“My friends just took off, why can’t we?" "What do you mean that we can't take all of our luggage on the plane?" “Why can’t we fly non-stop home from Aspen?"
Sometimes you can, and sometimes you can’t. Add this to the confusion: sometimes you can in the winter, but, not the summer. It’s as frustrating for pilots as it is for the passengers. However, because pilots are bound by simple laws of physics, it’s easier for us to accept.
There are 4 limiting factors that affect the ability to take off, and, how far we can fly. I’ll try and keep it simple.
1. Does the weight of the plane (including fuel, passengers and luggage) exceed the maximum structural limit of the plane?
2. Does the weight of the plane exceed the braking limits of the plane if a takeoff has to be aborted (think engine failure on the takeoff roll, an unsafe abnormality in the cockpit, or, even a deer jumping in front of the plane – it happened to a friend of mine)? Exceeding this “brake energy limit” could cause the brakes/tires to fail in a catastrophic manner.
3. Does the weight of the plane exceed a limit, that if an engine were to fail, the plane would not be able to stop in the amount of runway remaining?
4. Does the weight of the plane exceed a limit, that if an engine were to fail, prevent the plane from climbing steep enough to clear any obstacles on the remaining engine (think towers or mountains).
Number one is easy. Number two, three and four have other factors. Namely, each are weight-related, but it is weight in relation to the temperature and the altitude of the airport. A very simplistic discussion on physics would go something like: the hotter it is, or the higher the altitude, the thinner the air. The thinner the air, the worse the performance.
As for number two, the higher and hotter and heavier, brake energy limits can be exceeded faster.
As for number three, the higher and the hotter and heavier, the longer the runway we need.
Same goes for number four – high, hot and heave means poor climb performance on one engine. This is an odd one, because privately-operated planes do not have to meet the single engine climb performance that commercial or charter flights must meet. That’s why you sometimes see someone else take off if you when you can’t – they own the plane and you are chartering. However, charter rules are driven by safety, and I like operating under those rules, even when flying in private operations. A pilot buddy tells a story of watching a G-III taking off out of Aspen. It was clearly heavy, loaded with passengers and a lot of bags. A crowd of pilots had gathered to watch it take off. A non-pilot walked up and asked, “What is everyone watching?” My buddy responded, “Perhaps the biggest accident we’ve ever seen.”
So, how do you manage weight? You could ship excess bags home via UPS (I’ve seen this done). You could send a passenger home via the airlines. Or, often the best solution is to take on less fuel. BINGO! To achieve performance criteria, you sometimes cannot load as much fuel as it takes to get home – only enough to get you on your way and to make a fuel stop at an airport at a lower altitude, or, with lower temperatures or with a longer runway.
It’s not just Aspen. When I first started flying jets and was only a co-pilot, we took a transplant team to Provo, UT. Ringed in mountains, Provo is at 4,497 feet above sea level - much lower than Aspen at 7,837. My captain had not realized that the longer of the two runways was closed. The useable runway was not that short so he decided to fuel up for the long flight home. It was hot. I decided to crack open the charts to take a look at brake energy limits and climb gradients upon departure. We failed. We failed on brake energy and it was close on single engine climb ability. Solution: we had to wait two hours for the sun to go down and the temperatures to cool. The transplant team had rejected the organ and we were in no rush to get home – we were lucky that day.
It’s summer. I just read a story online that many commercial flights from Phoenix were cancelled due to the heat. Phoenix is “hot and high”. Above are the reasons – even airliners are not exempt from physics.
The good news is that pilots train for this. At Flight Safety, we have a “high and hot” day of training, complete with an engine failure drill, in the simulator, while taking off…from Aspen.