Estate Autunno 2000

FLIGHT PATH CONTROL - THE STABILIZED APPROACH

Riproporre articoli come quello che segue è fondamentale per mantenere attivo l'atteggiamento critico dei piloti nelle operazionio di volo. L'articolo di J.S. Clauzel, che pubblicammo nella edizione Estate 1999, è stato il primo esempio di scritto sempre attuale per la sicurezza delle operazioni, anche dopo vent'anni.

Anche questo interessante e "antico" articolo con cui ci "affligge" cpt B.S. Grieve (un nome che suscita movimenti scaramantici) è un riferimento sintetico, omnicomprensivo, che mette in evidenza gli aspetti essenziali che deve avere ben chiari in mente il pilota di linea quando è in fase di avvicinamento.

Il professionista che avrà avuto la pazienza e l'interesse di leggere anche altri articoli di questo numero e dei precedenti noterà, senz'altro, dei richiami e dei legami con circostanze e situazioni reali già illustrate ed analizzate.

Capt. B.S. Grieve

Operations Director, Britannia Airways

INTRODUCTION

The final approach and landing have accounted for nearly one half of all "CREW CAUSED" jet accidents over the last twenty years.

The purpose of this paper is to focus on the fundamental principles of the stabilized approach, and emphasise the importance of a disciplined adherance to standard operating procedures.

The statistics which are available are insufficiently detailed to establish exactly what the weather conditions were on every approach and landing accident, but it is reasonable to assume that, in the majority of cases, the crew were abIe to see the runway before reaching decision height. This is an important point as it highlights a dilemma that is regularly faced by pilots. It is, of course, that which tempts the pilot to sacrifice instrument scan and give preference to the external visual scan. This paper is an attempt to focus attention on important issues, and not meant to be critical of anyone in our Industry.

THE STABILIZED APPROACH

The importance of a properly stabilized approach is well understood by professional pilots. You do not have to be tremendously experienced to know that, there is no guarantee that you will touch down in the correct place if you allow the approach to become de stabilized. However, there is a dangerous anomaly which conspires to lure the unwary pilot into a false sense of security. It is simply that it is possible to pull off a smooth landing from an inaccurate approach and, conversely, a stabilized approach is no guarantee that the quality of touch down will be acceptable. Those pilots who “duck under” the glide slope towards the runway threshold, to ensure an early and smooth touch down will be able to relate to these remarks. Thay must be guided to change their ways if they are to remain, unidentified, by the accident statistics.

WHAT MUST BE DONE

The importance of a properly stabilized approach is a major training issue. It must be clearly identified by flight instructors when the pilot first joins the company and, thereafter, during reconversion. It must be reinforced, every year, on the annual line check and, for those airlines which are fortunate enough to have the agreement of their pilots on FLIGHT DATA MONITORING, through the post flight reading of the data applicable to each flight. The aim should be to convince the pilot that flying a stabilized approach should become second nature, in the same way as up elevator is, to becoming airborne, during take off.

HOW THIS MAY BE ACHIEVED

The flight instructors must concentrate upon the basic parameters of flight path control until it becomes second nature to the trainee.

There are FOUR key parameters which must be committed to memory so that it becomes possible for the pilot to read any situation, intelligently, and to respond with the appropriate skill without fail. To achieve this, every opportunity should be taken, during training sessions, to emphasise the value of these parameters:

· ATTITUDE

· THRUST SETTING

· CONFIGURATION

· VERTICAL SPEED

ATTITUDE will vary with Flap Setting, Indicated Airspeed and Weight. However, once the flap extension has been initiated, the attitude changes associated with the speed reduction will usually be small enough to ignore on most small and medium sized aircraft, of conventional layout. Large aircraft, or those with unusual aerodynamic features may require some attention to be paid to attitude during the flap/speed schedule, particularly if there is a large weight range available for landing. The solution lies with the flight instructors. Reference values for attitude must be committed to memory and applied during line operation.

THRUST SETTING obviously will vary with Weight, Head-wind Component, and the Flap and Landing Gear Configuration. Careful use of thrust, involving sufficient anticipation, will ensure that the flight path is easier to control because the trim changes associated with the changes in power will be reduced.

CONFIGURATION is of crucial importance to a safe and successful landing. Anticipation, in relation to the particular deceleration characteristics of the aircraft type, must sensibly be applied together with position information from navigation systems to ensure that a high degree of situation awareness is maintained during the intermediate and final approach. Any trim change must be anticipated .

VERTICAL SPEED will vary with Indicated Airspeed, Head- wind Component on the approach, and Approach Path Angle. For a nominal three degree approach path, the vertical speed will be 700 feet per minute in light winds. A strong head-wind or tailwind will have a noticeable effect on the vertical speed required to hold the approach path angle.

PRACTICAL APPLICATION

Once the pilot has fully understood the basic parameters, and demonstrated their application in the simulator, the lessons learned should be carried over into line operations. It is only at this stage of training that the lessons learned will be fully tested. In the real world, the factors which influence each approach are infinitely variable. The true test is satisfied when the pilot under instruction is able to read each situation and apply what has been learned.

In addition to the basic parameters already identified, a “framework” of STANDARDIZED PROCEDURES should exist, which will be achieved by the pilot when he applies the rules of FLIGHT DECK MANAGEMENT. For each type of approach, guidance should be given about when the approach must be stabilized. Normally, this will be a minimum of 500 feet above touchdown for all precision and visual approaches. In the case of a non-precision approach, careful consideration must be given as to what advice is provided about stabilization in relation to the decision height. Of course, this is only an issue in the case of the “step-down” technique. If a “continuous” approach is flown to decision height, then the 500 feet rule will still apply. FLIGHT DECK MANAGEMENT must stipulate that, “if the aircraft is not properly stabilized on a minimum of 5OO feet, then an immediate GO-AROUND must be initiated “.

There is another element, in the pilot's decision making process, which must be considered. That is, of course, our old friend the weather. It is not possible to anticipate every eventuality, but, if there is any significant crosswind present, or there is low cloud or poor visibility, then the aircraft should be stabilized on the approach by, at least, the outer marker or final approach fix.

No deviation from these requirements can be tolerated. The airline must accept that, from time to time, pilots will execute a missed approach. It is a decision that the pilots will know is accepted by their management.

Any influence from “commercial pressure” must not be allowed to dictate a course of action.

The object of being so pedantic is to remove as much risk as possible from the approach and landing phase of flight .

SIGNIFICANT FACTORS WHICH CAN INFLUENCE CREW PERFORMANCE

In my view, the following factors are sufficiently important, to be identified as having a significant influence , aside from basic flying skills, on the outcome of an approach and landing.

· COMPANY MANAGEMENT

· TRAINING EMPHASIS

· AIRFIELD CLEARANCE

· WEATHER

· AIR TRAFFIC CONTROL

· COMMERCIAL PRESSURE

· CONCENTRATION

COMPANY MANAGEMENT: There is no doubt that a good flight safety environment can only be created by sound management. The pilot group must be clear about what is expected of them. Flight Deck Management must be logical, clear and concise, and contain the basic framework of standardisation within which the pilot is expected to think and apply AlRMANSHIP, the basic credential of the trade. However, the overall responsibility for flight safety lies with Flight Operations Management.

TRAINING EMPHASIS: It is important to have clear objectives in the training department. Sufficient emphasis must be placed upon the fundamental requirements of flight path control, to ensure a repeatable level of performance. Emphasise that aircraft attitude is the most important parameter for each phase of flight. It is crucial to a stabilized approach. Be firm about the need to follow the guidance given in Flight Deck Management. It has been compiled by senior colleagues who are experienced, and have had time to consider their advice in the light of that experience.

Point out that a change of decision, at some late stage, could jeopardise the protection which standardized procedures afford. During the approach and landing phase, the trap is to allow yourself to be distracted. Having made the decision to execute a missed approach at decision height, and initiated the action, it is likely that the aircraft will descend below decision height or, in the extreme, land with an unsafe configuration if the decision to “GO- AROUND” is changed.

A good approach should ensure a good landing. Quality of touchdown is important, but a more important objective is to land in the correct place on the runway. The aiming point is normally at 1000 feet from the threshold. Set reasonable limits for the touchdown point, (within +300 feet and -100 feet of the aiming point) . This tolerance will need to be tighter for some of the shorter runways in an operation.

Point out that there is no such thing as a purely visual approach. It is not possible to guarantee a properly stabilized approach, to an accurate touchdown, through outside visual reference alone. During such approaches the outside visual scene is included in the normal instrument scan. Remember that you will notice a deviation, from the flight path, sooner through the flight instruments, rather than through any visual reference. Indeed, it may not be possible to notice a deviation, visually, until quite late on in the approach. The pilots must be reminded that elevators control flight path and thrust levers control speed. Workload will be reduced if the aircraft is kept in trim through the anticipation of each trim change.

AIRFIELD CLEARANCE: every airline’s operation can have “difficult airfields”. The reasons for such difficulty can be quite varied. It may be short runway length, or unusual slope. Perhaps there is intimidating surrounding terrain, or , merely a bad weather factor which includes challenging surface winds. Whatever the reason, these airfields must be categorised, and a training/check system introduced to reduce the additional risk caused by the degree of difficulty. This additional training gives the crews confidence and allows the management to sleep at night!

It is not a bad thing to have a number of difficult airfields in your operation. A good standard of professional skill achieved at these, ensures a better standard of operation overall.

WEATHER: Any type of weather situation will require some variation of the basic approach parameters which the pilot uses as a “framework” for each approach. Some situations, such as windshear, heavy rain or poor visibility , can be so severe that they make an attempt to land unwise, or even impossible. Weather minima do not always protect the pilot when these phenomena exist. A disciplined application of the basic teaching, may be the only salvation available.

Fortunately, for most of the time, the weather is not so extreme, but the pilot must appreciate the everyday effects of surface wind.

For example, a strong head-wind will require a lower rate of descent on the approach and may produce some windshear. Thrust handling will need to be more responsive than normal. During the flare, thrust reduction should be made more slowly, and left until later, otherwise the strong head-wind may kill the airspeed too quickly, resulting in a premature touchdown.This particular scenario will be more evident on some types of aircraft than on others.

A significant tailwind can also produce some windshear, although it will not be as much as with a head-wind because tailwind limits for landing are always less than head-wind limits. However, because of the tailwind induced higher rate of descent, the flare will need to be commenced a little earlier and the aircraft rotated to a slightly higher nose-up attitude for a good quality touchdown. In this situation, some quality of touchdown will need to sacrificed on those aircraft which are susceptible to tail-strikes. In either event, thrust should be handled carefully to avoid excessive float. A tailwind landing can be a demanding exercise.

Crosswinds promote additional considerations. A significant crosswind from the right can be associated with a tailwind component on the approach, whereas a significant crosswind from the left can be associated with a head-wind component on the approach. This is derived from meteorological mechanics which state that winds veer and increase with height. [NORTHERN.HEMISPHERE]

However, our modern Inertial Reference Systems show that local topography can have a modifying effect.

AIR TRAFFIC CONTROL: As the number of aircraft movements increases, A.T.C. are, increasingly, being faced with the problem of congestion. They have several ways of dealing with this. A sound plan of ordered flow is the usual goal, but, with the best will in the world, this will result at times when a large number of aircraft arrive at an airport all together, wishing to land. The previously ordered flow now has to be dealt with on a tactical basis. It is not uncommon to be instructed to “keep the speed up to the outer marker” or, “reduce to minimum approach speed”. The pilot's understanding of stabilized approaches is now liable to be tested to the full. He, or she, must balance the dynamics of the flight path with the requirements of the controller. The potential exists for a de-stabilized approach. The risk of a go-around being required, may be increased. The pilot will, quite naturally, not wish to return to the traffic congestion. He, or she , is faced with a dilemma. The discipline of standardized procedures provides the solution.

If the aircraft is not properly stabilized by 500 feet, then go-around. It will probably not make you popular with A.T.C., but you will endure to relish the title of “old pilot”. Remember, there are no marks given for being a “bold pilot” and, according to folklore, not much future in it either!

COMMERClAL PRESSURE: This is present in every successful airline operation. It is the pre-requisite for success. However, it is important for flight operations management to take a balanced view, and communicate that view to the pilots. Commercial pressure portrayed in the light of “commercial awareness” should become more acceptable. The pilots must believe that their management will support them if their decisions are made, sensibly, in the light of the best information available to them at the time, and will accept the fact that for some of the time, at least, the pilot's decision may not be optimum. It is important to recognise that the commercial pressure issue will need careful handling if you expect the flying discipline, established during training, to withstand the rigours of an everyday operation.

CONCENTRATION: This can be a particularly difficult subject on which to make comment. Difficult because, concentration is a human factor and a very personal process. Pilots must learn to give their devoted attention to flying accurately, while at the same time being very conscious of what is going on around them and their aircraft. At first sight this may appear to be a contradiction, but a closer assessment will reveal that there are, in reality, two levels of concentration required, one within another. There is the level of concentration focussed on flight path control, within a wider scope which embraces situation awareness. It is all too easy to become totally absorbed with flight path control at the expense of situation awareness. One of the benefits of “standard call-outs” is that they serve to keep both pilots in the information loop. In modern aircraft, situation awareness is aided by design. The HORIZONTAL SITUATION DISPLAY on E.F.I.S. equipped aircraft, together with G.P.W.S. have made a major contribution to reducing the possibility of controlled flight into terrain - a major category of approach accident. The pilots who fly these aircraft are better able to keep track of their position during this higher work load phase of flight, the approach and landing, than with those aircraft which only display raw data. The professional pilot will adjust to the type of equipment in his aircraft to ensure that he concentrates on the overview, in addition to that required for flight path control.

SUMMARY

There are many facets which contribute towards a stabilized approach. The correct emphasis placed during training, and a responsible management view are crucial. Also, it is vital that airfields with difficult approaches and landings are categorized, and a training and monitoring system established. The pilots must follow the teaching and the rules.

As a manager of flight operations I would like to leave you with some final thoughts. Managing such a discipline as flying requires specific techniques. Probably nowhere else in Industry is a manager responsible for such a large work force who are so elusive. That is why, it is so vitally important that a sound system of training and control is introduced to reduce the operating risk to the lowest possible level. When problems are encountered in an operation, or incidents occur, the first place to look is where the basic framework of control may have fallen down.

In this day and age of advanced technology, it is all too easy to ignore, or even forget, the fundamental principles of flight, in an attempt to concentrate upon fine detail. It could be that we believe we have progressed beyond that stage, and no longer need to bother with basic principles. You must be the judge of that.