The challenges for pilots when landing at hot and high airports
Everything becomes an effort when it gets too hot. Going for a run is hard work, focusing on a task takes extra concentration and a decent night's sleep is hard to come by. Do all this at altitude in a place like Johannesburg or Mexico City and these tasks become even harder. It's the same for flying an aircraft. Last week I wrote about how hot and high conditions affect our takeoff performance. Like with all things in life, what goes up must come down, so it will come as no surprise that high temperatures and elevation also require careful thought and planning when landing.
The effects of heat and altitude
Aircraft fly because of lift generated by the wings. The engines merely provide the forward thrust to drive the wing forwards. As air passes over the wing, the difference in pressure between the top and bottom surfaces, and the angle at which the air hits the wing, creates lift. However, not all air is the same.
The total amount of lift generated depends on two factors. The speed which the air passes over the wing and how dense that air is. Air density, in its most basic form, is how many air molecules there are in a given space. The more air molecules, the denser the air and the more lift is generated.
However, when air gets hot, the energy of the air molecules increases and they move farther apart. The further apart they get, the less dense the air becomes. It's the same for altitude. As altitude increases, the air pressure decreases and the air molecules move further apart, thus reducing the air density.
Individually, these two factors can affect the performance of an aircraft. Put the two together and they can cause some serious problems for the oblivious pilot. As a result, when landing into a hot and high airfield, we pay particular attention to the density altitude and adapt how we fly accordingly.
TAS -- IAS -- GS
Speed is speed, right? Not quite. When it comes to flying, there are three different measurements of speed that we must take into consideration: indicated airspeed (IAS), true airspeed (TAS) and ground speed (GS).
Indicated airspeed (IAS)
As the name suggests, indicated airspeed is the speed that is indicated to us in the flight deck. Probes on the outside of the aircraft called pitot tubes face into the oncoming air and measure the flow of air. This represents the speed which the air is moving over the wing. The IAS is important as it is this value which directly affects the amount of lift generated.
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However, IAS in effect tells us how many air molecules are passing over the wing. As mentioned above, the higher we go in the atmosphere, the fewer air molecules there are in a given volume of air. As a result, our IAS actually decreases as we climb into the thinner air.
What we really need is a speed that tells us how fast we're moving relative to the surface.
True airspeed (TAS)
The true airspeed (TAS) of an aircraft is how fast the aircraft is moving across the ground in zero wind conditions. Whilst this is of little use to us when thinking about lift, it becomes really important when managing the energy of the aircraft during the descent, approach and landing phases of flight.
Unfortunately, the aircraft is unable to calculate TAS directly. However, as it does measure the IAS and altitude directly, the system is able to calculate the TAS for us. It is then displayed on the screens, normally on the navigation display.
At sea level, TAS is pretty much the same as IAS. However, as the density altitude increases, TAS increases relative to IAS. Why? As IAS measures the number of air molecules passing over the wing, this value decreases with altitude. However, the speed at which the aircraft is moving over the ground is the same.
As a rule of thumb, TAS is roughly 20% faster than the equivalent IAS.
Ground speed (GS)
The ground speed (GS) is how fast the aircraft is actually moving over the ground and is what affects the total flight time. In its most basic form, the ground speed equals the TAS plus the wind. Therefore, if the TAS is 300 knots and you have a 100-knot tailwind, the GS is 400 knots.
It's for this reason that flights from Europe to North America take longer than the eastbound return. The prevailing jet stream winds travel in an easterly direction. As a result, we do our best to avoid these strong headwinds when flying westbound and look to utilize them when flying eastbound.
In February 2020, the tailwinds across the Atlantic were so strong that a 747-400 flew from New York JFK to London Heathrow in under five hours. By flying in the core of the 200 mph jet stream, the crew were able to use their TAS of around 550 mph and create a ground speed of around 750 mph -- an incredible 12 miles a minute.
Energy management
Managing the "energy" of a commercial airliner is key to a successful and safe landing. Carrying too much energy at certain stages of the approach increases the chances of a runway excursion.
When pilots talk about the "energy" of the aircraft, we are referring its speed and altitude in relation to its ideal position at that time. For example, 30 miles from touch down at a sea-level airfield, you'd ideally be around 10,000 feet and 250 knots.
Read more: The pilot’s view on the London Heathrow final approach
If you were in fact at 15,000 feet and 310 knots at this point, you would be carrying excess energy. This energy would need to be reduced, normally by use of the speed brake.
When making an approach to a hot and high airfield, there are two factors that can cause problems to the unwary: the effect of TAS and the thinner air.
The TAS trap
For the majority of the time when flying an aircraft, the primary speed of reference is IAS. It's the speed most prominently displayed to us and for good reason, as it is the speed that is most critical to flight safety.
Too slow an IAS and the wing could stall and stop producing lift. Too fast an IAS and the structure of the aircraft could be damaged. As a result, we are trained to focus on the IAS over any other speed.
However, referencing just the IAS when approaching a hot and high airfield could catch us out. In the example above, we may think that we're flying 250 knots at the 30-mile point, but the TAS, and as a result, the GS will be much higher. This gives us problems when both descending and turning.
Most approaches we fly are at a 3-degree descent angle. To calculate the rate of descent needed to achieve this, we multiply our groundspeed by five. However, as the TAS is higher, it follows that the ground speed will also be higher, resulting in a higher than normal rate of descent.
If a crew doesn't factor this into the descent planning, they could find themselves in a high energy situation.
Increased TAS also poses a problem when turning. Like in your car, when driving at a faster speed, it's more difficult to make a tight turn. There's a reason why you slow down for roundabouts. It's the same in the air.
As the TAS increases, the radius of turn also increases. Once again, if a crew is focused solely on the IAS and expecting the aircraft to turn relative to that speed, they are going to end up quite surprised. Making an approach into an airport like Mexico City, which is surrounded by 17,000-foot mountains and volcanos, this is a very real safety hazard.
Thinner air
If a crew have found themselves caught out by the problems with TAS and end up in a high energy situation, all they need to pull is just the speed brake and sort it out. Right? Unfortunately, there is a secondary sting in the tail from the high-density altitude.
We already know that the lack of air molecules reduces the lift but this also means that there is less friction between the aircraft and the air. As a result, the aircraft does not slow down as quickly as it would do normally.
When already in a high-energy situation, taking longer to slow down is the last thing that we want. So, in order to avoid this, we come up with a plan during the descent brief.
Landing roll and braking
Once on the runway, the story isn't over. The high-density altitude still affects us on the ground during the landing roll.
Firstly, as the TAS is higher than normal, the ground speed on touchdown is also much faster. In addition to this, the thinner air means there is less air resistance to slow the aircraft down. The result of this is that the brakes have to work much harder to hard to slow the aircraft down.
To help reduce the energy going into the brakes, we use the other deceleration system available to us, the reverse thrust.
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After touch down, we always activate the reverse thrust which deflects engine air forwards instead of backward, assisting the deceleration. There are two levels of reverse thrust: idle and maximum.
To keep the noise footprint to a minimum, on most landings we only use idle reverse. However, with the increased energy in a hot and high landing, we have to use maximum reverse to stop the brakes from overheating.
Mitigating the effects
A safe landing comes from a good approach and a good approach comes from an effective approach brief. Before starting our descent from the cruise, we will always discuss the major threats for the approach and landing. At a hot and high airfield, we will always talk about how we're going to manage the effects of the high-density altitude.
In the grand scheme of things, when making an approach into a hot and high airfield, the less energy you have, the better. It's much better to bring the aircraft in nice and slowly rather than be chasing the ideal energy profile all the way down to the runway.
As we know the effects of IAS vs TAS, we plan to fly a slower IAS at a given point. So, in the example earlier, instead of flying 250 knots IAS at 30 miles, we'll plan to be there 20% slower at around 200 knots IAS.
Even when flying to our planned energy profile, we will always still need time to decelerate, so this must be built into our plan. As this takes much longer at high-density altitudes, we must make allowances for this before we start the descent.
In normal situations, on the 787 we would normally allow around 10 miles in level flight to slow the aircraft down to the approach speed. When landing at the likes of Joburg, we will increase this distance a considerable amount, by around 50%. This gives us plenty of time in which to slow the aircraft down and gives us some breathing space should we arrive at the point a little high on energy.
The radius of turn is also something we have to make allowances for. The approach into Mexico City (7,500 feet above sea level) requires a tight left turn just five miles from the runway. If you're flying too fast, the turn will not be tight enough and you'll be unable to line up with the runway.
Bottom line
Landing into a hot and high airfield provides a number of challenges which are not evident to passengers. Without a decent plan, the effects of TAS and the thinner air can very quickly put you into a high energy situation which can be difficult to regain.
As a result, before every landing, we always conduct a comprehensive brief in the calm of the cruise. This not only enables us to identify the threats of the approach but also to discuss how we plan to deal with them.