How pilots adapt their takeoff plans at hot and high airports
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We’ve all been there, trying to run on a hot day. It’s hard work. If you’ve tried doing it in a city like Denver, Johannesburg or Mexico City, you’ll know how much harder it is at altitude. It feels like the air is thinner and it’s much harder to breathe. You’re not imagining this and it’s the same for aircraft.
Departing in hot weather at high altitude airports reduces aircraft takeoff performance. When operating in these conditions, pilots must pay close attention to the environmental conditions as they can affect the flight in more ways than you might imagine.
Aircraft fly not because of the engines, but because of the lift generated by the wings. In its most basic form, the faster air flows over the surface of a wing, the more lift is generated. However, in the real world, it’s not quite as simple as this.
The amount of lift created is very much dependent on not only the speed of air flowing over the wing but also the density of that air — in effect, how many air molecules are passing over the wing in a given time. The more air molecules passing over the wing, the greater the lift.
However, air is air, right? Not quite. Even though you may not be able to detect it, the density of air changes constantly as you move around the earth. It will even change in the distance of a few metres, even though that change is minimal.
Wind is the result of air moving from an area of high atmospheric pressure to an area of low atmospheric pressure. These differing areas of pressure result in varying air density.
So what affects the density of air? As we’ve just mentioned it, let’s start with air pressure.
Air pressure is basically the weight of a fixed column of air. If you squeeze more air molecules into that column, the weight (pressure) increases. Conversely, if you reduce the number of air molecules in the column, the weight (pressure) decreases.
To have a base to measure the actual weather conditions against, scientists came up with the International Standard Atmosphere (ISA). This assumes that using a location at sea level, the temperature is 15 degrees Celsius and the air pressure is 1013 hectopascals (hPa). The temperature is then assumed to drop by 2 degrees Celsius for every 1,000 feet increase in height.
Using ISA makes it easier to explain air pressure by using the example of barrels of water.
The ISA barrel contains a certain amount of water which makes it weigh 100 kilograms (as an example figure). If you add more water to that same barrel, the weight increases. If you remove water from that barrel, the weight goes down.
The barrel, or the column of air, remains the same. It’s the amount of water, or air molecules, which varies, in turn affecting the overall weight of the barrel.
The higher up you go in the column of air, the less air there is in the atmosphere above pushing down. This means that the air molecules can spread out even more, reducing the air density. At the top of Mount Everest, 29,000 feet above sea level, the air density is so low that it makes it extremely difficult to breathe.
The other factor which affects air density is the air temperature.
When air molecules are cold, they have little energy so are sluggish and slow. They sink close to the ground and make the air “thick” with air molecules.
As their temperature increases, their energy increases and they become more active. The more active they become, the more they move around and apart from each other. This in effect makes the air close to the ground ‘thin’ with air molecules.
As a result, the air on hot days is less dense than air on cold days.
For pilots, the whole point of understanding the effects of pressure and temperature is to appreciate the effect they will have on the performance of our aircraft. In order to do this, we combine the factors to determine the density altitude of an airfield.
Last month, I wrote an article on flying from London to Johannesburg, which is a classic example of a “hot and high” airfield. The airport sits 5,500 feet above mean sea level and temperatures in the summer can reach the mid 30 degrees Celsius.
To determine the density altitude, we use this equation:
Density altitude = airfield altitude + 1,000 feet for every 8 degrees Celsius above ISA
So, for a 28 degree Celsius day in Joburg, first we need to work out the difference from ISA. If we remember from above, at sea level the temperature is assumed to be 15 degrees Celsius, reducing by 2 degrees Celsius for every 1,000 feet we go up in height.
Using this logic, as Joburg airport sits 5,500 feet above sea level, we can work out that the ISA temperature should be 4 degrees Celsius (15 – 5.5×2). However, we already know that the actual temperature is 28 degrees Celsius, a difference of 24 degrees Celsius. We can, therefore, say that the temperature is “ISA + 24 degrees Celsius”.
Using the equation above we can then work out:
Density altitude = 5500 + (3 x 1000)
Therefore, the density altitude is actually 8,500 feet. This means that the aircraft is actually flying in air much thinner, and thus less efficient than we would initially think.
How it affects how we takeoff
Before every departure, we carry out a performance calculation to determine the most efficient way to get the aircraft airborne safely. On the 787 Dreamliner, we use the Onboard Performance Tool (OPT) to calculate the engine power, takeoff speeds and flap setting.
Whilst we don’t enter the density altitude directly, we do enter the data the OPT needs to calculate this — the airport and runway we’re taking off from, the air temperature and the air pressure. With this data, plus a few other parameters such as wind velocity and aircraft weight, the OPT is able to determine our takeoff performance.
However, hot and high operations have a significant effect on a number of aspects of a takeoff.
As the density altitude increases, the number of air molecules passing over the wing reduces, thus reducing the lift generated. As lift is a function of air density and how fast that air passes over the wing, the only way to overcome this is to go faster.
As a result, we will aim to use the longest runway available to allow us to increase the takeoff speeds and increase the lift. However, if the density altitude becomes too great, even the longest runways may not be long enough to accelerate to this faster speed.
Like with the wings, the lack of air molecules in hot and high scenarios also affects the efficiency of the engines. Jet engines operate by sucking air into the front, compressing it, igniting the oxygen component and then blowing the fast-moving hot air out the back to create thrust.
However, as mentioned before, when the density altitude is high, the air is much “thinner”. If we think back to our example of climbers on Mount Everest, the same principle applies. With a lower density of air, there are fewer air molecules passing through the engine. This results in less thrust being produced.
Once airborne, the effects of a high-density altitude are not over. With reduced engine power and lift generated from the wings, it only follows that we won’t be able to climb as quickly as normal.
This isn’t so much of a problem if the climb takes place over the water, for example when taking off from Los Angeles to the west. However, if there are trees, buildings and hills in the climb routing, we may have a problem.
The performance calculated by the OPT is actually more restrictive than you may initially think. Just because the aircraft could get airborne at a certain weight, say 230 tons, we don’t use that figure. Why? The engine failure case.
Everything we do as pilots is solely focused on safety. With that in mind, we’re always thinking of the “what if?” scenario and one of the most important “what ifs?” is losing an engine just as we become airborne.
Should this happen, we will need the remaining engine to be producing enough power to enable us to climb away from the ground and clear any obstacles in the initial routing.
As a result, the OPT will also take the required climb gradient into consideration when calculating our takeoff performance.
It’s for this reason why the longest runway may not necessarily be the best runway to use. Yes, it will allow us to accelerate to a fast speed, but if there are large hills in the initial climb out, it will limit our performance
A decision to make
The lack of lift and engine power leaves us with two options.
Option 1: Wait until the environmental factors change significantly to reduce the density altitude.
This will normally mean waiting for the temperature to drop. This may be possible in the evening, but in the middle of the day, it’s highly unlikely to change much. It’s for this reason that most long-haul departures from hot and high airports take place later in the day when the temperature has dropped.
Option 2: Reduce the aircraft weight.
All aircraft have a Maximum Takeoff Weight (MTOW) which is determined by the manufacturer. This is the maximum weight at which the aircraft can legally be for takeoff. For most normal departures, the MTOW is the limiting weight.
The Actual Takeoff Weight (ATOW), the weight of the aircraft when we start the takeoff roll, is calculated by the following equation.
It follows that the ATOW must always be less than the MTOW.
However, when departing from hot and/or high airfields, because of the lack of lift, reduced engine power and reduced climb performance, we may have to reduce the weight at which we can take off. This is known as a Performance Limited Takeoff Weight (PLTOW).
When arriving on the aircraft at a hot and high airport, the first thing we will do is enter the environmental data into the OPT to calculate our PLTOW. We then pass this weight onto the dispatcher who then has the challenge to ensure that the Actual Takeoff Weight is less than the Performance Limited Takeoff Weight.
So, with a full load of passengers and cargo, we could find ourselves in a situation where the ATOW is greater than the PLTOW. This gives us a problem.
If the ATOW is greater than the PLTOW, we need to reduce the weight of the aircraft. By looking at the equation above, there are only two ways of doing this. Either reduce the fuel load or reduce the payload.
Ultimately, we need a certain amount of fuel to fly to our destination, so there’s not a great deal we can do with this one. The only element which we can sometimes change is the destination alternate airport. By using a closer alternate, we may be able to reduce the required fuel by a couple of tons.
However, as we normally use the closest airport to our destination as the alternate, this is rarely an option. The only other choice we have is to reduce the payload.
The first element of the payload to be removed will be cargo. If this isn’t enough, next will be passenger baggage. Finally, in extreme circumstances, passengers will have to be offloaded.
Whilst an airline will do everything they can to keep passengers and their baggage from being offloaded, it’s our job as pilots to ensure that nothing comes before the safety of the aircraft and all those on board.
Taking off from hot and high airfields can seriously affect the performance of all aircraft. When departing in such conditions, pilots must pay careful attention to the conditions and adjust the aircraft weight accordingly. Taking off at too heavy a weight could have serious implications for the safety of the aircraft.
However, it’s not just on takeoff that hot and high conditions affect the performance of an aircraft. When flying an approach and landing, the density altitude makes a big difference in how the aircraft flies. If you’re not prepared, it can easily catch you out. In my article next week, I’ll be explaining what those pitfalls are and how we deal with them.
Featured photo by Alberto Riva/The Points Guy
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