When do a pilot know it is time for descent?
Question
When does a pilot knows it is time for descent?
Answer
Depending on the aircraft type, it is generally thought to be two different theories behind knowing when to decent from a given altitude.
If you fly a commercial aircraft with propellers, it will almost always be calculated based on the time to landing and company procedures to govern passenger comfort. This is because an aircraft with propellers have much more drag than a jet airliner, so it is able to decent much faster than a jet, without accelerating beyond limiting speeds. Since most propeller aircraft operate between the ground and 25.000 feet (8,5km) They would usually decent about 10-12 minutes before landing, and simply adjust the decent rate between 1500-3000 feet per minute. Again it depends on the type, and how well the cabin pressure can be maintained and so forth. It would probably not make much sense if I tried to explain the aerodynamics and technical knowledge behind, so I hope that will explain the basics, which is time and altitude for propeller aircraft.
When it comes to jet aircraft, it gets more complicated, as their ability to produce drag, and there by high decent rates, is very limited. In a jet you must also take into account the weight of the aircraft at decent and wind conditions, along with anticipated cloud formations with ice, to calculate your point of decent. This can be done by the aircraft computers, but every pilot flying a modern jet airliner is expected to be able to make these calculations and monitor them efficiently.
What I would be looking to calculate on a normal flight is a decent angle which will produce a economy decent. This is achieved at 260 knots for my aircraft type, a CRJ 900. What you do is basically check the weight of the aircraft before decent. Take headwind or tailwind into consideration and calculate or look up your angle of decent. Choosing the angle of decent will decide the speed with which the aircraft will decent with the engines running at idle. A higher angle will produce a higher speed. Using ice protection will increase the given speed for an angle unless you reduce the angle before the top of decent. This is just the basics and what I will be looking for is a distance from my cruise altitude, to the treshold.
This is a simplification of a complex understanding of aircraft aerodynamics, weather and general jet aircraft theory, and despite our best efforts, we often have to adjust these calculations to fit into the requirements of air traffic control and other aircraft in our vicinity. We do however always manage, and if we are short on the calculation we simply add some thrust and if we are missing some miles to achieve a decent we wish, we can always extend our flight path and add miles to the decent.
I hope you enjoy the answer, and should you have more questions, you are always welcome to write again.
Happy landings,
