Okay, so, while seated on a 757 a couple of days ago, it occurred to me that I hae no idea how they get a jet from 33,000 ft to ground level (except the actual landing procedure portion of the program where flaps and gear are pretty self-explanatory).
My question is this:
To return to terra firma from cruise altitude, does the pilot:
A) simply reduce throttle and allow the plane to “sink”?
B) actually point the nose down (and if so, how is airspeed kept from becoming an issue)?
Its a combination of both depending on how much you want to descend at the moment. For extended descents like you would encounter in a transition to landing (A) is a pretty good description of the strategy. Reducing the throttle does cause a descent from straight and level flight. The pilot couldn’t just point the nose down because that would increase airspeed. When transitioning to landing, you want to decrease airspeed and not increase it.
There is a counterintuitive saying about flying a plane. It says “Use the throttle to control altitude and use the control yoke to control speed.”
This is not entirely correct. Sometimes you do use the throttle to control speed and you can certainly use the control yoke to trade speed for altitude but it is roughly correct for extended climbs and descents.
There are plenty of pilots on this board so I will allow them to chime in too.
B and A both. If you reduce the throttle and descend (not sharply), you can keep a fairly constant speed. Your descent occurs over a pretty long distance, IIRC.
No, it is not akin to a stall. A stall occurs when airflow over the wing gets disrupted by low airspeed, becomes turbulent, and introduces rapid loss of lift and unstable flight dynamics like a wing pitching up or the airplane going into a spin.
What you are describing is perfectly normal. I have heard it described as a “mushing effect”. The wings have perfectly normal airflow over them, they just aren’t producing enough lift to keep the plane at a constant altitude so it sinks.
I said above that it is a combination of reduced throttled and having the nose pitched slightly down by the elevator. They really do work in tandem but the throttle is more important in this case. What the pilot aims for is an equilibrium in rate of descent and airpseed. You could descend using just the throttle but you would be in a mess if you simply pointed the nose down. Airspeed would build until you overstressed the aircraft in the worst case.
This is actually a more in depth question than you probably planned. If you are really interested in it, the non-fiction aviation classic “Stick and Rudder”, found at any large bookstore, explains this topic clearly and in depth.
In the simplest of terms, think of a plane having four forces acting upon it at all times (look at this picture). An aircraft loses altitude if the lift is less than the weight and loses speed if the drag is greater than the thrust. To reiterate what has been said, a pilot can either use less throttle or angle the nose down. The best way is using both these methods at the same time. Each aircraft has different procedures for the descent, with differing airspeeds and such.
I hope this doesn’t turn into Bernoulli versus Newton GD but that is not quite right. An aircraft generates less lift than weight in both steady climbs and steady descents. Climbing and descending is more a matter of thrust and drag. FWIW.
As usual, good answers get followed by the nitpickers, here’s mine:
At a constant rate of descent, the lift on the airplane will equal the weight (roughly speaking, without going into vector diagrams), not be less as some have said. When the lift is less than the weight, the descent rate will be increasing. The same is true (in reverse) for climb.
The forces have to be in balance for constant speed, whether that’s level, in climb, or descent.
What that means is that, in order to descend gently and gracefully, the pilot will (usually) reduce thrust (or engine power). This will momentarily cause the airplane to slow down. This reduced airspeed will, temporarily cause the lift to be reduced, and the airplane will begin to descend. When the desired descent rate and airspeed are achieved, the pilot will make some more small adjustments to re-balance everything (at a lesser power setting, and probably at a lower airspeed), and the airplane descends at constant rate.
If you used your method B in the OP to descend, it would certainly work, but as you guessed, airspeed would build. To avoid that, you’d have to make a dramatic decrease in power. If you’re really in a hurry to get down, you’d deploy flaps, spoilers, and even landing gear to make the airplane as draggy as possible, and reduce the power practically to zero. In an airliner, you’d only do this kind of thing in an emergency (like loss of cabin pressure or fire).
I only pick 'em because I have had to refute a fair amount of mistaken aerodynamics in my life. And Race, a vector diagram was just what I was picturing.
That being said, your reply was right along the lines of what I first considered posting; in particular the part re balancing of forces for unaccelerated flight. I think yours is a very well put answer. MM
I’m going to sidestep the whole aerodynamic discussion and try to answer this:
And the answer is: a little of both. For sake of argument, let’s say we’re hand flying the airplane (ie autopilot off) at 33,000 feet and Mach .78. ATC tells us to descend to 20,000 feet.
Now, the airplane is nice and trimmed up with the power set to maintain level flight at Mach .78. If all I do is reduce power the airplane will seek it’s trimmed airspeed and will nose over all by itself. Usually you help it out a little, but not much - just a gentle push to get it started. Now that we’re descending, the RATE of the descent at the trimmed airspeed is controlled by the power. Pull the power to idle and the nose will drop until your trimmed airspeed can be maintained with idle thrust. Modulate the power and you can control your descent very nicely. We’ll ignore any small trim changes associated with changing from flying Mach to IAS in the descent and just say that if you keep the airplane trimmed for the airspeed you want it will fly it, with only minor deviations.
It really is no more complicated than that - for a fairly rapid descent, pull the power to idle. For a shallower descent, leave some power on. Now, if you PUSH the nose over and hold it there, the airspeed will build very rapidly. You will have to trim nose down or you’ll be fighting some very strong nose-up force on the yoke as the airplane tries to return to it’s trimmed airspeed. These kind of wild rides are discouraged.
