What, you may ask, is a middle altitude? A wiseacre might say that it is the airspace between low and high altitudes and he would be right.
The highest level for the middle might be Flight Level 250. The aircraft certification standards change above that altitude. Or it might be Flight Level 230 where air traffic control airspace sectors generally change from low to high. Or it might be Flight Level 180 where the airspace rules change and Victor airways become jet routes.
From a physiological standpoint, Flight Level 180 is a good level to pause and think before climbing higher.
Anyone who has done altitude chamber or other training to examine the effects of hypoxia knows that at 18,000 feet you can lose oxygen flow or cabin pressure and function for an adequate period of time to make a plan and get to a lower altitude without declaring an emergency. Up the ante to 25,000 feet and the time of useful consciousness can be very short, depending on the pilot's physical condition. Even someone in perfect shape might not have much more than a minute. That would be every bit a full-blown emergency.
Because the vast majority of the new general aviation airplanes that might use the middle altitudes are not pressurized, the allowable use of nasal cannulas for oxygen up to Flight Level 180 might define the top of the middle altitudes for most pilots. So, from 10,000 feet, where Mode C becomes a requirement and which is a good altitude to start considering oxygen use, up to 18,000 feet might define the middle altitudes for most general aviation pilots flying unpressurized piston airplanes.
I would hasten to add that it is legal for a pilot to fly at 12,500 feet (or between 12,500 and 14,000 for up to 30 minutes) without supplemental oxygen. That might be legal but for most pilots it is probably not a good idea. You can get a bad case of the stupids after a few hours above 10,000 feet.
All this is becoming more important because turbocharging is quite popular. Where airplanes are offered with and without turbos, 70 percent of the new airplane buyers typically opt for the turbo version.
Many of the new turbos are certified up to 25,000 feet, some to 20,000 feet. How to best get your money's worth out of this?
First, you have to be realistic. A piston airplane with turbocharging will have a true airspeed increase of from 1.6 to 2 knots per thousand feet of altitude above 6,000 to 8,000 feet where the natural engine can produce full cruise power. That is not much when you consider time to climb and, if flying with a headwind, the increase in wind with altitude. If your turbocharged airplane will do, say, 180 knots at 8,000 feet that means it will do 187 or so at 12,000 feet. That is an increase but it won't change your life.
Wind is a huge part of the bargain when seeking value from turbocharging, too. From years of doing this I learned that wind is stratified. Look at the wind forecast and it will often show a linear increase in velocity with altitude. Equally often, it doesn't work that way. This is of most consequence when westbound, into the wind. All airplanes are fast with a tailwind.
On the typical day with a headwind, the least velocity will be found in the lower levels, where it is turbulent. Climb above the turbulence with a headwind and there's a spike in airspeed and/or rate of climb as the airplane moves up into the smooth air. That's because the airplane is climbing into an increasing headwind. That is also the major velocity increase that will be seen in climbing from about 1,000 feet agl into the smooth air.
Once in the smooth air, the velocity increase with altitude is not likely as pronounced as the forecast would suggest. The difference between, say, 12,000 feet and 16,000 feet might well be handled by the increase in true airspeed with altitude, as long as the air remains smooth. If you are trying to go higher when westbound without a turbocharger, the true airspeed will drop with altitude and you'll probably believe the wind forecast.
The next big increase in headwind with altitude will be announced, usually, by wind shear turbulence. This might just be jiggles. Often in the windy season there is an area of high speed wind within a jet stream. We used to call them jet cores but somebody came up with finer terms. The National Weather Service forecasters now refer to this as a "jet streak" or a "jet max." I like jet streak the best and will use this term forevermore.
Jet streaks can wander down into the middle altitudes, and even if they don't the wind can increase beneath the jet streak. So if you are above the low-level turbulence, climbing with a headwind, and there's any turbulence or a spike in rate of climb as you go ever higher, the groundspeed is likely to decrease. GPS makes all this so easy to see.
This brings us to what I think is one of the biggest advantages of turbocharging. We all like to get places faster, but I have always been willing to give up some speed for comfort, and this is where turbocharging shines.
I have spent a lifetime crisscrossing the eastern mountains. Without turbocharging it was often a bumpy ride in the windy season. With turbocharging most flights are in smooth air. Usually, but certainly not always, 12,000 feet will give a smooth ride when westbound over the mountains when the wind is blowing. If there are lake effect snow showers and the route is far enough north to be in air containing those showers, 16,000 to 18,000 feet might be required to stay up out of the ice and in smooth air. If possible, planning a route that stays upwind of the lakes would likely find lower cloud tops.
Things you can't escape in the middle altitudes are the up- and downdrafts that come with a strong flow over the mountains. These will definitely be there and where the air might be smooth, the pilot workload can be high.
If you are flying above what is called the critical altitude for the turbo system, a characteristic of the system enhances the effect of up- and downdrafts. At the critical altitude the wastegate in the turbo system is closed, meaning the full output of the turbo is being used. That turbo output is affected by airspeed so if, for example, you fly into a downdraft it will also affect the power output of the engine --in the wrong direction. You'll have less power in downdrafts as the airplane slows to try to climb in the sinking air, and more in updrafts where you must nose over to hold altitude.
Another power question must be addressed. It is possible to fly airplanes without turbos in the middle altitudes. In recent times there have been several accidents where control was lost of Cirrus airplanes with normally aspirated engines flying in the middle altitudes, usually because of structural icing. The airframe parachute on a Cirrus won't do much good if control is lost and the airspeed builds beyond the chute's limit speed, and some of these airplanes came to a bad end.
Pilots flying normally aspirated airplanes at altitude have to acknowledge that the higher you go, the less margin in the operation. Get close to the ceiling of an airplane and get a little ice, or fly into a downdraft, and control could be lost. If the autopilot is in the altitude hold mode, the autopilot could stall the airplane. Certainly when an airplane is flown within 10 percent of its service ceiling, there's not much wiggle room.
Back to airplanes with turbos. The higher you fly, the hotter the engine will run. That means more wear. Also, there might be manifold pressure restrictions with altitude and some airplanes have reduced airspeed limits with altitude. Any limitations are important and should be heeded.
When everything is considered, the best use of a turbo is to fly higher only when there is some advantage-meaning more tailwind or more comfort. Vaulting up to the airplane's maximum certified altitude on most flights, and betting your life on the flow of oxygen, just doesn't make a whole lot of sense.
