by A. Howard Hasbrook
There's an old saying that "a good landing requires a good approach" and conversely a poor approach means a poor landing. ..." and basically these are correct. The approach is the primary key to putting the airplane on the ground where the pilot wants it, at the right speed and in the proper attitude.
What is the "secret" of making a good approach? It's keeping the airplane on a constant - angle descent, with the flight path lined up with the centerline of the runway and intersecting the runway near the desired touchdown point. That path also must be of sufficient length to give the pilot enough time to make the necessary corrections to get "her in the groove" before it's time to flare and to recheck his gear - down-and-locked lights.
To keep the airplane on a straight flight path to the runway requires control of several variables. These are aircraft speed (horizontal and vertical) and heading (usually, control of the latter is complicated by crosswind). The need for speed control cannot be overemphasized. It is absolutely essential, not only from the point of view of making precise landing approaches, but also in relation to providing adequate aircraft performance and control during routine and emergency situations.
We have seen more approaches - and subsequent landings - messed up by poor airspeed control than by any other factor. This is because changes in airspeed result in changes in lift, groundspeed, vertical speed, flight path profile, flight path angle and in the subsequent point of touchdown. Changes in airspeed usually occur because the pilot fails to maintain a constant pitch angle for a given thrust condition. Such pitch changes, regardless of whether they're pilot induced or produced by rough air, cause changes in lift, and in vertical speed, resulting in an undulating flight path.
We have also seen excess speed creep into the approach - increasing the chance of overshooting - simply because the pilot failed to realize that a constant throttle setting will result in a constant increase in power as the aircraft descends into more dense air ... a thousand foot decrease in altitude will increase manifold pressure about 1 in., producing a significant increase in thrust. Hence, to maintain constant speed, thrust (not throttle position) should be held constant along with constant pitch attitude.
Once a pilot has the proper airspeed and thrust numbers memorized and nailed down, he can devote the bulk of his attention to analyzing the essential visual cues for maintaining the desired flight path to the runway.
Although most pilots are not consciously aware of these cues, the decreasing distance between the top of the runway and the horizon, the uniform visual widening and lengthening of the runway triangle, the straightness of the runway image - or its lean - and the speed and direction of flow of the intervening terrain toward and past the pilot (as seen in his peripheral vision) are all used to evaluate progress of the approach. And, this evaluation capability can become extraordinarily efficient through practice, so long as the pilot knows what to look for. For example, he can, through psychological reinforcement during daytime approaches, come to associate decreasing altitude with the illusion of increasing groundspeed seen through the sides of his eyes. As the terrain flows past the aircraft, the more rapid the flow, the lower he expects his altitude to be. Conversely, a decrease in flow speed will mean a higher altitude.
Thus, during an approach in a strong headwind, a pilot may inadvertently descend below the proper glide path because of his impression he is too high. (The reverse of this visual illusion can occur during a downwind turn at low altitude. This has caused pilots to pull up - and stall - because the increase in ground speed gave them the impression they were losing altitude.)
Constant widening of the runway outline, as a function of decreasing distance between the aircraft and the runway, is another important cue used by the pilot to assess the correctness of his approach (see Figure 1). However, to be used effectively, it has to be combined with the progressive vertical lengthening of the runway image, as well as with the decrease in the vertical distance between the horizon and the top of the runway. Since these are the only cues available at night, it is probable that the functional relationship of these cues are the ones which the pilot most often used to maintain a straight line descent to the runway.
Unfortunately, not all runways are of the same length and width - varying in length from 1500 to 15,000 ft. and in width from 50 to 200 ft. Since possible combinations of runway width-to-length are many, it becomes apparent that the use of runway outline cues is not a simple task, because of the various ratios involved. For this reason, pilots should be very cautious, and very alert, when making an approach to an unfamiliar airport, particularly at night (see Figure 2).
Expansion theory (illustrated in Figure 5) relates to the apparent outward movement of terrain from the point where the flight path intercepts the runway. The author uses the no-vertical motion area in the same way to determine the intersection point. Everything above this area appears to move toward the horizon, everything below it toward the aircraft or obliquely downward. The consecutively larger trapezoids illustrate the appearance of the runway as it grows uniformly larger as the aircraft flies nearer on an ideal approach path. The appearance of the runway from consecutive points on an actual oscillatory flight path is illustrated in Figure 1.
As noted before, with constancy of airspeed, pitch attitude and thrust - and of wind - the flight path, and its angle, will remain constant. This, in turn, will also result in a constant rate of change in the angular and dimensional relationship of the runway image. This constancy, as well as that of increase in apparent ground speed, are valuable visual cues.
If a pilot has difficulty in flying consistently good approaches, he may need to look more attentively for these cues. One way is by investigating the runway scene visually while a pilot companion flies a series of approaches from the right seat - using straight flight paths as well as others with rather wide vertical and horizontal variations - until the observer becomes visually aware of the rate and size differences in the appearance of the runway during the correct and incorrect types of approaches. Without the distraction and responsibility of flying the airplane himself, the visual variations in rates of change of runway size, and angular spread and changes in ground flow velocity should soon become vividly apparent.
Another problem that some pilots encounter is that of trying to visualize the proper flight path angle to the point of intended touchdown.
On numerous occasions, we have seen private and commercial pilots start their descents so far from the airport that the flight path if continued at the same angle would have intercepted the ground a mile or more short of the runway.
When approaches are made from two or three miles out, an error of only a few degrees in the flight path angle will result in large under or overshoots. As illustrated in Figure 3, if the approach path is begun at about 525 ft. above airport level at a distance of 10,000 ft. from the runway, a 2 deg flight path, if continued without correction, would put the touchdown point almost 1/2 mi beyond the far end of the runway. On the other hand, a 4 deg flight path angle would put the aircraft into the ground 1/2 mile short of the runway unless a correction was made soon enough. At night, incidentally, the surrounding terrain cues showing need for such a correction are often quite meager and almost undetectable - which could account for the rather large incidence of VFR night landing approach accidents, in which aircraft hit short of the runway.
Thus, it is obvious that the pilot must be able to determine his flight path to the desired spot without having to make corrections later on in the approach.
Some pilots say they use a spot on the windshield as a form of gunsight to initiate and then hold a constant flight path angle. However, if one examines this technique in detail, some of its problems become readily apparent. For example, unless one is operating in extremely smooth air, aircraft pitch angle will normally vary a few degrees with average pilot handling. At a 30 inch distance (windshield spot to pilot's eye) only a 2 deg change in aircraft pitch will involve about a 1/2 inch movement of the spot; a movement that would be most difficult to nail down against the runway in any kind of turbulence - which exists almost constantly near the ground on warm or windy days. In addition, vertical movement of the pilot's head and eyes add error to this method. Thus, using a windshield spot as an aiming device can easily cause the pilot to overshoot or undershoot, since a one degree error can put him half a mile or more on either side of his intended landing spot.
In choosing an approach angle best suited to ordinary conditions, it should be kept in mind that the glide paths in most VASI's and ILS's are set at 2 1/2 deg to 3 deg. Approaches made at these angles with conventional, fixed-wing aircraft result in airspeed and vertical speed envelopes that provide adequate control, reasonable landing gear loads at touchdown and safe rollout distances. Therefore, unless one is contemplating operating into very short strips over high obstructions, it would seem desirable to use about 3 deg flight path angle during visual approaches as a routine matter, so as to develop a constancy of visual reinforcement from the cues used during previous landing approaches. However, it should be remembered that all runways are not necessarily level (see Figure 8).
To obtain full value is developing and acquiring such cue reinforcement, it follows that approaches of reasonable length should be employed. Obviously a pilot doesn't need four or five mile long airline type approaches, but neither should he bend her around onto final right over the threshold unless he really knows his plane, the approach terrain, and also wants to scare his passengers. If he uses an approach speed of 90 mph, a 2 mile approach in calm air will provide about 1 min 20 sec to get everything squared away before touchdown - not very much time, particularly at a strange airport, where there are no familiar cues to help unravel the situation. A simple way to set up a 3 deg glide path entry is to start the approach descent 2 mi out, at an altitude of about 550 ft above airport elevation. However, if a 1 mile approach is desired, the pilot can cut the figure in half and line up with the runway at an altitude of about 275 ft; a constant descent rate to touchdown from these altitude points will follow a 3 deg slope.
Another factor that can sabotage the best intended approaches and landings is crosswind and crosswind is a fact of life most of the time, regardless of how many runways are available. Crosswind during an approach can be handled by using one or a combination of several methods. One method, of course, is to set in the required crab angle. The difficulty with this type of wind correction is that variations in wind, as altitude diminishes, require constant changes in heading. And changes in heading, take appreciable time, time which may not be available.
Another method is to use sufficient slip (toward the windward side) to compensate for crosswind drift. This requires less time for heading changes but requires a fairly high degree of proficiency in cross-control. This technique is favored by many pilots because it keeps the airplane's longitudinal axis (centerline) lined up with the runway, and requires no last second de-crab maneuver just before touchdown. (The slip method also saves tires and helps keep the windward wing down.)
However, regardless of which method is used - and sometimes, in conditions of heavy crosswind, a combination of both must be used - the amount of drift correction required can be detected easily by visually noting whether the aircraft is aligned with the theoretical centerline of the runway. A visual cue that may be used to detect alignment relates to whether the runway image leans (see Figure 4) to either side of vertical, or stands straight up. If the runway leans the approach path is not in line with the center of the runway - and sooner or later, an "S" turn will have to be made in order to land on the runway centerline.
Visually determining where the flight path intersects the runway can be difficult unless one knows where and what to look for. Some instructors refer to it as the center of the expansion pattern ... an area of no movement around which all portions of the terrain and runway expand or move outward (Figure 5); in our study of the subject, the interception point seems to lie in an area above which the runway seems to move vertically toward the horizon and below which it expands toward the approaching aircraft. Essentially, it is an area of the runway that has no apparent vertical motion. Once a pilot has become consciously aware of this visual cue and can use it with some degree of accuracy, the chance of over or undershooting decreases.
However, for those who have difficulty in seeing this no movement area, the old timehonored technique of noting whether the runway threshold raises or lowers with respect to the aircraft's nose may be used to obtain a rough estimate of whether one is under- or overshooting. This procedure, of course, requires a constant pitch attitude as well as a constant (fixed) location of the pilot's eyes in relation to whatever portion of the airplane he may be using as a reference point. For example, stretching upward to see over the nose can change the pilot's vertical viewing angle by several degrees, comparable to the visual effect of changing the aircraft's pitch attitude by a like number of degrees.
During poor visibility conditions and particularly during night approaches, a pilot can make doubly sure he doesn't underrun his glide path by checking the altimeter and the vertical speed indicator periodically. He should set his own VFR minimums relative to the airport elevation, making sure he doesn't hit the 50 ft mark until he's over the runway threshold. Also, using a descent rate not in excess of 400 to 500 fpm helps to prevent an inadvertently steep flight path. Even on clear, but moonless nights, an approach into a black hole airport out in the boondocks can be extremely hazardous unless the flight instruments are scanned systematically until reaching the runway - because of the visual illusions involved.
Flaring the airplane (gradually rounding out the flight path to one that is parallel with the runway) is not difficult if the pilot knows where the ground is. If he doesn't, he's in trouble. Some student and private pilots try to overcome this lack of knowledge by driving their plane into the runway, which is hard on the nose gear, and eventually on the pocketbook.
Flare cues are primarily dependent on the angle at which the pilot's central vision intersects the ground (or runway) ahead and slightly to the side (see Figure 6). Unfortunately, the why of this intercept angle is not very well understood. However, it's been demonstrated in tests that if a pilot looks constantly at the far end of the runway during his intended flare, he may not flare at all. This is probably because, for example, a vertical distance of 10 ft between his eyes and the ground only subtends an angle of one eighth of a degree, measured at the end of a 5000 ft runway, and his eye has difficulty resolving (seeing) changes in such a small angle. To detect a left variation in vertical distance between his wheels and the runway, would then require his visual detection of one-eightieth of a degree change in angle - an impossible task!
On the other hand, if the pilot looks at the runway at a point too close to his plane, he'll see nothing but a blur of passing runway surface or he'll have the illusion that he's lower than he actually is. In either case the aircraft will probably drop in hard as it runs out of flying speed.
Although many pilots think that flare and landing cues are primarily dependent on two eyed (binocular) depth perception, the cues used most are those related to changes in runway or terrain perspective and to changes in size of familiar objects near the landing area, such as fences, bushes, trees, hangars, and even sod or runway texture.
With a little practice, monocular (one-eyed) vision works just as well as the two eyed variety in putting an airplane down safely - and smoothly. For the disbelieving, it might be interesting to note that - according to current FAA medical records - 4005 one-eyed persons hold valid FAA pilot certificates. Of these, 75 had first class medicals, 674 held second class and 3256 held third class (student or private pilot) medical certificates, and their safety record is just as good as that of their two-eyed brethren.
Many pilots who have good success in flaring at the proper altitude and maintaining their wheels a few inches above the runway until eventual touchdown do so by directing their central vision at a shallow downward angle of from 10 to 15 deg toward the runway. As shown in Figure 7, maintaining the same viewing angle causes the point of visual interception with the runway to move progressively rearward toward the pilot as the airplane loses altitude; this rate of rearward movement may be an important cue in assessing the rate of altitude loss. Conversely, forward movement of the visual interception point will indicate an increase in altitude, and would be interpreted to mean the pilot had increased the aircraft's pitch angle too rapidly, resulting in an overflare. Location of this visual interception point in conjunction with assessment of flow velocity of nearby off - runway terrain, as well as the similarity of appearance of height above the runway ahead of the aircraft (in comparison to the way it looked when the aircraft was taxied prior to take-off) may also be used to judge when the wheels are just a few inches above the runway.
To recap - consistently good landings require constancy in flight path angle and airspeed. To attain this consistency, keep alert to the visual cues that are necessary to the task ... if a pilot's having trouble with his landings, it's a sure bet he's not looking in the right place at the right time.
This article, which originally appeared in the Business and Commercial Aviation Magazine, August 1971 issue, has been reprinted courtesy of BCA and with permission of the author, A. Howard Hasbrook. Although the original article appeared in 1971 the message it conveys is still valid. Mr. Hasbrook's flying career began in the mid 1930's and covers the broad spectrum of aviation from cubs to jets including sales, flight test, crop dusting, airline flying, flight instruction, check pilot, inventor, designer, accident investigator, research pilot and writer.
Visual cues - you may not even be conscious of them - are what guide you to a touchdown, and they can be deceptive if you don't know how to read them
The purpose of this series of publications is to provide the flying public with safety information that is handy and easy to review. Many of the publications in this series summarize material associated with the audiovisual presentations used in General Aviation Accident Prevention Program activities. Many of these audiovisual presentations were developed through a cooperative project of the FAA, the General Aviation Manufacturers Association, and Association member companies.
Comments regarding these publications should be directed to the Department of Transportation, Federal Aviation Administration, General Aviation and Commercial Division, Accidents Prevention Staff, AFO-806, 800 Independence Avenue, S.W., Washington, D.C. 20591.