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...(continued)
When you fly into cold rain, you're doing a lot more to cause cooling issues, especially on the surfaces exposed to the rain, than you are by pulling the power back on a descent. When you land and shut down the engine, the engine temperature climbs, with a lack of cooling airflow, and a lack of oil circulation. Some parts begin to cool, while other parts continue to heat up. When you apply power for takeoff, a significant thermal change occurs. One doesn't normally expect to see cracking during these times, but it can rarely happen. Likewise, pulling the power to idle at altitude is poor practice, but one shouldn't normally expect one's engine to begin cracking and falling apart. Thermal abuse of the engine, however, is to be avoided, and it's always a good idea to baby the engine to some extent.
There are strong arguments to be made against partial power climbs and operation at too low power settings, but there's also no need to do idle descents if one plans ahead. There's no need to worry about speed brakes in the skymaster; it's draggy enough as-is. The airplane is one big speed brake. It's got two propellers to do the job, and struts, and if you need it, the gear does very well, too. If you're worried about shock cooling and plan to carry power while extending speed brakes, it's a noble thought, but baseless in reason. Save your money.
In a large turbofan airplane, retarding power at the wrong time or adding power at the wrong time can cause fan case rub. Pulling back power while nosing over into a descent, for example, can cause fan case rub and deterioration of the rub strip and the fan disc. Retarding power initially, losing a little speed, then making it up by dropping the nose makes more sense, though it's not commonly taught insofar as I've seen. Likewise, in the piston airplane, retarding power slightly before beginning the descent, easing over into a gentle descent, then eeking the power back a little at a time once established in the descent, makes sense, and is good airmanship and power management in a turbocharged piston airplane.
If you're used to descending in the flight levels, you know that in a turbojet airplane a descend at lower altitudes, say FL270 and below, can usually easily be done at idle and as a straight forward descent. At higher altitudes, especially approaching FL410 and above, one generally just makes small power reductions, starts with a small rate of descent, and comes down maintaining one's mach number. The initial part of the descent is a ginger one with respect to descent rate, speed, and power.
A similar approach can be taken to descents in piston airplanes; reduce the power a minute or two before beginning the descent, then start down and recover the speed. A minute or two later reduce power a little more, and increase descent accordingly to maintain speed. With a little finesse, there's only a small incremental change occurring thermally. In this way one can keep speed up throughout the descent, then leave the power alone when preparing to set up for the approach in the terminal area. Level off, the airplane slows to the desired speed without having to monkey with the power, and as the airplane naturally slows, the cooling airflow decreases, the engine begins to warm somewhat, and one can begin configuring for the terminal area arrival and approach/landing.
No need to throw out speed brakes.
Barometric is the manifold pressure setting that corresponds to the altitude at which one is operating. At sea level with the engine shut off, read barometric pressure on the manifold pressure gauge; it's close to 30" just as the altimeter is 29.92 at sea level of a standard day. At five thousand feet, no power on the engine, sitting on the ramp, the manifold pressure gauge reads about twenty five inches of mercury. This is barometric. At 10,000', barometric is 20".
Aside from the general rules of reducing power an inch to two inches per thousand feet or per minute, try to abide by the rule of thumb of keeping power at barometric or better, until needed lower for work in the terminal area (landing). This doesn't mean the initial power reduction should be right to barometric depending on one's cruise situation, one may be operating well below barometric from the outset, and may need nothing more than dropping the nose and starting the descent. Convsersely, operating at a high power setting, one needn't pull power back to barometric initially, but should make a reduction of an inch or a few inches before starting a gentle descent.
If you happen to be flying a geared engine, there are good reasons to keep the power up and keep the engine driving the propeller, because despite what some will tell you to the contrary, allowing the propeller to drive the engine through the reduction gears can lead to a very early demise in your engine. The issue is not as critical with non-geared engines, and is hotly debated by some, but it's still a good operating practice to keep your power up high enough that the slipstream doesn't end up driving the engine through the propeller; keep the engine driving the prop, instead. If your power is high enough to do this, and you aren't making large, rapid power changes, then you're in good shape.
Another good habit to get into when landing is not to through the propeller forward. Too often I see people run a checklist on the downwind or on final, and when they get to propeller, they push the prop all the way forward. The engine wraps up in speed and screams, a surge is heard, and the engine has pointlessly been abused. Wait until the power is pulled back far enough that an RPM increase won't occur, before advancing the propeller.
Once on the ground, don't be in a big hurry to shut down. This is as true of turbojet airplanes as turbocharged airplanes. Give at least five minutes between reaching idle power, and shutdown. If you have to sit in the tie downs or sit at the end of the runway (off the runway, obviously) after landing, then do it; keep the oil circulating through the engine and turbo bearings while they cool, before shutting down. Shutting down too soon is a great way to coke the turbo bearings and ruin the turbocharger, as well as the engine. Don't do that.
If you're trying to slide into Aspen, Colorado, you might benefit from the speed brakes. Otherwise, for the most part they'll simply be crutches for poor power management, and are unnecessary. You can save money and do without the brakes, just fine, by planning ahead.
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