To most of us, a runway is a runway. It’s a strip of tarmac that’s used when an aircraft takes-off, and lands. We all know that runway length is important, after all it’s fairly obvious to all of us that you can’t land a BBJ on a small grass strip. But manufacturers and pilots alike will tell us that there’s much more to a runway than its length.
As well as being able to stop, or take-off on a runway, two other key factors are the environment: most notably the air temperature and and altitude of the runway. This is why Europe bound flights from Mexico City leave so late at night. It is because Mexico City is ‘hot and high’. The airport is 7,316ft above sea level, combine this with year round high temperatures and the engines and engine performance is negatively affected.
When we look at engine performance, the first thing we see is the thrust rating. This is important as it tells us how much power is produced at take-off to help the aircraft get off the ground in the first place. This thrust level isn’t used used throughout the flight though, it’s most commonly used for departure, and I’m sure most of us have noticed during a flight that as soon as we’re comfortably off the ground that the thrust is reduced.
This of course means that the aircraft isn’t as powerful as it was at the moment of departure, however momentum has been gained by that point, so the lower thrust level is all that’s needed to keep us aloft. Often throughout a flight the pilot in command will select a higher thrust setting, but this will be when the aircraft needs to gain speed, or climb to a higher altitude.
The ISA rating is often next to the thrust rating, and this is equally as important as the rating itself, as it tells us up to what point the engine will perform at its best. ISA stands for International Standard Atmosphere, and it’s most commonly followed by the plus sign and a number.
In simple terms this tells us at what point, plus ISA, in temperature the engines will perform as designed to, but of course nothing is ever as simple as that.
Take the Legacy 500, the winner of the prestigious Corporate Jet Investor aircraft of the year award. Its twin Honeywell HTF7500E engines are capable of producing 7,036 lb of thrust each, at ISA +18. As ISA is calculated at sea level (SL) and with the air temperature at +15 degrees, that means that the engines will perform their best up to + 32 degrees. Which is simple enough to grasp at seal level, but once you start adding in the how high above sea level the runway is, things get a little bit more complicated.
ISA is not only calculated at sea level and at +15 degrees, but it’s also calculated at an air density of 1.225 kg/m³ at an atmosphere pressure of 1013.25hPa. Without getting too involved in weather, hPa is an acronym for Hectopascals, which is itself equal to 100 Pascals.
As we go above seal level, the air density changes, it gets thinner. In humans this means that the thinner the air is the harder it is to breath as there’s less oxygen for us to take in.
For every 1000ft above sea level, the temperature is calculated to drop by 1.98 degrees, although this is usually rounded up to 2 degrees. Not only does the temperature drop, but the air pressure and air density drop as well.
Air pressure drops slower the higher up you go, although the air density drops remain relatively constant at 0.4 to 0.2 kg/m³.
All this means that at Mexico City, at 7,000ft, the air temperature is calculated at 1.1 degrees. This means that for the Legacy 500 to achieve its highest thrust setting, the outside temperature would need to be below 19.1 degrees.
Engine performance is only one of the attributes that affect an aircraft’s landing and take-off performance. Both of these are critical in deciding which airports can or cannot be used.
When calculating the distance needed to land an aircraft two data points are used: Landing Distance Required (LDR) and Landing Distance Available (LDA). LDR is how much runway is needed for the aircraft to land on, whereas LDA is the length of the runway. For obvious reasons the LDR cannot be greater than the LDA.
Ideally the calculated actual landing distance will fall below the LDR to give an additional safety margin.
If reverse thrust is used on landing, then engine performance considerations discussed above must be factored in.
Manufacturers like to keep the LDR down as much as possible, as it means that the aircraft can use smaller airports. That of course is one of the key drivers of business aviation, being able to use smaller airports closer to your actual destination.
But in less developed parts of the world runways are often shorter anyway as they don’t need to cater for larger commercial airliners. In these cases then the lower the landing distance required the better.
Below is a short table outlining the landing distances required for current in production and future large cabin business jets:
|Manufacturer||Aircraft Type||Landing Dist (ft)||Landing Dist (m)|
As well as the performance characteristics of the aircraft and its engines, there are other factors that come into play with runways as well, and these are mostly to do with navigational aides, the most common of these are ILS and Localisers.
ILS stands for Instrument Landing System, and is a ground based system that helps pilots land aircraft safely. The system uses a series of radio beam and high-intensity lighting arrays to transmit lateral and vertical guidance to an aircraft as its landing. Without an ILS an aircraft would be unable to land in reduced visibility.
Runways are categorised by three different types of ILS used, with the third category split into four subsections. The category of runway tells the pilot at what height they need to make a decision on a missed approach if they can’t see the runway, as well as the minimum amount of visibility a pilot must have while on approach.
CAT 1 has a decision height of 200ft and a runway visual of 1,800ft.
CAT II has a decision height of 100ft and a runway visual of 1,200ft.
CAT III has no decision height but a runway visual of 1,2000ft
CAT IIIa has no decision height but a runway visual of 700ft
CAT IIIa has no decision height but a runway visual of 150ft
CAT IIIc has yet to be approved for operation anywhere, but it proposes that an aircraft be able to taxi in zero visibility
The localiser is a set of directional beams that help the pilot line-up with a runway. Several beams are transmitted from a station just beyond the end of the runway, with several beams going slightly to the left, and several slightly to the right.
By calculating the distance between the beams, an aircraft can correct its track to ensure that its correctly lined up on the runway centreline for landing.
So the next time you’re at the end of a runway waiting to take off, think about the calculations that the pilot has to take, and look around to see what equipment is also installed. After all, a runway is more than just a strip of tarmac.