FROM LANDING COMPULSION TO CONTROLLED FLIGHT INTO TERRAIN. WHAT PILOTS NEED TO KNOW
(a cura di Aldo Pezzopane)
Il sentiero di volo che percorre un aeromobile dal termine della fase
di crociera al punto di contatto con la pista dell'aeroporto dove l'equipaggio
intende atterrare, ha una configurazione geometrica ottimale. Le caratteristiche
di questa traiettoria [pendenza e velocità] devono garantire la
sicurezza dell'operazione, la fluidità di progresso dell'aeromobile
in sintonia con il traffico e [ma questa deve essere una conseguenza e
non un obiettivo] una ragionevole economia in termini di bilanciamento
tra esigenze di orario e consumo di carburante.
Scostamenti macroscopici da questa traiettoria sono il primo avviso che il pilota deve aumentare la sua concentrazione sulla condotta per recuperare la corretta configurazione di traiettoria. La percezione della posizione spaziale in relazione a questa traiettoria virtuale è fondamentale e distrazione, disorientamento, perdita del quadro, sono stati, nella maggior parte dei casi, l'innesco dell'abbandono della traiettoria corretta, terminato con eventi finali accidentali o catastrofici. Le uscite di pista e gli impatti al suolo con velivolo controllabile [Controlled Flight Into Terrain]. Quelli che vi proponiamo sono elementi di riflessione sui fattori di queste due tipologie di eventi.
L'industria aeronautica
propone un continuo e pressante progresso tecnologico ma i problemi che
vengono incontrati in volo dai piloti sono sempre gli stessi, come indicano
diversi incidenti accaduti anche recentemente. Per questo appare quanto
mai attuale il contenuto dell'intervento effettuato dal Comandante Homer
Mouden ad un seminario internazionale della Flight Safety Foundation,
tenutosi in Canada oltre vent'anni or sono e frequentemente ripreso e
riproposto nei Bulletins della stessa Foundation per molti anni a seguire.
L'analisi di Mouden è diventata un "pezzo" storico per la sua completezza
e per l'equilibrio delle sue considerazioni. Ogni pilota di linea che
abbia incontrato situazioni analoghe a quelle descritte non può
non venire catturato dalle argomentazioni e dallo stile chiaro, pragmatico
di Mouden e leggerlo tutto d'un fiato.
LANDING COMPULSION
by Captain L.Homer Mouden - Manager, Flight Safety - Eastern Air Lines (1978)
When each of us boarded a flight to come to this seminar assumed that the aircraft would land at the Ottawa International Airport. That was the whole purpose of the flight. If, during a seemingly routine approach, the engine suddenly surged and airplane rotated for a pull-up, every passenger in the cabin would have responded, some with apprehension and others with annoyance. Strangely enough, very few reactions would have been relief. If the go-around resulted in having to go to an alternate with the usual inconvenience, no matter how mild, very, very few of the passengers reactions would have been free of irritation.
A pilot, who encountered a severe wind
shear condition while on approach, successfully executed a go-around and
then proceeded to his alternate. While deplaning at their alternate airport,
several passengers chastized the captain for making them ride a bus from
there. Why does this occur? Maybe airline landing reliability has preconditioned
the passengers to consider anything other than routine arrival at their
intended destination as unacceptable.
Of more than academic interest is the question as to whether such factors
influence a pilot's judgment when suddenly faced with a decision to land
or to go-around. What are some of the factors that, subtly or otherwise,
influence a pilot's judgement? Impact on the flow of air traffic is certainly
one of them.
At the busier airports in our present Airways
Traffic Control system, a go-around affects much more than just that aircraft.
The system itself is basically geared to the anticipation of a landing
from each approach. The spacing, of following aircraft, and the departure
from feed-in points, are all immediately affected as soon as an approach
is not completed to a landing. It is difficult for a controller, who has
observed several hundred consecutive landings without a single go-around,
to keep mentally oriented for an incompleted one on every approach. A
pilot making 50 to 60 landings at 10 to15 different airports, month after
month, without experiencing a go-around, may become pre-conditioned to
expecting all approaches to terminate in successful landings. The last
go-around that many pilots made was from a point half a mile or more out
on the final approach when some aircraft failed to clear the runway as
soon as had been anticipated.
Other factors that may affect a pilot's decision are fuel awareness, passenger
inconvenience, and the effect on schedules. But the human factors are
the ones that exert the most pressures on a pilot's judgment. Perhaps
of all the human factors the most subtle and yet the must insidious are
pride, complacency and unawareness. Everyone is affected
by these to varying degrees in every action he takes or every judgement
he makes. Personal pride can have a contributing or detracting impact
on our actions or judgments. Each of us needs to constantly reevaluate
those things in which we take particular pride to see if that personal
pride is valid or vain.
During a pilots' meeting some 35 years
ago, a senior pilot stated with some considerable pride, "I haven't missed
an approach during the last four and a half years". When I heard this
I recalled three approaches that he had missed during the last three winter
months while I was flying as his copilot - only we had landed each time!
Under the same criteria, I'm sure that every one of us who has flown an
airplane has also missed an approach out of which he has landed. And I
suspect also, that each of us has secretly wished afterward, that he had
gone around on some of them. The vast majority of us were successful at
that specific time by virtue of some luck, and perhaps, some skill, rather
than good judgment. An old military axiom which says, "Use your good
judgment to avoid situations that require you to demonstrate your superior
skill" has been adapted to modern aviation. At no time is it more
applicable than during the final approach.
What are the factors which cause a pilot to continue an approach to land
when his subsequent evaluation of his own flight tells him that he should
have gone around? Obviously, there never is a single, simple answer any
more than there is a single contributing factor to most aircraft incidents
or accidents. Neither is there any way to determine the number of flights
which landed successfully, but where good judgment would have dictated
a go-around. The U.S. airlines complete 4,000,000 landings a year and
yet, if only one attempted landing ended in an accident, which could have
been avoided by a go-around, the ratio is too high.
That captain who 35 years ago had not missed an approach in four and a half years was flying an airplane with an approach speed of 70 mph that could normally be stopped without difficulty within 1,500 feet from the touchdown point. The minimum ceiling for an approach from the low frequency range was 400 feet. He had 40 seconds and 8/10 of a mile for manoeuvring after breaking out of the cloud base. However, there was no precise directional or vertical guidance, so the primary reason for not being able to land when weather was a factor in those days was the inability to fly on instruments with precision from the low frequency range to the point at which visual contact at minimum ceiling limitations would permit visual alignment for the landing.
Today, a captain making a routine CATII
ILS approach to a 100 foot decision height at 140 knots wilI have only
five to eight seconds (11 to 13 seconds to touchdown) from the decision
height to the runway in which to analyze and evaluate all factors while
covering another 240 feet each second. This in itself is a major contributing
factor to a recognized tendency for pilots to continue to a landing from
situations which might subsequently be evaluated as having justified abandoning
the approach and making a second attempt.
Even the interpretation of the decision height itself has been a significant
contributing factor to this tendency to continue the approach. Part 1
of the U.S. Federal Air Regulations defines decision height as follows:
Decision height, with respect to the operation of aircraft means a height
at which a decision must be made, during an ILS or PAR instrument approach,
to either continue the approach or to execute a missed approach.
Since the definition identifies decision height as "…the height at which
a decision must be made…", and since neither the human thought process
of the pilot nor the response time of the aircraft are instantaneous,
the continuation of the landing procedure, even under marginal conditions,
may seem more feasible at the time than the execution of a go-around from
that particular situation. Even Newton's First Law of Motion, that "an
object continues in a uniform straight line unless affected by some outside
force", seems easiest to cope with until that external force is about
to become the ground.
The definition and physical identification of the decision height has
become a subject of considerable controversy. Over ten years ago, Mr.
Joseph Ferrarese, then Chief of the FAA's Operations Division, attempted
to clarify this in a paper presented at the SAE - National Aeronautics
Meeting and Production Forum in which he stated:
"…decision height is the limit to which a pilot may descend before deciding
to continue his approach to a landing by means of visual aids and cues,
or to execute a missed approach…This is not to say that the pilot waits
until he arrives at the decision height before deciding to land or to
go-around. The decision making process begins at the time the ILS approach
is initiated and continues while the approach is in progress… It therefore
becomes evident that while the decision height is an exact point in space
at which the pilot makes an operational decision, the information he requires
to make this decision has been accumulating for a considerable time, and
it would be incorrect to assume that all aspects, of this decision must
be formulated and assessed at one critical instant on the approach".
There are two factors in this decision-making that merit further consideration.
One involves the "visual aids and cues" that the pilot acquires during
the last few critical seconds of the approach. These require almost instantaneous
analysis and evaluation while he is making the decision to continue an
approach to a landing or to execute a go-around. The other factor involves
the acquisition of timely information. This is one of the recognized major
problems, particularly during rapidly-changing weather conditions. Inaccurate,
delayed or absent information can have a significant impact on a pilot's
ongoing evaluation of a situation.
What we are really talking about is adequate communication. If a subsequent
evaluation of an unsuccessful landing indicates that a go-around would
have been a preferred manoeuvre, it would also indicate that somewhere
prior to that landing there may have been a break in the informational
loop which caused the pilot to make the incorrect decision, i.e., communications
breakdown, or, it may have been one of those other factors which we previously
mentioned, such as deviation from established procedures, failure to compensate
for external forces, or more particularly, failure to recognize the situation.
While all of this may sound as though we are, making excuses for the pilots,
it really is for the purpose of putting in focus the fact that during
an approach to land with a modern jet airplane, the pilot - a human being
- is physically manoeuvring or monitoring a vehicle traversing the airspace
at about 230 feet per second while the ground is rushing up to meet him
at a rate in excess of 10 feet per second. As all this is going on, the
human computer, the brain, is having to process all the bits of information,
evaluate their impact on the situation and produce a print-out called
"pilot decision".
Psychologists tell us that the process
of making decisions creates stress within the individual but once a decision
has been made, there is a definite feeling of relief. Thus, even If that
decision subsequently is determined to have been a wrong one, there may
be a tendency to stay with it regardless of the consequences. We are not
talking about the individual who deliberately ignores his own good judgment
for some overriding reason which at the time seems to justify the risk
involved. We are discussing the influencing factors which affect the individual's
decision-making process.
Factors which assist in the making of good decisions are time and accurate
information. A more effective decision-making process can take place if
the maximum amount of useful information has been provided with sufficient
time to assimilate, evaluate and act upon it. When Joe Ferrarese stated,
"…information a pilot requires to make an operational decision has been
accumulating for a considerable time…", he recognized that adequate communication
is fundamental to assuring that the required information for making a
correct operational decision is available. This is equally true relative
to those operational decisions controllers must make. The supplying of
real-time information, which cannot be misunderstood, continues to be
one of the major problems facing the aviation industry. Nowhere is such
information more essential than during an approach under marginal or rapidly-changing
meteorological conditions.
There are enough accidents and incidents each year to indicate that the
failure to go-around from some approaches being made under the marginal
or unknown weather conditions is a continuing problem. But what can we
do about it? Is the compulsion to land a realistic problem? Perhaps it
would be more realistic to consider it as a normal human reaction to a
particular situation. Then, if we agree that there are many landings made
or attempted that should have been go-arounds, and I believe that everyone
including the pilots will concede this, we must determine the basic as
well as the specific reason for this.
Bob Burgin presented a most thought-provoking paper last year. I was in
complete agreement with it except for two things, the title, "The Missed
Approach" (Review 3/4 1977), and the many references to the terminology
"missed approach". You may recall that the term "landing expectancy" was
defined and discussed from the pilots' and the controllers' viewpoints
but all references to incompleted or non-landings were referred to as
"missed approaches".
The connotation of "missing" includes failure or inferior performance!
Webster's Collegiate Dictionary defines miss as "to fail to hit, reach,
or contact; to fail to obtain; to discover or feel the absence of" - all
negative concepts with a connotation of non-performance or inferior achievement.
Psychologically, we are using the wrong terminology when we refer to "missed
approach". Were you ever duck or goose hunting with a friend when both
had an opportunity for a good shot? He made a clean kill while you watched
one lonesome feather drift down. Do you remember your feeling of frustration,
chagrin and perhaps even some bit of anger as with scarcely concealed
glee, he asked, "You missed that shot?" The implied failure to achieve
was all tied up in that one word "missed".
Incidentally, the aviation industry has also, unintentionally, contributed
to another psychological impact on pilots through the adoption and use
of the term "nonprecision approach". All too many pilots have been influenced
by this terminology into flying an approach, without the benefit of electronic
vertical guidance, with a lesser degree of precision than they would when
making an ILS approach. Even though a "nonprecision" approach usually
has higher minimums, it requires a much more precise control of the aircraft
and a higher degree of pilot concentration than when flying a straight-in
ILS approach.
Every despotic ruler knows the psychological impact on the minds of people through the use of terms to which a different meaning has been applied. Thus, a "Democratic Republic" might actually be very dictatorial in structure. And many of us are being subtly preconditioned to the acceptance or belief in the validity of "giant size" boxes of soap or cereal, or "super savings" sales of items that are marked down only from the price to which they were raised last month. We should be very careful about the misuse of mind-influencing words. I am convinced that the aviation industry has contributed to this problem.
Recently the Chairman of the National Transportation Safety Board in the United States sent an open letter to the presidents of forty-two U. S. air carriers. One of the points discussed was The Prudence of a Missed Approach. In summarizing the discussion on this subject, he concluded with "Management should insure that the pilot understands that the missed approach manoeuvre is a legitimate manoeuvre under such conditions". Actually, his discussion dealt with the same subject we have been discussing previously - the inability of the pilot to continue the approach to a landing for reasons which may or may not have been within his control.
It is estimated that we are flying in excess
of four million instrument approaches each year throughout the world.
During the last ten years, in just the United States alone, almost eighteen
million approaches have been made at ATC controlled airports. But it has
been impossible to obtain a reasonably accurate estimate of the number
of those approaches in which weather was a factor, that have not been
continued to a landing.
The FAA conducted a survey during the first three months of 1974 of the
number of "missed approaches" compared to the number of operations under
Instrument Flight Rules that were reported. Since most turbojet aircraft
are operated under IFR procedures, irrespective of the meteorological
conditions, there was no way to determine in how many of these go-arounds
weather was a factor. The available results of the survey also did not
identify which were general aviation or air carrier approaches or go-arounds.
The statistics of the rather incomplete survey indicated that 1.7% of
all reported approaches resulted in pilot-initiated go-arounds. When this
percentage factor was applied to Chicago O'Hare's 273,000 arrivals during
that year, it would have meant that almost 5,000 flights had gone around.
However, since this same 1.7% in the survey also included all go-around
initiated by pilots irrespective of the reasons, the statistics did not
identify the issue being explored, namely, those situations in which weather
was the contributing factor to the pilot's decision to go-around.
A personal survey was then conducted among pilots on one airline as to
the number of "missed approaches" because of weather that they could recall.
They were queried as to how many of these were within the past year, five
years and ten years. Each was also asked for his total airline hours and
for all estimate of the total number of landings he had made during that
time. Of some one hundred pilots questioned, very few would even attempt
an estimate of their number of landings, but over 80% felt reasonably
confident that they could recall the number of times they were unable
to land because of weather after having made an approach.
By dividing the average duration of each flight segment on this airline
into the pilot's total airline flying hours, a rough estimate of his total
landings was obtained. Even after arbitrarily applying a factor of two
- since we recognize the fallibility of the human mind - the ratio of
abandoned approaches to total landings ranged from one per fifteen hundred
to one per four thousand. The average of all these came to slightly less
than one in twenty-six hundred.
Before this survey was completed, it became readily apparent that the
infrequency of incompleted landings from an approach was one of the major
reasons why pilots and controllers became preconditioned to a landing
expectancy. Another interesting aspect of this survey was that over 70%
of the pilots placed a majority of these incompleted approaches as having
occurred during the first half of their airline career. No attempt has
been made to rationalize whether this is the result of more landing aids,
better weather information, a change in the pilot's confidence in his
own capabilities or whether the traumatic impact of the first few occurrences
left a more vivid impression.
Irrespective of the relative infrequency of completed landings, the statistics
of accidents/incidents following a final approach are higher than the
aviation industry can continue to accept. Therefore, the real issue is,
what can be done to reduce the number of unsuccessful landings following
an approach? Some actions which could improve the accident/incident ratio
include:
Provide the pilot with real-time accurate wind and weather information
throughout the approach to landing.
Install full electronic and visual guidance to all runways.
Develop aircraft and engines that eliminate the necessity for unrealistic
noise-abatement procedures which require operation of the aircraft outside
parameters for which it was designed.
Improve ATC's capability for expeditiously handling a flight whenever
a final approach is not continued to a landing.
Eliminate the connotation of inferior achievement by pilots whenever an
approach does not result in a landing, by:
- discontinuing the use of the terminology "missed approach",
- accelerating co-operative education of pilot to recognize that going
around from a discontinued approach is a fundamental part of the approach
procedure itself, and not an indication of failure.
- insuring that pilots include the go-around potential as a part of every
approach irrespective of known weather conditions.
- insuring that the ATC personnel and system include the possibility of
a go-around in every approach clearance.
Anticipate the necessity for and provide a continuing and updated
educational programme for pilots, ATC controllers and weather personnel
on all pertinent aspects of the preceding activities.
The very purpose of each flight is to safely and expeditiously complete
a landing at its destination. No one can be more interested in the successful
achievement of this goal than the pilots. The entire commercial aviation
system must be co-ordinated to help them accomplish this. But during the
final phase of each flight the pilots are charged with the responsibility
and authority for determining that this approach can be completed to a
safe landing; if not, they must go around! We must insure that all pilots
are motivated to be mentally and physically prepared for the possibility
of going around from every approach, and particularly during those approaches
under reduced instrument meteorological conditions.
If a go-around occurs in less than one out of every thousand successful landings, it is hard to remain motivated to such an expectation. The entire aviation system must recognize this and provide the maximum capabilities in real time weather information, expedited re-entry procedures into the final approach pattern, and a co-ordinated programme for keeping the pilots aware of the go-around potential. The goal should not be to abandon a greater percentage of the approaches. Rather, all segments of the industry should be working toward eliminating the necessity for ever going around; but with pilots, controllers and passengers recognizing that the possibility of a go-around is a fundamental part of every approach, for the purpose of every flight is to land safely and successfully at its destination.
Anche se l'analisi di
Homer Mouden verte principalmente sull'interruzione dell'avvicinamento
in condizioni meteorologiche marginali, le considerazioni sulla difficoltà
riscontrata nei piloti ad attuare una opportuna riattaccata, un "go-around",
sono applicabili totalmente agli avvicinamenti "non stabilizzati". Per
i lettori con meno familiarità con il gergo aeronautico precisiamo
che un avvicinamento non è stabilizzato quando l'aeromobile, che
si trovi a poche miglia dal contatto con la pista, in genere due o tre
minuti prima, non ha la configurazione prevista [carrello esteso e flap
estesi come richiesto], non è nel campo di velocità richiesto
[dal peso attuale, dalle condizioni di vento, ecc.] e non è su
una traiettoria di discesa coerente con il sentiero di avvicinamento [rilevabile
dagli strumenti] che generalmente è intorno a 3°, una pendenza
del 5%.
Per i piloti proponiamo una visualizzazione molto schematica delle condizioni
che possono predisporre ad avvicinamenti destabilizzati o che evidenziano
errori di pianificazione nella discesa. Questa visualizzazione è
sostanzialmente acquisita da qualsiasi professionista consolidato e, a
grandi linee, rappresenta una raffigurazione delle modalità di
attuazione dell'avvicinamento alla pista di atterraggio da parte del pilota.
La linea che va dal punto
di normale inizio discesa [TOD] al punto di contatto in pista [TDP] ha
una pendenza che corrisponde, per la parte iniziale, ad un rapporto di
uno a tre,
tra la quota [in migliaia di piedi] e la distanza [in miglia nautiche]
dalla pista con l'aggiunta di una decina di miglia per la decelerazione.
In altre parole da 31.000 ft di altitudine di crociera la discesa deve
essere iniziata intorno alle 100-110 miglia nautiche dall'aeroporto. Peso
dell'aeromobile, componente di vento, uso di impianti antighiaccio e altri
fattori comportano variazioni intorno a quest'ordine di grandezza di distanza
in rapporto alla quota.
Discese iniziate molto prima per ragioni di traffico aereo possono essere
effettuate su pendenze inferiori o richiedere livellamenti intermedi mentre
discese iniziate in ritardo possono essere corrette con l'uso di aerofreni
o, se non è possibile effettuare un percorso diretto, richiedono
delle circuitazioni per perdere quota.
In ogni caso, al di là di una certa posizione spaziale che dovrebbe
coincidere con un punto preciso della segmentazione d'avvicinamento [in
genere la transizione tra segmento di avvicinamento iniziale e segmento
di avvicinamento intermedio], non devono esistere deviazioni significative
di configurazione e di velocità. Questo avviene fino alla intercettazione
del sentiero di discesa di 3° [Final Approach Point] sul quale l'aeromobile
dovrebbe procedere come se fosse su dei binari. Ovviamente l'operazione
di avvicinamento alla pista non è semplice come questa schematizzazione
potrebbe far credere. Le condizioni meteorologiche [windshear, turbolenza,
forte precipitazione] ed il conseguente stato della pista [ristagno d'acqua
e varie altre amenità come i depositi di gomma] rendono il quadro
alquanto complesso e stressante per il pilota e possono rendere estremamente
difficile stare sui "binari". A questo riguardo il discorso di Homer Mouden
si coniuga perfettamente con l'articolo di J.S.Clauzel già presentato
su questo sito: Documento:
Awareness of stresses associated with approaches and landings under marginal
conditions - J.S. Clauzel
(english)
Questo è un
ampliamento grafico della precedente illustrazione che comprende i segmenti
di avvicinamento.
Esso ci permette di indicare l'area a sinistra della pendenza ideale come
zona favorevole alle uscite di pista, dato che gli incidenti di questo
tipo sono stati il risultato, quasi nella totalità, di un percorso
in volo con caratteristiche di ritardo nella discesa e nelle azioni di
rallentamento ed assunzione di configurazione.
La zona a destra, invece, anche se non identifica un "avvicinamento non
stabilizzato", indica spesso, nel caso di perdurare della condizione di
deviazione dalla traiettoria, una vera e propria mancanza di consapevolezza
della posizione da parte del pilota.
Ovviamente avviene in
condizioni strumentali o di notte, dopo un avvicinamento iniziale "flat"
o "shallow" ed è dovuta a molteplici fattori ampiamente individuati
in anni di investigazioni ed analisi degli "accident" denominati CFIT
[Controlled Flight Into Terrain]
Abbiamo già parlato di questo tipo di eventi che nel passato hanno
funestato anche le compagnie nazionali. Tipico, nel ricordo di molti che
sono ancora oggi in attività, il disastro del DC9 avvenuto durante
l'avvicinamento a Kloten (Zurigo) e ultimo della serie il disastro dell'ATR42
a Pristina.
Ecco alcune interessanti considerazioni in merito espresse in un voluminoso
report su questi eventi realizzato dalla Flight Safety Foundation.
CFIT factors
Flight Crew Complacency
Complacency can be defined as self-satisfaction,
smugness, or contentment. You can understand why, after years in the same
flight deck, on the same route structure to the same destinations, a flight
crew could become content, smug, or selfsatisfied. Add to this equation
a modern flightdeck with a well -functioning autopilot, and you have the
formula for complacency.
Here is an example of flight crew complacency. The flight crew is flying
an arrival. They get a nonstandard clearance to descend to a lower altitude,
in an unfamiliar sector. Suddenly, the GPWS warning sounds: "Pull up!
Pull up!" The flight crew is not sure what to do, because they have never
experienced this before. They may hesitate to pull up, or they may ignore
the warning-with disastrous results.
In this scenario, the GPWS warning may not have registered with the flight
crew. They have flown into this airport hundreds of times, but because
of complacency, their brains may very well have disregarded aural and
visual cockpit warnings.
At the other extreme, flight crews may also be exposed to continued false
GPWS warnings because of a particular terrain feature and a GPWS database
that has not been customized for the arrival. The flight crew becomes
conditioned to this situation since they have flown the approach many
times. This can also lull the flight crew into complacency, and they may
fail to react to an actual threat.
Note: The newer versions of GPWS can be programmed by the manufacturer
for specific airfield approach requirements, so that these nuisance warnings
are eliminated.
• Know that familiarity can lad to complacency.
• Do not assume that this flight will be like the last flight.
• Adhere to procedures.
Procedural Factors Associated With CFIT
Many studies show that operators with established, well thought out
and implemented standard operating procedures (SOP) consistently have
safer operations. It is through these procedures that the airline sets
the standards that all flight crews are required to follow. CFIT accidents
have occurred when flight crews did not know the procedures, did not understand
them, and did not comply with them or when there were no procedures established.
More than one CFIT accident has occurred when the flight crew delayed
its response to a GPWS warning during IMC.
If an SOP had addressed this situation and provided the flight crew with
specific guidance, maybe an accident could have been avoided. In the absence
of SOPs, flight crews will establish their own to fill the void in order
to complete the flight. Some crews think the weather is never too bad
to initiate an approach! It is the responsibility of management to develop
the comprehensive procedures, train the flight crews, and quality control
the results. It is the responsibility of the flight crew to learn and
follow the procedures and provide feedback to management when the procedures
are incorrect, inappropriate, or incomplete.
- Do not invent your own procedures.
- Management must provide satisfactory SOPs and effective training to
the flight crew.
- Comply with these procedures.
Descent, Approach, and Landing Factors
CFIT accidents have occurred during departures, but the overwhelming
majority of accidents occur during the descent, approach, and landing
phases of the flight.
CFIT accidents make up the majority of these accidents.
An enlightening analysis of 40 CFIT accidents and incidents was accomplished
for a 5-year period, 1986 to 1990. The airplanes' lateral position in
relation to the airport runway and the vertical profile were plotted.
(Figures 1 and 2). One of the interesting things is that almost all the
position plots in Figure 1 are on the runway centerline inside of 10 mi
from the intended airport. The vertical profiles shown in Figure 2 are
also significant. The flight paths are relatively constant 3-deg paths-right
into the ground! Most of the impacts are between the outer marker and
the runway.
The geographical Iocations of CFIT accidents during the 1970s show a different
pattern than those in the late 1980s and 1990s. During the 5 year period
from 1972 through 1977, there were 75 CFIT accidents or incidents. Twenty-five
of these accidents/incidents were greater than 8 nm from the runway. The
preponderance of the remaining accidents/incidents were inside the middle
marker. However, for the period 1986 to 1990, the distribulion of accidents/incidents
was relatively even. This difference may be the result of improvements
made in runway approach aids that took place during this time period.
Additional ILS were installed, as well as runway approach lighting systems.
Continued capital investment in runway precision approach and lighting
systems needs to be made worldwide.
- Know what approach and runway aids are available before initiating an
approach.
- Use all available approach and runway aids.
- Use every aid to assist you in knowing your position and the required
altitudes at that position.
Most CFIT accidents occur during non-precision approaches, specifically
VOR and VOR/DME approaches. Inaccurate or poorly designed approach procedures
coupled with a variety of depictions can be part of the problem. Figure
3 is an example of an approach procedure produced by different sources.
There are documented cases that the minimum terrain clearances on some
published approach charts have contributed to both accidents and incidents.
For more than a decade, a worldwide effort has been under way to both
raise and standardize the descent gradient of non-precision approaches.
There are gradients as little as 0.7 deg in some VOR approach procedures.
ASRS report #254276 illustrates the hazard of shallow approaches coupled
with other confusion associated with the procedure design. In addition
to the shallow approach gradients, many approaches use multiple altitude
step-down procedures. This increases flight crew workload and the potential
for making errors.
- Study the approach procedure(s) before departure.
- Identify unique gradient and step-down requirements.
- Review approach procedures during the approach briefing.
- Use autoflight systems. when available.
There is more than one standard for approach procedures in the world.
The U.S. standard is Terminal Instrument Procedures (TERPS).
The ICAO standard is Procedures for Air Navigation Services-Aircraft Operations
(PANS-OPS), and the Russian Federation uses still another.
Flight crews, therefore, may be exposed to different standards and different
margins of terrain clearances.
-Study anticipated approach procedures before departure.
-Know that there are different approach design standards.
Different approach procedure charting requirements and printing can
also make it more difficult for flight crews to safely fly an approach.
High elevation obstacles and terrain surrounding airports have been annotated
on charts for years, but the actual terrain has not been depicted. Slowly,
the publishing and printing organizations for aeronautical and approach
charts have begun to use color and depict terrain or minimum safe altitude
contours. Recently, some of the larger international operators have started
printing their own customized charts that include these features. This
greatly helps the flight crews to recognize the proximity of high terrain
to the approach courses. Hopefully, this will result in fewer accidents.
Unstable approaches contribute to many CFIT accidents or incidents. Unstable
approaches increase the possibility of diverting a flight crew's attention
to regaining better control of the airplane and away from the approach
procedure. A stabilized approach is defined by many operators as a constant
rate of descent along an approximate 3deg flight path with stable airspeed,
power setting, and trim, with the airplane configured for landing.
- Fly stabilized approaches.
- Execute a missed approach if not stabilized by 500 ft above ground level
or the altitude specified by your airline.
In some modern glass-cockpit aircraft, the flight guidance system
has the capability to display flight path vector/flight path angle. Use
of this mode enables a stabilized approach to be flown at the required
slope during a non-precision approach, with automatic correction for the
effects of wind.
Flight management systems also have the capability to provide a computed
profile for a non-precision approach. Required conditions for the use
of lateral and vertical navigation functions for this purpose are that
the approach profile is included in the database, that it is verified
in accordance with obstacle clearance criteria, and that the FMS accuracy
is confirmed to be high.
The use of these techniques, in conjunction with the autoflight system,
reduces crew workload and should ensure a higher level of safety. Procedures
specific to the airline type are given in the applicable Flight Crew Operating
Manual. Crews should be adequately trained, either in the simulator or
in flight, to use the procedures associated with these features.
- If a non-precision approach is necessary, use the recommended flight
guidance system function to fly a stabilized profile at the required angle
whenever possible.
- Continuously monitor position and track by reference to the basic approach
aid(s).
Figura 1
Figura 2
Per ulteriori note e informazioni su argomenti correlati al contenuto di questi due documenti in inglese vi rimandiamo ai seguenti articoli già pubblicati sul nostro sito.
- Eziologia di un incidente: l'impatto dell'A320 Air Inter sul monte Saint-Odile
- Il disastro di Mont Sant-Odile (seconda parte)
- Malpensa (1968) e Frosinone (1971) - ricordo di due incidenti
- Incidenti aerei in Italia negli ultimi trent’anni
- Overrun a Genova - commento al rapporto finale
- FLIGHT PATH CONTROL - THE STABILIZED APPROACH (english)
- Eziologia di un incidente in avvicinamento a Malpensa del luglio 1992 (italian & english version)
