The landing phase is one of the most critical phases of flight and most accidents and incidents
occur during this critical phase. Among many other things, the landing distance calculation is one of
the most essential calculations that must be performed in order to ensure a safe landing.
In fact, many regulations concern the landing distance of an aircraft. So, with all this in
mind, let’s examine landing distance, as well as how pilots calculate this, in today’s video.
For a transport category aircraft certified as a large aeroplane, the landing distance
calculation commences from 50 ft above the runway threshold and ends when the aircraft
comes to a complete stop. This 50-foot distance is called the screen height and is a fixed value
for all aircraft certified as a large aircraft - as per EASA regulations under the category CS-25.
The landing distance can be divided into two parts: an airborne section and a ground run.
To be more specific, the airborne portion of the landing starts at 50 ft, where the
aircraft is at the correct approach and landing speed, known as Vref (landing reference speed),
over the runway threshold. It ends when the main wheels of the aircraft touch the ground.
This marks the beginning of the ground roll, which ends when the aircraft comes to a stop.
The total of this is considered an aircraft's required landing distance,
but is also known as Actual Landing Distance (ALD). A.L.D’s are tested by the manufacturer
during the aircraft certification process and are mentioned in its flight manual.
The Landing Distance Available (LDA) concerns a given aerodrome runway. This represents the
distance from the point on the runway at which an aircraft can commence its
landing to the point where the surface can no longer bear an aircraft's weight.
At almost all airports, the landing distance is from one runway threshold to the other.
A runway with a displaced threshold cannot be utilized for landing distance calculation.
As noted by the FAA, a displaced threshold is a threshold located at
a point on the runway other than the designated beginning of the runway.
When it comes to landing distance requirements, there are two conditions to consider.
One is the calculation of landing distance when the aircraft is
dispatched, and the second is done inflight right before the landing.
First, let’s examine landing distance assessment at the time of dispatch.
According to regulations, when an aircraft is dispatched to an airport, it must be able to
land within a certain percentage of the landing distance available at the field.
For a turbojet aircraft, it must be possible to make the landing within 60% of the Landing
Distance Available (LDA). And for a turboprop, it must be able to make the landing within 70%
of the LDA. However, for turbojet aircraft, an extra 67% of its Actual Landing Distance (ALD)
must be added to the final landing distance calculation, and 70% for turboprop aircraft.
This multiplied aircraft ALD is known as the Required Landing Distance (RLD).
Is this a little confusing? Let’s use an example to make things clearer:
An Airbus A320 is to be dispatched to an aerodrome with a runway length of 8000 m.
According to the aircraft performance for a dry runway, it has an ALD of 1100 m.
So, we have to ask, can the A320 be dispatched?
60% of the runway’s 8000 m length is 4800 m. This is its LDA.
As per Airbus data, the A320’s actual landing
distance is 1100 meters. 67% of this would be 737 m.
Therefore, the Required Landing Distance for the A320 would be
calculated as 1100 + 737. The outcome is 1837 m.
Our calculations show that the aircraft has an RLD of 1837 m, which is far less than 4800 m.
In this case, the aircraft can safely be dispatched to the airport without issues.
For a turboprop, the calculations are similar, but the numbers are different as it must be able
to land within 70% of an airfield's LDA, and the turboprop's RLD is also its original ALD plus 70%.
For wet runways, 15% of RLD must be added. If we take our previous example,
the initial RLD is 1837 m. So, when we add 15% of 1837 m - which is 276
m - to 1837 m itself, then the wet RLD equals 2113 m.
When it comes to contaminated runways, if the flight manual has specific data
for landing distance with different contaminants, those values must be used.
The FAA notes that a runway is considered contaminated whenever quote “standing water,
ice, snow, slush, frost in any form, heavy rubber, or other substances are present.”
When the manufacturer provides contaminated ALD,
this must be converted to an RLD (67% of the contaminated ALD),
and then 15% of that contaminated RLD must be added to get the final measurement. If
contaminated data is not available, then wet runway performance must be used.
Let’s now look at landing distance at the time of arrival
The Landing Distance at the Time of Arrival (LDTA) is a more recent
addition to regulations. The purpose of LDTA is to give pilots a more realistic landing distance.
Traditionally, the Actual Landing Distance (ALD) given by the manufacturer in the
flight manual is considered the base performance of the aircraft.
But the ALD is not realistic for the following reasons:
It is calculated only on a dry runway. It assumes a touch-down rate of 480 ft/min.
It assumes full manual braking after landing. Speed is considered under standard conditions.
It does not usually consider the effect of runway slope. And finally,
It is performed by a test pilot whose primary aim is to reduce the landing
distance as much as possible so that the aircraft can be marketed well.
Test flight performance data cannot be achieved in a routine flight because:
Runway conditions change. The touch-down rate is lower than
480 ft/min; For reference, a typical landing descent rate rarely even reaches 300 ft/min.
Full manual braking is not used in order to reduce brake wear. Additionally,
most large aircraft use auto brakes.
Standard atmospheric conditions do not exist in real life, so speed variations can occur.
Changes in the wind can cause adjustments in landing technique and distance.
The runway slope is a factor; and finally, Airline pilots perform landings with varying
skills, and the idea behind a safe landing is not to reduce the distance as much as possible.
An LDTA, thus, helps pilots to calculate their landing distance much more correctly. These days,
the ATC can give the pilot accurate information on the
runway condition based on a Runway Condition Assessment Matrix (RCAM).
Using the RCAM, the pilots are provided Runway Condition Codes (RWYCC).
These are calculated every third of the runway. The RWYCC codes are numbered from 1-6.
These numbers account for how good the braking performance is. For instance,
5 is considered good, 3 is considered medium, and 1 is considered poor.
ATC or the airport authorities issues the code as number/number/number for example, 6/5/4.
Each number accounts for a different part of the runway. In the case of our example,
the first section has a braking action of 6, the second has 5, and the third has 4. Pilots can
choose to use the lowest number as the braking action or the mid-number when a code is given.
The mid-number is applicable because most of the braking during a normal landing occurs in
the middle area of the runway. If it’s a heavy aircraft, then the last part of the runway may
also be applicable. Ultimately, this will be decided by the commander of the aircraft.
So, there’s our quick guide to understanding various landing distances,
and how pilots properly calculate safe distances for their missions.
What are the best and worst landings you’ve ever
experienced during your travels? Share them by leaving a comment down below!