We regularly see non-turbocharged piston singles cruising in the 4500-6500-foot range, even when wind and weather aren’t operational considerations. Meanwhile, a few thousand feet higher, the ride’s better—as is visibility—there’s better comm and navaid reception, and likely a lot less traffic. So, why do some pilots of personal airplanes prefer to cruise at lower-than-optimum altitudes? Why do others go as high as they reasonably can for the trip length? Is the extra time and fuel worth climbing a few more thousand feet?
It all depends, of course, on variables like terrain, weather and headwinds. But there few downsides, and several upsides, to adding a few thousand feet to what you might consider a “normal” altitude for you and your airplane. It’s common to find the extra time and fuel consumed climbing are a wash, thanks to higher block-to-block speeds. It also depends on the airplane; the calculations and considerations applying to normally aspirated engines change when turbocharging is introduced, and usually are even more in favor of climbing.
The bad
That’s not to say climbing to a higher altitude doesn’t have a price. One way to measure the pros and cons is a basic computation of the time and fuel required. Using relatively common flight-planning software is one way to determine if the costs outweigh the benefits. Meanwhile, available performance data easily can tell us the gallons and minutes required to climb to, say, 5500 feet msl, and then continue to 9500. When comparing results, it’s easy to forget most of the time and energy expended to get to 9500 is used getting to 5500. After we’re at 5500, continuing the climb another 4000 feet simply doesn’t cost much.
Once we’re at altitude, however, it’s common to start gaining back our investment in time and energy. That’s because we can eke out a higher true airspeed for the same power. Alternately, we can fly at the same true airspeed we might use at a lower altitude, but do it with a reduced power setting.
Depending on the terrain and trip lengths, flying at altitudes where supplemental oxygen is either required or a good idea can be a downside. Carrying supplemental O2 requires tanks and equipment, plus accessories like pulse oximeters and refilling stations. It all costs money, which we could be putting into the fuel tanks. Meanwhile, oxygen bottle can be heavy, as can installed storage and supply systems, eating into useful load.
Using oxygen can be uncomfortable, too, especially for passengers. But those are the extent of the downsides. Typical supplemental oxygen use has several benefits, among them restored visual acuity, when compared with flying at the same altitude without O2, especially at night. Pilots also typically report arriving at their destination less fatigued and better prepared to deal with approaches to minimums, delays and other arrival-related tasks. A key factor is how long you remain at altitude: Spend four or more hours above 10,000 feet msl without O2, and it’s likely you’ll feel it.
Another concern about cruising at higher altitudes can involve the amount of time it would take to get back on the ground if “something bad” happens. Generally, the presumed scenario involves an in-flight fire, or perhaps a medical problem (brought on by the higher altitude?), and there’s a need to be on the ground right now.
But in-flight fires are rare, and airplanes aren’t falling out of the sky because their pilots have a medical problem. Instead, having some extra air under you can come in very handy when a single’s engine quits. The additional altitude translates into both energy and time, allowing longer glides and greater opportunity to find a suitable landing area. Yes, greater altitude means having to plan your descent. Deal with it.
One of the only real downsides to cruising at relatively high altitude is a normally aspirated engine’s reduced power output, which can be a factor near terrain. Trying to outclimb a downdraft isn’t fun, but can be avoided with careful planning. A more powerful engine won’t hurt, and neither will a turbocharger. If it’s a concern, restrict your high-altitude flying to calm days away from terrain.
The good
We’ve already touched on some of the upsides to cruising at higher altitudes, including greater true airspeed (TAS) on the same fuel. The rule of thumb we were taught is that TAS increases two percent for each thousand feet of altitude. In other words, an airplane that cruises at 100 KTAS at sea level (which admittedly is a silly comparison, since no one cruises at sea level…) can pound out 120 KTAS at 10,000 feet, presuming it can make the same amount of power. That’s a pretty good deal. It’s an even better deal if there’s a tailwind.
Tailwinds, however, can be problematic. The geometry can work out so that what by rights should be a slight tailwind actually works out as no help at all, or even a slight headwind. That’s because we have to turn into the wind to correct for it, and there’s a roughly 120-degree arc behind us from which the wind can blow actually translating into a groundspeed boost. Anything outside that arc is a wash, at best, at least as far as tailwinds are concerned. However, it still may pay dividends to be at a higher altitude and accept little-to-no tailwind versus remaining at a lower altitude with a low tailwind. Again, that’s thanks to higher TAS or lower fuel burn, your choice.
While we’re on the subject of winds aloft, let’s also talk about weather. Generally, you’ll find a smoother ride at a higher altitude, it’ll be cooler in the summertime and you can be above the puffies.
Meanwhile, it should go without saying that cruising at higher altitudes eliminates most worries about running into something. That “something” can be terrain, an obstacle or even another aircraft. How high you need to be to avoid everything depends on where you are, of course. The main worry is starting out over flat land and heading toward rising terrain. If you’re not paying attention…well, surprises aloft are uniformly bad. What traffic there is can be moving faster than you might be accustomed; the 250-knot speed limit at and below 10,000 feet msl obviously doesn’t apply up here.
More good news: Additional altitude can mean better communications, especially when over remote regions. And you’ll be able to receive distant ground-based navaids better, too. If you’re doing it right, getting a DME lock from 200 nm out means you’ll be there in an hour or so.
Finally, ATC radar coverage usually is pretty good at altitude. You’ll likely be in an ARTCC’s airspace most of the time and there can be fewer ATC sectors to deal with, which means fewer frequency changes.
Practicalities
To reliably and safely cruise in, say, the low teens, there are a couple of things to keep in mind. First, the airplane, the oxygen system and the pilot have to be in relatively good shape. Above 10,000 feet msl, the airplane is required to have at least a Mode C transponder. As of January 1, 2020, you’ll need to meet the FAA’s ADS-B OUT requirements. Some bottled water never hurt anything, and you should at least review the aeromedical guidance so you know what to look for in the way of signs warning of hypoxia and other altitude-related conditions. Your airplane’s cabin heat system can be found lacking, so a blanket or warm clothing can come in handy, especially in the winter.
Operational planning extends to ensuring the weather at altitude will make your plans practical ones. For example, it doesn’t make much sense to accept a 40-knot headwind at 12,000 feet while there might be only 20 knots on the nose at 8000.
Getting back down also requires planning. In fact, the higher indicated and ground speeds of a lengthy descent from the teens usually help make up for the reduced speeds encountered in the climb. Using your EFB’s or GPS navigator’s ability to calculate a top-of-descent point when nearing your destination can be a lot of fun, too.
At the end of the (sometimes long) day, climbing to the low teens—or even just to 10,500—can pay some dividends on your next cross-country. It certainly won’t pay to go there in a Skyhawk for a one-hour hop, but when flogging a high-performance single or a light twin for a few hours, getting some air underneath you early and often can be addictive. Enjoy the clear weather, the smooth ride and the view.
Just be sure to bring along some supplemental oxygen and water, and plan your descent. If you’re lucky, you’ll literally be able to look down on people flying the same airplane type and route as you are, and watch as you go whizzing by at a higher groundspeed and reduced fuel flow.
