In Avoiding Other Aircraft, we highlighted the cruise altitudes available to pilots depending on the type (VFR/IFR) and direction of flight. With this post, we’ll take cruising altitudes a step further and examine how to choose the best altitude for existing circumstances. As you might imagine, a variety of factors affect the altitudes pilots and dispatchers ultimately decide on. Let’s get started.
The Tropopause: Finding the Sweet Spot
Have you noticed how so many airliners tend to level off near 35,000 ft (“F[light] L[evel] 350” in aviation parlance)? This popular flight level is far from coincidence. The most congested altitudes for enroute airliners result from the performance advantages associated with the Tropopause.
The Tropopause is the boundary between the Troposphere, the lowest atmospheric layer, and the Stratosphere. Its height varies with the earth’s curvature, ranging from around 24,000 ft at the poles to approximately 56,000 ft near the equator. In the Contiguous 48 US States, the average Tropopause height is roughly 36,000 ft. A few benefits of operating near this altitude include lack of general aviation (slow) traffic and the ability to summit most weather. However, the performance advantages of Tropopause-area flight are the primary reasons for the deluge of jets at these heights.
Atmospheric Pressure: As you probably know, atmospheric pressure decreases as altitude increases. This decrease in pressure diminishes engine performance, but results in two significant advantages: 1. Total aerodynamic drag on the aircraft decreases, and 2. The lower the air density, the less fuel is required by the engines. Operating in these fuel-efficient altitudes saves airlines several million dollars each year in fuel expenses alone.
Temperature: While I’ve stated that decreased atmospheric pressure does diminish aircraft performance, this engine-robbing reduction in pressure is partially offset by the cooler temperatures aloft. Cold air, with its relatively low energy, tends to condense. As temperatures decrease with increases in altitude, the natural tendency of this cool air to compress helps counteract the overall rate of decreasing atmospheric pressure. This cooling of air with increases in altitude is a significant benefit for jets, but is only an option up to the Tropopause.
The Game Changer: Besides marking the top of virtually all weather, the Tropopause also denotes the end of decreasing temperatures with increases in altitude. Above the Tropopause, temperature actually increases with altitude, which rapidly diminishes aircraft/engine performance. Above the Tropopause, significant performance reductions eliminate virtually all benefits to be found at higher altitudes.
Overall, the Tropopause is the sweet spot for airline operations. Reduced aerodynamic drag, low fuel consumption, minimal (if any) weather, and the absence of slow aircraft all increase the efficiency of flight at this level. With this many benefits, it should come as no surprise when your captain announces, “We’ll be cruising along today at 35,000 ft.”
While the Tropopause usually offers the best overall conditions for airliner flight, at times it’s impractical/unwise to climb to the altitudes around FL 350. Let’s look at some reasons why it’s occasionally better to choose altitudes not in the neighborhood of the Tropopause.
Winds: Winds have a general tendency to increase with altitude. Depending on the direction of flight, this can be a huge blessing or a significant curse. As a tailwind, performance and speed work in the flight’s favor. As headwinds, Jet streams (often in excess of 100 knots) lengthen flight time and burn significantly more fuel. With strong headwinds aloft, it’s often better to seek a lower altitude without the gales.
Length of Flight: Short flights often negate the advantages of going high. For instance, airplane engines burn more fuel while climbing than in cruise. It doesn’t make sense to spend 30 minutes at climb power to spend 10 minutes in cruise. In many cases, the lengthy climb easily erases any performance savings of the short cruise. For passenger comfort, a period of level flight will also be more tolerable than a flight profile that resembles an inverted V.
Pilots and airline dispatchers usually have many options when deciding on a cruise altitude. For the reasons outlined above, the Tropopause if often a good choice. However, at times conditions make flight at lower levels much more practical.