Flying instructor Charlie Huke explains the importance of considering variations in weight, altitude and temperature (WAT) not only when taking off, but also in landing – something many pilots fail to appreciate.
I used to work with a chap called Freddie. He was one of our ops guys, and a fanatical motorsport enthusiast. He had his own rally car, a souped up Mk2 Ford Escort. Every penny and spare minute Freddie had was spent on this car. The only problem was that his dedication and commitment was not matched by his success. Rarely, if ever, did Freddie finish anywhere near the podium. He had a little trophy cabinet in his garage that was totally bereft of anything impressive. Nevertheless, Freddie was quite sanguine about this. His reasoning was that his car went best on Thursdays, and as there were never any Rallies on a Thursday he was never going to win. If ever they did hold one on a Thursday though, he would wipe the field!
But, whilst not linked to the calendar, there certainly would have been days on which Freddie’s car performed better than normal (and so to would his competitors’) albeit it may not have been that noticeable. With a small aeroplane though, the day to day variation in performance can be quite significant, as changes of weight, altitude, temperature, and surface condition can have a profound impact. Although this is covered in all the text books, it is an unfortunate fact that many pilots fail to appreciate the magnitude of performance variation with such changes until it’s too late and they’ve frightened themselves! And to be fair, I understand why. During training all flights tend to be at the same, or very similar, weight and C of G. The vast majority of the syllabus is conducted from the same airfield, so elevation doesn’t change and it’s rarely truly hot in the UK. Consequently it’s rare to actually see any meaningful real world impact of weight, altitude and temperature (WAT) during training. Yes, the aircraft goes a bit better solo, but that’s often all that is really considered. However, any number of bent aeroplanes stand testament to the fact that it is crucial to consider WAT, and not only taking off. It’s just as important when landing.
In essence, the colder and denser the atmosphere and the lighter the aircraft, the better it will perform. With Freddie’s Escort, the power delivered by the engine depends upon the weight of the fuel/air mixture on each power stroke−the denser the air, the more power delivered. And it’s exactly the same for your aeroplane. However the car stays on the ground, so doesn’t need to generate lift as an aircraft must. Flying surfaces (wings or rotors) also perform better in a dense atmosphere. So with the aircraft increasing temperature and altitude (less dense air) has a double impact on performance. Both engine and airframe struggle.
All modern production aircraft, and some homebuilts will have takeoff performance charts. They’re generally easy to use, and look like a graph. You start with the temperature, follow the line to your pressure altitude, and then generally slide down a line allowing for headwind (or up for a tail-wind). You can then read off the takeoff distance required from the side. But it’s imperative that you understand what you’re getting. Usually it is a distance from brakes-off to an imaginary fifty-foot barrier, at a given weight, configuration and from a level, paved runway. If you use the chart for a weight less than your actual takeoff weight, you will pay the penalty. As you will if you assume it is ‘to fifty feet’, when in fact some charts just give the ground run!
Similar charts will allow you to calculate landing distance required. Note too, that any variation in configuration (flaps etc) or deviation from normal operating technique will almost certainly result in increased distance used. For aircraft without such data−many vintage aircraft and most homebuilts−you will need to establish sensible figures for yourself by trial and error, and don’t just follow the herd! Just because you have been told the same type as you’re flying has operated out of a strip, doesn’t mean you can. What was the wind and WAT on his visit? Are your tyre pressures a bit low, do you have the same propeller, and is yours clean of bugs? No two days are identical nor too are any two supposedly identical aircraft.
Having now got an accurate distance required for the prevailing WAT, you need to consider the surface. As I don’t have space here to cover it, check the text books: wet grass, runway slope and soft ground all have a published figure to factor the initial distance from the performance charts. If, for example, it’s both uphill and soft, you have to apply both factors. And then for extra safety, multiply it by 1.33 for taking off and 1.43 when landing. Ten minutes spent playing with performance charts and applying factors for snow etc will be eye-opening for many pilots. Even with apparently small WAT and surface changes you can easily double the distance required for many aircraft. And it’s not just a desktop exercise, the very real number of holes in hedges made by people used to flying two-up from a big airfield when they visit a small grass strip four-up is impacting everyone’s insurance premium. If Freddie’s car doesn’t perform (other than on Thursdays) all it means is no trophy, if your aircraft doesn’t go as usual the impact (excuse the pun) is far greater!
Finally, and although it is usually less important for single engine light aircraft than multi-engine commercial types (where one-engine-out ceiling is often limited), consider en route performance. Not just range and endurance, but the ability to clear topography. Few light aeroplanes have charts for this, but I remember years ago with an old tailwheel Robin 220, at Ruoms in the bottom of the Ardeche gorge in France. We had very unfavourable WAT, but the chart said we could take off, which we did−it took almost twenty minutes to climb out of the gorge though! And one last thought: all of this is utterly pointless if you don’t know the runway length!
