1. TO CONTINUE THE APPROACH OR NOT
Once past the OM or point ’D’ the requirement, that the reported visibility must read at or above prescribed minimum is no longer applicable. Instead this applies to flight visibility. This implies, that the see to land concept must be well defined and the consequences well understood.
See to Land Concept
In the see to land concept, the pilot is required to continuously assess the possibility of a safe landing and roll-out on the basis of visual reference.
The see to land concept is applicable to most company instrument approaches.
In case of an operation without a Decision Height, there is no need for visual reference. Such an operation relies on automatic flight control systems such as automatic landing, automatic roll-out and accurate ILS signals. Any requirement for RVR is only needed to ensure visual reference for safe taxiing and/or safe roll-out following a ’no DH’ landing.
See to land may seem obvious. However, it proves to be difficult to describe. This is clearly reflected in USA legislation, which was based on NTSB (USA National Transport Safety Board) research and recommendations. It appeared, that approach and landing accidents are the largest single cause of air carrier fatalities (41%).
ILS CAT II and III approaches, where special equipment, training and approach procedures are used, resulted in a good safety record.
A large number of the approach and landing accidents occurred when the reported weather conditions were better than CAT II, but less than 1200 ft ceiling and 5000 m visibility. Of these the majority involved ceilings of less than 600 ft and visibility less then 2500 m. Among the factors causing or contributing to these accidents the NTSB cited: continuation of descent below the descent limit with inadequate visual cues, unrecognized altitude loss or descent rate, disorientation, collisions with obstacles well below the normal descent path, loss of sight of the runway while below the descent limit and other factors related to lack of adequate visual reference.
This has led the FAA to revise its rules in 1981. As company used FAA rules and associated background information as an important source for the development of the approach minima calculation method, some selected information is given below.
Operation below the descent limit
No pilot may operate an aircraft at any airport below the descent limit or continue an approach below the descent limit unless:
- The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal manoeuvres and such a rate of descent will allow touch down to occur within the touch down zone of the intended runway;
- The flight visibility is not less than the visibility prescribed for the approach procedure being used;
- At least one of the following visual references for the intended runway is distinctly visible or identifiable to the pilot:
the approach light system, the runway or its markings or lights,
the touch down zone (or its markings or lights), etc.
(further requirements are subject to non-precision, ILS CAT I, II or III operation).
Some comments
– ’A normal rate of descent to allow touch down to occur within the touch down zone’
This is a.o. intended to ensure that both the adequate visual reference is obtained and that the aircraft is established on a normal glide path for landing.
Flight Visibility
’The flight visibility is not less than prescribed for the procedure being used’
The visibility referred to is the visibility from the aircraft. This flight visibility is continuously required until touch down. It is an important factor in preventing disorientation and inadvertent descent below the glide path. How the pilot should measure the flight visibility in the dynamic situation during the approach cannot be precisely answered. Nevertheless it remains a concept, that provides for the necessary safety during approach to land.
’One of the following visual references for the intended runway is distinctly visible and identifiable’
The requirement clarifies, that operation below the descent limit may not be based on unauthorized landmarks like towers, haystacks, buildings, etc.
The words distinctly visible and identifiable denote the intention, that the visual cues be discrete and unmistakably identifiable.
Example: Descent below MDA or DH/DA may not be based on a general glow of lights through a layer of fog, where the visual references themselves are not discretely identifiable.
Further discussion: When flight visibility is marginal
Flight visibility (defined as ’the visibility from the cockpit’) can deviate substantially from reported weather conditions.
As a consequence of the ’see to land’ concept flight visibility must continuously be assessed while below the descent limit.
Safety investigations (NTSB) learn that in marginal visibility pitch control is the most difficult. Correct visual pitch assessment needs sufficient visual reference. If the flight visibility is less than prescribed for the applied approach procedure, the visual means for safe pitch control are in principle insufficient to proceed for landing.
Vigilant visibility assessment will certainly be made in expected marginal conditions such as an approach on the basis of a PIREP or increments below.
However, also in less obvious conditions the visibility from the cockpit could be substantially less then expected.
(i) Threshold
In actual operation, in case of a precision or non-precision approach, there is no requirement to see the threshold before descending below the DA/MDA.
Lowering the limits to CAT II and deeper into CAT III will drift us away from the difficulties of the see to land concept while increasingly relying on more sophisticated automatics and ground facilities.
(ii) Expanding visual segment
When approaching in a homogeneous atmosphere, the segment visible on the ground will rapidly expand. However, if the flight visibility decreases, expansion of the visual segment will be delayed or will not occur at all.
This could result in loss of pitch orientation, even when having the threshold insight.
A consequence of the see to land concept is that a go-around is required if the flight visibility is below its prescribed minimum for landing.
A below minimum visibility report is received beyond OM or Point ’D’
When approaching in rapidly changing weather conditions a report received beyond OM or point ’D’ could give a visibility value below the one which authorized continuation of the approach beyond OM or point ’D’.
The rules do not require a go-around in such circumstances. The flight may descend to the limit to have a ’look and see’ if conditions at DA or MDA allow a continuation to land or require a go-around.
To continue the approach following a below minimum report might be experienced as logical, when executing a non-precision approach on the basis of a general visibility report.
The pilot might reconsider this in case a below minimum report is received when a lower limit ILS is made, especially when using a DH below the alert height.
The course of action following a below minimum roll-out RVR report could be different depending on circumstances. E.g.: current position, aircraft type, weight and condition, available automatics, runway length and braking action, wind, etc.
If deciding to go around, be advised it may be initiated from any altitude (also refer to guide 6.1.9 Missed Approach).
NOTE: The chances of receiving a below minima report beyond OM or point ’D’ can depend upon local RVR reporting practices.
Optical illusions
Pilots are reminded, that the decision required of them at the descent limit is made using the weakest chain of the tool system: the human eye.
Visual illusions are not uncommon, when human senses receive seemingly contradicting information. Being aware of suspect conditions can enhance decision making at and below the descent limit. Before some conditions are high-lighted, which can lead to illusions, a general statement might be appropriate:
Instrumental tools such as flight guidance systems and RVR devices do not suffer from visual illusions.
2. VISUAL ILLUSIONS
Most of our approaches are based on the see-to-land concept. So, depending on the approach procedure, some visual flight will be required. This visual portion is very short for ILS CAT II, but extended visual flight may be required in the case of a non-precision approach. Visual assessment of the situation at the descent limit and flight by visual reference thereafter may go wrong unintentionally because of visual illusions.
Being aware of suspect conditions can help to prevent ending up in a hazardous situation. Visual illusions may be caused by one or any combination of the following features: fog, rain, sloping approach terrain, sloping runways, featureless terrain, runway lighting intensity, darkness, black hole effect and (blowing) snow. We will deal with some of these features in the following paragraphs.
Rain
Heavy rain can affect the pilot’s perception of distance by defusing the glow of the approach or runway lights and causing them to appear less intense. This may lead the pilot to suppose they are farther away. On the other hand, only a little scattering due to water on the windscreen can cause the lights to bloom and double their apparent size. The result is, that the pilot believes to be closer to the runway than he/she actually is, leading possibly to a premature descent.
Light ray refraction as a result of rain on the windscreen can lead a pilot to believe he is above or below the correct glide path or left or right of the centreline.
Errors in altitude perception can be 200 ft at a distance of 1 NM from the threshold.
A heavy rainstorm moving towards an aircraft can cause a shortening of the pilot’s visual segment.
This can produce the illusion, that the horizon is moving lower or the aircraft pitching-up. A natural response would be to lower the nose.
Black Hole effect
This illusion can occur on a clear night with no visible horizon. The aircraft approaches the runway over the sea or other featureless, unlit terrain towards an aerodrome with bright city lights behind it. Visibility is so good that there is little need to rely on the instruments except to check the airspeed.
The straight-in approach is totally uneventful until the aircraft lands short of the runway, possibly by several miles.
What could have gone wrong?
Tests have sown that under these circumstances a pilot relying on a visual approach will tend to fly along the arc of a circle centered above the pattern of city lights with its circumference contacting the terrain. Such a path results from maintaining a constant visual angle subtended at the eye by the nearest and farthest city lights. When deceptive conditions are present, such as up-sloping city terrain, this kind of approach path can go to critically low altitudes. The lack of foreground lighting results in the pilot being denied important closure information without his/her awareness and consequently the aircraft lands short.
Blowing snow
Anticipate a lack of visual cues to effect runway identification. Approach and runway lights will provide some identification. Runway markings and the contrast between the runway and its surroundings will be lost in the whiteness. Depth perception may be difficult. Especially in the polar regions be aware of the white-out phenomenon, which can even occur in a crystal clear atmosphere.
In crosswind conditions blowing snow may cause the pilot to select the wrong crab angle and to deviate from the centreline.
Fog
Is the aircraft tracking correctly and is it on the correct glide slope in the normal approach attitude? When approaching in fog with only some approach lights visible at DH, the pilot is able to assess correct tracking. But what about glide slope deviation?
Without really being aware of it, it appears that there is a particular spot on the window screen, which the pilot uses as reference for e.g. the aiming point on the runway. It is located on the windshield on a line from the pilot’s eye to the distant point towards which the eye is moving. The location of this spot on the window appears to be rather steady, regardless of the aircraft attitude. In other words, if it is located over e.g. the horizon, the pilot is aiming at level flight.
During ’normal visibility’ approaches, the aiming spot on the windshield will be located over the aiming point on the runway. Also during normal visibility approaches, pilots become accustomed to a particular approximate relationship between the apparent width of the runway and the extent to which the aiming point appears to be ’elevated’ above the threshold.
Finally runway perspective can give an indication of too high or too low.
Limited visual segment
With only some approach lights in view and no touch down area an experienced pilot is able to project the estimated location of the still obscured aiming area on the windshield spot. However it takes training to do that, because it is unnatural.
If the touch down zone is not available as an aiming point and the approach lights are only partly visible, the natural reaction would be to lower the nose to position the aiming spot on the windshield just beyond the far end of the approach lights. On this spot the pilot is accustomed to project the aiming area of the runway.
In other words, approach lights provide little or no information regarding glide slope deviation. Moreover, they can produce the illusion, that the aircraft is too high. As visibility becomes less, this effect becomes more serious.
Through extensive training the pilot is prepared to recognize and cope with a limited visual segment. Good crew co-ordination procedures assuring continuous monitoring of ILS raw data and full use of the autopilot capabilities contribute to a safe operation.
However, visual illusion can become an important factor in low visibility conditions even if the pilot is well trained. This may be the case when the atmosphere is not homogeneous. Whereas most training is done under homogeneous atmospheric conditions, strong illusion may be evoked if the visual segment does not develop according to the expected pattern.
Visibility may rapidly change along the flight path in patchy fog.
Wind shear can make the situation worse. There seems to be a strong probability, that combinations of glide slope deviation and pitch variations resulting from wind shear can deceive the pilot into thinking he/she is on the desired flight path.
Pitch orientation is extremely difficult if the familiar pattern is not encountered. In such situations the NTSB noted unrecognized altitude loss or descent rate, collisions with obstacles well below the normal descent path, failure to monitor or cross check altitudes, ….. etc.
Cases are known, where illusions were so strong, that even the autopilot was disconnected and the approach continued by visual reference: touch down was short of the runway.
An unfortunate decision as the autopilot does not suffer from visual illusions!
3. NO AUTOLAND (TCH)
’No Autoland (TCH)’ may be noted on the approach chart
’No Autoland (TCH)’ may be noted on the approach chart. This refers to appropriate AOM directives to abandon the ILS glide path timely so as to cross the runway threshold above the ILS glide path.
The reason is that the ILS glide path crossing height over the threshold is too low for one or more aircraft types, and no usable underrun area is available.
The action to effect upward glide slope deviation must be conducted visually
The action to effect upward glide slope deviation must be conducted visually. This requires adequate visual reference. With the purpose to reduce the chance of visual illusion substantially, company requires that in these circumstances the runway threshold must be visible at the descent limit.