Principal Air - Flight Training / Charter in Canada

Principal Air - Flight Training / Charter in Canada, Learn to Fly


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Abbotsford International Airport
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The Glide

“Touch us gently, Time! Let us glide adown thy stream.
Gently, - as we sometimes glide through a quiet dream!

- Barry Cornwall – 

Gliding is a manoeuvre rarely practiced by most of us once flight training is completed. Even during pilot training, gliding is often dealt with in a somewhat perfunctory manner. The forced approach exercise is, by far, the most commonly failed item on flight tests according to Transport Canada statistics. I certainly notice when checking out even very experienced pilots that gliding often proves a challenging endeavour.

Understanding the basic mechanics of the glide and incorporating gliding practice into our routine as a pilot can improve both skills and safety. Competence requires two factors: training and practice. Hopefully, each of us will have had adequate training at some point in our pilot careers. Practice to maintain and improve skills once training is completed is up to each one of us. The thought of having an engine failure 10 years after the last time I made an opportunity to practice gliding doesn’t instil confidence.      

In the not too distant past, power-off landing approaches with light aircraft were standard practice and provided gliding practice on each landing. The reliability of aircraft engines has improved greatly and, with both light and heavier aircraft, the power-on, stabilized approach has become standard. This practice has made landings a more certain endeavour, but it has reduced the opportunity for pilots to practice gliding on a regular basis.

For the most part, we have to consciously seek out opportunities to practice this very important skill to keep ourselves current and competent.

The three major factors we can consider as affecting glide are aircraft weight, angle of attack, and wind. Aircraft weight is an interesting factor in relation to gliding. We discussed this aspect last month when we talked about descending.

In normal flight, there are four forces acting on an aeroplane: lift, weight, thrust, and drag. In a glide, we have the unique situation of dealing with only three forces: lift, weight, and drag. Thrust is absent. In a true glide, our engine is stopped; no thrust is being produced.

If we draw the little triangles to help understand the forces active in a glide, we notice the aircraft assumes a nose down, descending path. Lift acts upwards at right angles to the direction of flight; drag acts opposite and parallel to the direction of flight; weight acts at right angles to our altitude, directly between the centre of gravity of the machine and the centre of gravity of the earth.

A portion of the weight vector, what we call its horizontal component, acts not as thrust but as though it was thrust, parallel and in the same direction as our line of travel. The steeper our angle of descent, the more weight provides assistance as though it was thrust, overcoming drag and increasing airspeed. The greater the weight of the aeroplane, the more assistance we derive. In a vertical dive—hold on to your hats—all our weight would act as though it was thrust.

Our glide ratio, the ratio between distance over the ground and altitude lost, is equal to our lift-to-drag ratio (1). In order to maximize the distance we travel over the ground in a glide, we must achieve the best possible lift-to-drag ratio (2). We may remember something similar from our study of flight for maximum range where we also must achieve the best lift-to-drag ratio (3).

The angle of attack for best lift-to-drag ratio is fixed. Since lift must equal weight in a steady state, non-accelerated manoeuvre, the only variable left for us as pilots to manipulate is airspeed (4).

Very few aeroplanes are fitted with an angle of attack meter so we use airspeed as our primary source of information regarding angel of attack. When gliding, we achieve best lift-to-drag angle of attack at different airspeeds depending on the weight of the machine. A heavier machine will achieve best lift-to-drag at the same angle of attack as a lighter one but will do so at a higher airspeed.

Regardless of weight, two identical machines starting at the same altitude in the same environmental conditions will arrive on the ground at the same spot, having traveled exactly the same distance over the ground. The heavier machine, however, will arrive more quickly. It’s sort of counterintuitive, but there it is and we have the math to prove it.

Wind is a very important factor when gliding and should be considered carefully when making a plan to arrive at a given point on the surface. We all know wind affects ground speed, how fast we travel over the surface, but does not affect airspeed. As William Kershner points out, in the event of an emergency, we will probably be better off not overcomplicating an already complicated situation (5). However, increasing airspeed slightly when gliding into a headwind or reducing it slightly when gliding with a tailwind will increase distance traveled over the ground.

An easy Rule of Thumb to keep in mind is to increase glide speed into a headwind by one-half the headwind speed. In most cases, particularly if there is any turbulence, it is not practical to slow the aircraft down very much in a tailwind as we are already flying at low speed and do not wish to risk a stall.

With a headwind, time is the enemy. We want to minimize the amount of time the headwind is working against us. With a tailwind, time is our friend. 

In an emergency, a better plan would most likely be to pick a landing site that does not require us to get every possible inch out of our glide if that is an option.

There are two basic types of glide most often considered: the minimum sink rate glide and the maximum distance glide. Gliding for minimum sink rate is used to achieve maximum time in the air at the sacrifice of distance. This can be a useful manoeuvre in particular situations. You might find yourself without power over flat ground, water, snow or a marshy area where altitude is difficult to judge and arriving at a particular point is not be a priority.

The speed for minimum sink rate glide is closely related to the speed for maximum endurance. The match is not exact because, even at the minimal power setting for maximum endurance, the propeller is providing thrust and increasing the airflow over the inside portion of the wings and the aircraft’s tail section. With power off, the thrust and slipstream produced by even minimal power is absent and the propeller itself, if it is allowed to windmill, creates significant drag.


The minimum sink rate glide has both the advantages and disadvantages of low airspeed. On the plus side, we will have as much time as can be obtained to assess our situation and take positive action and we will arrive at the surface at as low an airspeed as possible. This can be of great benefit to our survival: impact increases with the square of the speed at contact. When gliding to land, perhaps at night or in low visibility, on a surface that may be difficult to assess, low speed is all to the good. Float pilots may be able to relate this process to glassy water landings when it is challenging to assess altitude above the surface prior to contact. The key is to establish a low and stabilized rate of descent at low airspeed consistent with maintaining control of the aeroplane and a low rate of descent.

On the down side, control problems may easily arise in turbulent conditions necessitating a higher airspeed which reduces time in the air.  

A basic Rule of Thumb for the minimum sink rate glide for single engine, fixed gear aeroplanes is to use 1.1 x Vs (KCAS) factored for weight (√aircraft weight/gross weight). Not something we would want to calculate moments after the engine quits on a dark night over the ocean, but something we could spend a moment with on a rainy day in the comfort of our living room or office and perhaps record on our checklist for future reference in the event of need.

The maximum distance glide is probably a more commonly used number. It is closely associated with the speed for maximum range, but, as with the minimum sink rate glide, is not exactly the same. Your machine’s POH will normally provide a generic number you can learn and use in the event you want to practice or must execute a glide prior to a forced landing.

The basic Rule of Thumb for the maximum distance glide for single engine, fixed gear aeroplanes is to use 1.3 x Vs (KCAS) factored for weight. Once again, I can’t see myself pulling out a calculator in the event of an engine failure, but playing with the numbers in a safe place where time is not a factor does seem to help increase appreciation for where those numbers come from.

As an example, a quick look in the POH for our 1976 C-172 tells me “best glide speed” is 65 KIAS (66 KCAS) with flaps up and 60 KIAS with flaps down. Going back to the Rule of Thumb, assuming gross weight, I calculate glide for maximum distance with a most forward C of G as 65 KCAS. With a most aft C of G, it calculates as 68.9 KCAS.

Averaging the two numbers comes out to 66.95 KCAS, call it 67 KCAS. Pretty close to Cessna’s numbers. Just for the fun of it, I can factor for weight and find, at 2100 lbs., maximum distance glide will be executed at 64 KCAS (rounded off); at 1900 lbs., it is 61 KCAS (rounded off). Not terribly significant differences, but there they are.

On a practical note, practicing the glide will keep required skills and techniques current and provide an opportunity to improve those skills. It also allows us to develop a greater sense of confidence in performing the manoeuvre should it become necessary or useful. Playing with the machine for practice when we are not faced with the stress of an emergency allows us to get a much more solid handle on the whole problem and develop a much clearer sense of what can or must be done to maximize the performance of a particular machine. 

A policeman doesn’t want to start learning how to use his pistol moments after being confronted by an armed felon; a school doesn’t want to start practicing fire drills immediately following the discovery of smoke billowing from the furnace room.

It’s an excellent idea to prevent your next gliding practice session from occurring just following an unexpected stoppage of that little fan up front years after the last time you tried it out. How much fun does a person really need, after all? Enjoy.


  1. Distance/Altitude = Lift/Drag. The sides of similar, right triangles are always proportional.
  2. L = ½ CLpV2S; D = ½ CDpV2S. L/D = CL/CD, all other factors cancel.
  3. The coefficient of drag is equal to the sum of the coefficient for inducted drag (CDI) plus the coefficient of parasite drag (CDP). The CDI changes with angle of attack. The CDP changes with the shape of the aircraft, for example if we deploy flap or landing gear. There is only one angle of attack in any configuration that produces the maximum number for the L/D ratio.
  4. L = ½ CLpV2S. The variables over which we have control as a pilot are the Coefficient of Lift, CL, which we change be adjusting angle of attack, and the airspeed, V. Air pressure, p, at any given altitude, and wing surface, S, are fixed.
  5. Kershner, William K., The Advanced Pilot’s Flight Manual, Iowa State University Press, 1994, page 111.