but necessary substitute for the real, much more effective, horizon. (When I denigrate this instrument as crappy, believe me, I mean no disrespect. The invention of this gyroscopic instrument has made instrument flight possible and vastly expanded our flight horizons. I mean only to elevate the real horizon in our mind.)

Rather than provide these numbers, we will learn to derive them. “Give a man a fish; teach a man to fish.”

So, early on in the process of learning how to control the airplane on instruments, we will complete one of these charts I describe. I could simply hand you the numbers I have derived from many hours in our training airplane and tell you to memorize them, but I would rather work with you to derive them. Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime. Our aim is not simply to find the numbers for our airplane. Our larger purpose is to learn how to find these numbers for any airplane we may fly. In the process, we will learn much about aerodynamics and about the interrelationship of power and angle of attack. (We often refer to attitude as if it is angle of attack, but they are not the same thing. Pitch or attitude is a useful shorthand – “pitch up to slow down, pitch down to speed up” – but we must understand that it is actually something different and there are times when the distinction is important, even critical. More on that anon …)

The understanding of these aerodynamic principles and relationships is crucial because the numbers we derive serve merely to put the airplane near our given performance target, which may or may not be near enough and which we may or may not have time to correct. If we do need to achieve that precise performance and if we do have the time to make the correction, our knowledge of aerodynamics, learned in part from the process of deriving these numbers, will enable us to achieve the necessary precision.