Estate 1999

Questo lungo articolo fu pubblicato quasi vent'anni or sono sul Notiziario ANPAC, il periodico dell'Associazione dei Piloti, per iniziativa del Com.te Giovanni Riparbelli e fu da me riproposto in una pubblicazione tecnica interna di compagnia (Gruppo Alitalia).

Quando lo lessi la prima volta fui confortato da quanto espresso da J.S. Clauzel che confermava quanto avevo appreso con l'esperienza della realtà operativa di dieci anni di comando sul DC9-30. In seguito, su altre macchine di corto e di lungo raggio, fino agli ultimi dieci anni (prima di appendere il volantino al chiodo nel 1997) sul Boeing 747, ho avuto modo di verificare che il messaggio era sempre attuale.

Raccomando ad ogni professionista di leggerlo attentamente, di portareselo dietro e di rileggerselo e meditarlo, come il breviario per il prete.

Potrebbe salvare delle vite.

acpezzopane

 

AWARENESS OF STRESSES ASSOCIATED WITH APPROACHES AND LANDINGS UNDER MARGINAL CONDITIONS

di J.S. Clauzel

Flight Safety-DACO-1981

Abstract

While the total numbers of fatal accidents during approach and landing are decreasing, those occurring in marginal weather conditions appear to be increasing. Accordingly, this paper presents a comprehensive examination of the factors that contribute to this type of accident; namely, weather conditions, aircraft performance, airport characteristics, and flight crew human factors. Effects of control inputs as related to stall margins, visual illusions, and other phenomena associated with marginal weather conditions are addressed. Recommendations are offered on crew conditioning, with the position token that, during an approach, when there is sufficient doubt in the minds of the flight crew, a go-around should be viewed as a mark of good judgment. Further, it is recommended that each airline carefully review this area of operations with respect to training and policy

Introduction

The Operational Flight Safety group at Douglas Aircraft Company has been investigating several recent accidents, incidents, and near accidents which have occurred during landing attempts under marginal approach conditions. One of these was a major disaster, and the potential for more is there. None was associated with any aircraft system malfunction. Three of the incidents will be presented for purposes of illustration, although these are only a few of the many available.

It is apparent that common factors in each of these occurrences were abnormal touchdowns and the decision to attempt to land the airplane under marginal approach conditions. Although extensive research has been conducted on this subject and numerous papers written, it is apparent that we in the aircraft safety business still have much work to do. Based upon statistics which show that almost 50 percent of all air carrier accidents occur during approach and landing and that many more are very close calls, we must conclude that we have not yet reached the person in the cockpit in an effective way. As always, the key to an effective safety effort is cockpit awareness of the hazard combined with a concise definition of the problem and clear guidelines for dealing with it. While it is true that many of the most important decisions in the cockpit must be made in the proverbial split second, the decision arrived at is more likely to be the right one if the crew is properly prepared for it ahead of time.

In most cases, an accident, incident, or near accident during approach or landing occurs under marginal conditions because the flight crew elects to continue the approach even though things are not quite right, obviously hoping to catch up. It is the author's conviction that missed approach procedures are not being utilized when, in fact, weather, airport, aircraft, flight crew conditions, or a combination of these factors dictate that they should be utilized. Seldom is any single factor in aircraft operation the sole cause of anything. Rather, the entire complex, interwoven pattern must be considered. An event which occurs minutes or even hours before a landing may, in the final analysis, make the landing unsafe. This paper will review the various facets of marginal approach conditions, including the weather, the airport, the aircraft, and the flight crew, with the view that flight crews must be made aware of possible pitfalls and be encouraged to execute a missed approach when it is prudent to do so, rather than to unnecessarily risk a hazardous landing.

Statistical review of approach and landing fatal accidents

To assess the significance of the effects of weather on fatal accidents in approaches and landings, statistics from Flight International from 1975 through 1980 were reviewed. During this six-year period, there were 136 fatal accidents, of which 51 (37.5 percent) occurred during the approach and landing phase of flight. Of the 51 fatal approach and landing accidents, 10 were listed as being related to adverse weather conditions. These 10 accidents thus represent approximately 7.4 percent of all fatal accidents and close to 20 percent of all fatal approach and landing accidents. From the author's own compilations, we know that there were many more accidents and incidents in this category that were not fatal, plus even more very close calls.

Figure 1 shows the relationship between all fatal accidents and those fatal accidents occurring during approaches and landings. The approach and landing accidents are separated into weather and non weather-related contributory causes. The figure represents a smoothing of the data by using best-fit log-normal regression lines to enable an interpretation of the general trends of the data. This reveals some significant trends in fatal accidents and, more specifically, in approach and landing accidents. First, it appears that during the past six years the total number of accidents along with the approach and landing subset are decreasing. The number of non-weather-related approach and landing fatal accidents also appear to be decreasing, but at a greater rate. However, we now find that those fatal approach and landing accidents where weather is a factor exhibit an opposite trend and that rate is, in fact, increasing.

In the analysis of marginal weather-related approach and landing accidents, there are several major factors which should be considered. The first concerns the specifics of the weather conditions at the time of the accident; the ceiling, visibility, cloud cover, fog, wind shear, etc. The second factor is the aircraft. Were there any system malfunctions which altered the flight characteristics? Were there items on the minimum equipment list (MEL) which might increase workload? Were there basic flight or control characteristics which might influence aircraft performance in marginal conditions? The third factor concerns the flight crew. How well prepared were they to cope with the unique conditions they encountered? Did they have sufficient sleep the night before? Are they on medication, either prescription or non-prescription? What kind of marital, financial, or other social problems do they have? Then there is the airport at which the landing is to be made.

Runway length, width, grooving, approach aids, lighting, and other items each play an important part in an approach and landing under marginal conditions. These factors will now be analyzed in more detail.

What constitutes marginal approach conditions

The effects of adverse weather conditions confronting an aircraft have been well covered in available literature, and much has been written about frontal activity along with wind shear, rain, fog, scud, and the like. While these phenomena need not be described here, it should be recalled that weather can make the aircraft's environment not only hostile, but deadly. On one hand, weather can rapidly affect the entire air mass through which the airplane is moving, thereby changing the dynamic responses of the airplane; and, at the same time, it can produce marginal visibility, and further change the effective dynamics of the airplane through delays or misinterpretations in pilot ''feedback", especially during the critical maneuvering to land after sighting the runway during an instrument approach. And even with today's sophisticated weather satellites and weather measuring and advisory systems, the crew must still make the final decision of whether to land or go around.

There is little pilots can do about rapidly changing air masses, with respect to wind shear, for example, aside from avoiding the condition or carrying sufficient speed {energy) so that when the shear is encountered, with application of proper technique, the aircraft will continue to fly. Similarly, there is little a pilot can do about reduced visibility except to cope with it as best he can. Unfortunately, there is no practical method of measuring the most important parameter "slant visual range", which is the pilot's maximum visibility over the aircraft nose toward a ground-based object. This can be deduced from other parameters that are measured such as the prevailing visibility at the airport, ceiling, and runway visual range. However this leaves much to be desired, especially in scud or other non-homogeneous visibility conditions near the ''decision height", or minimum descent altitude. The runway could be in sight at the decision height, but the visual situation could change drastically in a moment so that when a pilot breaks out of overcast conditions a second time, he could find the visual approach slope indicator (VASI) reading in the red zone, and shortly thereafter hear the sounds of impact with the tops of trees. The moral of this story is: when in doubt, execute a missed approach.

Accurate, up-to-date airport weather information is vital for safety during landing operations.

The airplane's contribution to a marginal approach is, in some cases, obvious. Yet, in some cases, it is not so obvious - such as when dispatch with a particular system inoperative may later contribute adversely to the marginal approach. The basic avionics configuration of a particular model alone can be restrictive if the airplane's instrument flight capability is insufficient to provide the crew with the proper tools for the prevailing conditions.

Crew members' knowledge of the airplane's performance capabilities can also be an important factor, depending on their knowledge of the hazardous condition. In a severe wind shear, for example, we recommend that during a rapid decrease in headwind or increase in tailwind, which tends to decrease indicated airspeed, thrust and pitch attitude be immediately increased to maintain an acceptable airspeed and flight path. Power should be immediately advanced to the maximum allowable setting, and a missed approach should be initiated when this type of an encounter occurs at low altitude. Airspeed should be traded down to the stick shaker speed (but not below), if necessary, to prevent ground impact.

Turbulence is another reason for selecting higher final approach speeds for better controllability. Most airline operators require their pilots to modify their approach speeds so that in conditions of gustiness or turbulence, speed increments are added. Pilots are generally told to apply these increments above 1.3 Vs and to fly at the higher speed during the approach in order to minimize the risk of stall or loss of control as a result of turbulence, gust, or wind shear. The effect of control inputs during extensive maneuvering close to flare speed especially during turbulence, will be dealt with in more detail later.

In addition to turbulence, wake vortex -or more accurately, tip vortex- during an approach can greatly influence the approach path. A twin-jet passenger-liner on a training flight rolled completely over and crashed, fatally injuring four, as the result of an encounter with a wake vortex generated by a wide-bodied aircraft at Forth Worth. The mechanics of wake vortex generation are fairly well documented, and adequate protection should be afforded by proper separation of aircraft. Another form of turbulence, sometimes found very close to the ground, is generated by vortices shed off large buildings near runways. At some airports, new, large hangars have been built for wide-bodied jets, and even a light breeze can cause disturbing vortices to be shed into the aircraft's flight path. Pilots should be aware of the condition by becoming familiar with airport layouts where prevailing winds cause the problem, and determine how much more speed to carry on final approach in order to compensate for it.

Another factor in marginal approach conditions is the so-called visual illusion. This can adversely affect the pilot's perception of the approach scene. The touchdown distance can be greatly influenced by optical illusions which can take many insidious forms, especially on a visual approach to a sloped runway. The pilot approaching a runway sloped up-hill or even a level runway preceded by up-hill terrain, will have an above glide-path illusion that makes him think he is high -the normal glide slope will appear too steep- but he may, in fact, be low, flying a flat approach, and thus be exposed to a short landing. The opposite effect will occur during an approach to a down-hill runway, or over down-sloping terrain, and the pilot will tend to fly a steeper approach and thus be exposed to landing long or even overshooting the runway. Hazards near the runway may produce detrimental psychological effects if the runway is short, and if the pilot attempts to touch down close to the threshold, the danger could be increased.

Rain on the windshield can interfere disastrously with a pilot's assessment of his progress toward touchdown. Rain causes objects to be obscured by an overall blurring effect. Light dispersion through the layers of water on the windshield can cause the horizon to appear to the pilot as a gradient blur. Pilot assessment of touchdown is also adversely affected by runway lighting, color contrast, and other characteristics.

Thus, approaches to brightly lighted runways at night have resulted in touchdowns short of the runway. The effect is greatly increased in clear air or when the approach zone is not lit. Irregularities in runway surfaces can cause a runway to appear much shorter, especially if the pilot loses sight of the far end of the runway because of a ground rise between the aircraft and the far end. When the color of the runway approximates the color of the surrounding terrain, problems in depth perception occur.

Examples are the snow-covered runway and the dimly lit runway. But even less obvious conditions present severe problems in pilot perception, resulting in overshoots and undershoots.

The experienced pilot has trained his eye to subtend an arc of some 2-1/2 to 3 degrees during an approach, the angle related to the horizon which determines the proper touchdown aiming point. But the pilot will automatically adjust this angle downward, applying it from the far end of the visual segment; thus, in the absence of other guidance or cues, insidiously aiming himself short whenever only part of the runway is in view.

One of the most consistent statistics that emerges from extensive studies of causative factors in aircraft accidents is that 50 to 70 percent can be attributed to errors made by the flight crew. A typical breakdown is shown in Figure 2.

We already know that of all accidents and incidents, approximately 50 percent are in the approach and landing phase. In this classification, slightly less than 70 percent are relegated to human error with the following cited as major causal factors.

1. Failed to follow approved procedures.

2. Improper operation under instrument flight rules.

3. Misjudged speed/altitude.

4. Spatial disorientation.

5. Inadequate supervision of flight.

The term "human error" is, however, misleading in that the "erring" is usually equated to failure to properly perform a given task. If the demands of the task exceed the pilot's capacity, then it can be said that he has not erred.

In reality, the performance has been within human limitations. The actual error may be looked upon in two ways:

1. The system has failed to recognize the pilot's limitations

2. The pilot has failed to recognize his own limitations.

In either case, steps must be taken to either increase the pilot's capacity by training or reduce the demands, for example, by executing a missed approach in order to avoid an accident-prone situation. The former is a long-term solution and the latter an immediate one. In general, aviation can be considered as a high-stress occupation for flight crews. They are involved in critical tasks in which human errors can be catastrophic and they are subjected to adverse environmental factors such as low humidity, widely varying temperatures, and noise. In addition excessive social problems may result from family separation, irregular work periods, and schedules that amount to an almost permanent stand-by status. The effects of constantly having to resynchronize their body rhythms, which have been disturbed by time-zone changes and irregular work/rest patterns, and the sleep disturbance and deprivation which results from these living patterns must also be taken into consideration. This is especially true at the end of a long day, as shown in Figure 3, when the very high stress Ievel demands of an approach and landing in adverse weather conditions may often be experienced.

Obviously, a large number of factors associated with the physiological and psychological makeup of the flight crew tend to degrade or limit human performance capability.

Table 1 lists a number of these which, when taken in the context of marginal weather conditions, tend to advance the incident toward the accident.

While it is not the purpose of this paper to detail all of the stressors placed upon flight crews, the manifestations of several will be explained to provide some understanding of how these stress factors affect performance at crucial phases of flight.

For example, fatigue, which is difficult to measure accurately or quantify, manifests itself in symptoms which can be clearly seen and felt, such as:

1. Decreased attention.

2. Slowed and impaired perception.

3. Impaired thinking.

4. Decreased motivation.

5. Decreased physical and mental performance.

Fatigue has a tendency to lower a person's assessment of the acceptable level of his performance. It lowers the level of quality which he or she believes should be attained in task performance.

Another stress factor to be considered, which has effects similar to fatigue, is the use of various medications (prescribed or over-the-counter) by flight crews in the treatment of the common cold or to induce sleep. It should be realized that a large number of these medications affect the crews' task performance in a detrimental way and have predictable side and after effects. Some can cause allergic reactions when used in combination with alcohol.

Others can cause very serious side effects long after ingestion. Use of over-the-counter antihistamines to alleviate symptoms of the common cold should also be approached with care. The case is cited of a pilot taking Sudafed, a nasal decongestant, which resulted in postural dizziness associated with "graying of vision". This situation reemphasizes the potential hazard of self-medication for upper respiratory tract problems by flight crew members.

Thus, flight crews must be acutely aware of their own limitations in assessing their proposed actions in a high-stress marginal approach and landing situation.

Why try to salvage a marginal approach?

Approaches are made under marginal conditions many times throughout the world every day with the crew and the airplane in complete control. The question is: Why try to salvage a marginal approach? What to do when the approach itself becomes marginal? The question "to land or not to land'' is posed to every airline pilot many times during his career. As the aircraft nears that altitude known appropriately as decision height or minimum descent altitude the pilot must judge the situation in order to decide whether to continue the approach to landing or to initiate a missed approach. This is the time when the pilot must assess the environment around him by evaluating the cues and information available. It is also when the pilot feels the stress of added pressure from his management and passengers to complete the landing at the scheduled destination and the ultimate responsibility for the safety of passengers, crew, and the aircraft. The goal of every flight is to get the airplane safely on the runway, at the scheduled destination if at all possible.

There are obvious pressures to press on and get the "wheels on the ground". For one, there is the commitment of the crew to maintain the on-time arrival so that the crew and the airline will look good in the schedule records and there is also the recognition of the unspoken needs of the passengers for a landing at the scheduled destination without the delays and inconveniences of landing at an alternate airport. Additionally, there is a general desire on the part of crew and traffic controllers alike to avoid having to carry out the procedures for a missed approach.

The pressure to continue also reflects an "ego" involvement associated with:

1. I can make it.

2. Why go around, I made it last time.

3. The aircraft ahead made it, why can't I?

4. I have a reputation to maintain.

5. Etc.

Add to this, the desire to get home and relax with the wife and children at the end of a long day or arrive in time for some planned activities, and scene is set for the high-stress marginal approach and landing accident or incident.

When pilots feel the added pressure to continue they should also consider the positive reasons for executing a missed approach. It is the pilot's responsibility to determine if the approach and landing are authorized under the existing weather minima in order to attempt a landing free of risk or danger. If the approach is being made in marginal conditions and it becomes necessary to execute a missed approach, it is because the pilot chose not to expose the passengers, the crew, the airplane, and himself to the hazard or danger that could possibly lead to loss or injury. In some situation, the pilot may determine that continuing the approach is unsafe or that he has doubts about the situation and environment. Marginal conditions sometimes cause the pilot to become rushed or to "get behind the airplane'' as when ATC directs the pilot to turn the aircraft in close. A jet aircraft operating at low power in a high-drag configuration, descending at excessively high rates on final approach in marginal weather represents a prospective accident if the pilot is hurried or the approach is unstable. His job, the company's safety record, and the lives of people on the aircraft and on the ground, including his own, are at stake when the pilot chooses to continue an approach or make a landing that should not be made.

The pilot's judgment is the factor that determines whether or not to land. The pilot must determine if the conditions are such that a marginal approach exists, and to do this, he must have good prior flight preparation, knowledge of instrument presentations and sensitivity problems, and an understanding of meteorological illusions in order to form an opinion by weighing evidence and then comparing the situation with previous flight experiences.

Flight planning, especially during marginal weather, should begin before takeoff and be reaffirmed throughout the flight before the descent is made.

This provides the pilot with a detailed mental picture of the entire instrument approach procedure and runway environment before the approach is initiated. It enables him to fully concentrate on instrument interpretation and the actual execution of the approach. He should have a complete understanding of the missed approach procedures and be prepared to execute them. Not only is knowledge of the instrument presentations required, but changing instrument sensitivity while converging on a course often causes rather wild gyrations by pilots who have waited too long to properly establish their position on the glide slope and localizer. Trying to come back on course, when close-in will aggravate the problem because of instrument sensitivity and the lower altitude and proximity to the runway. Thus, the precise execution of instrument approaches requires much practice. The pilot must also learn not to reject instrument references. To overcome the natural tendency to depend on spatial references, pilots must discipline themselves to trust instrument visual references and translate them into precise, safe instrument approaches.

As we have stated, weather often produces convincing illusions. Combinations of meteorological conditions and reflecting lights can produce deceptive illusions that often cause pilots to lock into false horizons, produce vertigo, and adversely influence the pilot's spatial orientation.

Many accidents and incidents have occurred after the pilot looking out called that he had the ground, the approach lights, the runway lights, or the runway "in sight", and the pilot flying the aircraft made the transition or was trying to make the transition from instruments to visual flight. Even after the runway environment has been sighted, the pilot is far from home free.

As the aircraft nears the decision height, the pilot has been accumulating the information in order to evaluate the situation and make an operational decision: to land or not to land?

At this time, descent and approach awareness are coordinated between the pilots, with the pilot not flying normally making necessary callouts. At decision height, the callous for "at minimums" is made. A wise pilot will have double-checked the decision height and computed and memorized the appropriate threshold altitude. The pilot flying will call out with a "missed approach" command if visual cues are insufficient. If things do not look right, going around and getting back up to where there is time and altitude to think things out is a very wise move, especially in marginal conditions. We must remember that it is the pilot's place to use good judgment; to assimilate, evaluate, and act upon the situation. Pilots must be reminded that executing a missed approach, when necessary, is a mark of good judgment!

Some technical aspects of approaches and landings under marginal conditions

Much has been said about margin over stall and coefficient of lift during all kinds of approaches: smooth air, high headwinds, crosswinds, turbulence, and so forth.

Equally well covered are the various formulas for speed additives for these conditions along with wind shear and the techniques for coping with these phenomena. The reason speed additives increase controllability and margin over stall is obvious: the faster you go, the higher you are above stall speed and the greater the availability of aerodynamic forces for control. However, simply because one flies at 30 percent above the stall speed and adds 10 knots or so on top of that for gusts, he might not have all the margins he thinks he has while maneuvering to maintain a wings-level attitude plus trying to line up with the runway while in turbulence.

Figures 4 to 6 depict the degrading effect of spoiler deflection on the available margin over stall, the load factor to stall, and the change in pitch attitude or angle of attack to maintain constant lift during lateral-control applications for a typical jet transport in the landing configuration.

The percent increase in the stall speed, pitch attitude, or angle of attack, to maintain constant lift, and the load factor to stall are presented as a function of control wheel deflection. One hundred percent control wheel deflection represents full lateral control application (full asymmetric ailerons and full spoiler deflection on one wing panel). It should be noted that these three factors all are additive when the control wheel is deflected so that for any given instantaneous wheel deflection, the resulting degradation in margin is not one or the other of these functions, but all three combined. For example, on a typical jet with maximum landing flaps, a 50 percent deflection would result in a 4 percent increase in stall speed, reduce the load factor to stall to about 1.58g from 1.7g, and require a change in angle of attack to maintain a constant lift of approximately 2 degrees. All this not only effectively lowers the margin to stall to 21 percent instead of the 30 percent plus 5 knots the pilot thought he had, but it also limits the angle to which a bank turn can be made before stalling and requires a commensurate increase in thrust and pitch to maintain the same lift.

We believe that the foregoing is not generally known by flight crews and may offer an explanation as to why such a large number of accidents and near accidents have occurred when the airplane landed short, landed hard, or hit the ground while attempting a missed approach under turbulent and/or otherwise marginal conditions.

The obvious message here is that flight crews should be made aware that excessive maneuvering, whether due to poor lineup or heavy turbulence, should be avoided during approaches, landings, or missed approaches. Considerable information is available on the subject of nonprecision approaches, and there are many methods and rules of thumb available for determining position awareness during such approaches. Suffice it to say, for the purposes of this paper, that flight crews should be made aware of and strongly encouraged to use whatever means, aids, and techniques are deemed appropriate by their management to enhance their ability to follow a safe descent profile to the runway after departing from the final approach fix. Such procedures, if adhered to, will add greatly to the safety of approaches during poor visibility and/or otherwise marginal conditions.

Summary and recommendations

This paper has examined numerous factors in an attempt to understand the apparent increasing trend in numbers of fatal accidents during approach and landing under marginal weather conditions. In analyzing this type of accident, it must be realized that, in addition to the weather, consideration must be given to the aircraft characteristics, the airport at which the approach is being made, and flight crew human factors. All these parameters are important elements with their own margins which contribute to accident causation.

The weather, with its associated ceilings, cloud cover, rain, fog, and wind shear, provides for a continuous change in the visual environment, producing marginal visibility and illusion requiring constant awareness by the flight crew.

Crew response to this dynamic environment can also be compounded by various MEL items and inflight equipment failures. Crew knowledge of an airplane's performance characteristics is essential. Airport characteristics, such as sloped runways, lighting, and color contrast, are additional factors which give rise to visual illusions that must be compensated for.

Finally, consideration must be given to the physiological and psychological makeup of the flight crew. Many factors such as loss of sleep, fatigue, age, lack of proper nutrition, the pressure of ensuring a landing at the scheduled destination, and other problems contribute to individual performance degradation.

In order to reverse the apparent increasing trend in fatal approach and landing accidents in marginal weather, the following recommendations are made:

 

1. Establish a cockpit awareness program on the subject. Make sure the flight crews are aware that this regime of flight needs their attention.

    

These training programs should stress the need for being prepared to execute a go-around should circumstances dictate.

Management attitude toward missed approaches is most important. No one wants to encourage unnecessary missed approaches or overly cautions flight crews. Yet it is infinitely better to take it around and try again, that to try to salvage a marginal approach and then have to salvage what is left of the airplane and its passengers and crew.