Aircraft cruising speed

You’ve probably heard the term “cruising speed of an aircraft” many times. But have you ever wondered what it really means? Most people who use this term don’t actually understand its meaning. There’s a lot of interesting information ahead, and not just about aviation.

What is Cruising Speed

In simple terms, cruising speed is the speed an aircraft can maintain for a long time to cover a sufficient distance while using fuel optimally at a specific altitude.

In other words, it’s the speed that provides the most optimal balance between fuel consumption and flight range.

For passenger aircraft, this means the speed is sufficient to quickly transport passengers or cargo, but not so high that fuel consumption significantly increases the cost per kilometer.

The thing is, as speed increases, air resistance — and thus fuel consumption — grows non-linearly, with a particularly noticeable spike in resistance when approaching the speed of sound (the sound barrier). That’s why civilian aircraft fly at speeds of 0.8–0.9 Mach, just below the speed of sound.

After all, for a civilian aircraft, it’s important not only to use minimal fuel per kilometer but also to reach the destination quickly enough to complete multiple flights within a given time. Typically, for a passenger aircraft, the cruising speed is about 80% of the maximum speed.

For military aircraft, the situation is similar. For a bomber, it’s important to deliver as many bombs or missiles as possible over the longest possible distance. The speed must not be too low (to avoid interception or being shot down) or too high, so as not to burn fuel too quickly.

For a fighter jet, the math is the same, but it’s usually about the time during which the fighter can perform its tasks, rather than the range.

So, cruising speed is the optimal speed for the task an aircraft typically performs.

Why Is It Called Cruising Speed?

The term cruising speed, of course, applies not only to aircraft.

After all, there are no cruisers in aviation — it’s not the navy. So where did the term come from, and what did it originally mean?

A cruiser is a ship designed for cruising operations, such as hunting enemy merchant ships, patrolling, or protecting one’s own trade routes. The ship must stay at sea for a long time and operate independently.

kruisen is a Dutch word meaning “to sail along a route.”

Thus, the term “cruiser” referred to a ship that could embark on long voyages while remaining fast and maneuverable enough to chase and sink someone.

Two key points stand out: long enough and fast enough.

Maximum Speed

Maximum speed is straightforward — it’s the absolute maximum an aircraft can achieve. But there’s a catch.

Typically, maximum speed is reached under ideal conditions, such as at a specific altitude, with partially filled fuel tanks, and without certain loads — just to test whether it’s possible to accelerate to a certain limit.

In real conditions, no one tries to break the sound barrier near the ground because the atmosphere is too dense. Statements from journalists or bloggers often only highlight a misunderstanding of what’s really happening.

Maximum speed can also be limited by flight conditions (aircraft weight, altitude, maneuvers performed), the age of the aircraft (the airframe may be worn out, making it risky), and other constraints.

So, maximum speed often exists only “on paper.”

Economic Speed

This one is really simple. It’s the speed at which fuel consumption is minimal. The main goal is to save fuel. How long it takes to get there doesn’t matter.

The key is to either fly the maximum distance, use the least amount of fuel, or stay in the air as long as possible.

For example, the World War II patrol aircraft PBY “Catalina” could stay in the air for up to 20 hours. PB stands for “Patrol-Bomber.” Its task was to patrol a designated area for as long as possible to detect submarines, ships, or conduct rescue operations.

The “Catalina” still flies

Interestingly, the aircraft, which first took to the skies in 1935, was used as a firefighting aircraft until 2014. Even now, many examples still fly, though as private aircraft in enthusiasts’ collections.

How Does Cruising Speed Differ from Economic Speed?

Economic speed is used for maximum fuel efficiency. Cruising speed is for an optimal balance between all parameters of a specific task.

What’s the Difference?

There’s a great example in history.

The legendary reconnaissance aircraft SR-71 Blackbird had a maximum speed of 3.3 Mach and a cruising speed of 3.2. This means that at an altitude of 25 kilometers, the “Blackbird” could sustain a speed of about 3,500 kilometers per hour for a long time.

The fastest aircraft in history

Meanwhile, the equally famous Soviet interceptor MiG-25 achieved a maximum speed of 2.8 Mach, or 3,000 km/h, at an altitude of 21 kilometers.

The MiG-25’s task was precisely to intercept the American reconnaissance aircraft (or the prospective bomber XB-70 “Valkyrie” at the time, which was never adopted).

It might seem that the speed difference isn’t significant, especially since the MiG didn’t need to shoot down the target with a cannon.

There are R-40 missiles, which have a speed of 5 times the speed of sound, or 5,700 kilometers per hour. Add 5,700 + 3,000, and you get a speed sufficient to catch and shoot down any aircraft.

Interestingly, how many SR-71s did the MiG-25 intercept? None at all. And it’s not because American reconnaissance aircraft were afraid of interception.

For the MiG-25, a speed of 2.8 Mach was the maximum it could achieve. Flight regulations allowed maintaining this speed for only 5 minutes; after that, the speed had to be reduced, or the airframe and cockpit canopy would overheat, potentially leading to depressurization. Overheated fuel could also cause serious problems.

The temperature limit was set at 290 degrees Celsius. At this temperature, touching the canopy, even with gloves, was dangerous.

Interestingly, if you closely examine abandoned MiG-25 aircraft, you can see that their glazing has a yellowish tint and is very cloudy—a result of repeated overheating of the material.

So, the MiG-25 couldn’t intercept its target precisely because the target was moving at cruising speed, while the MiG was at its maximum speed. This meant the interceptor could only fly at that speed for a very short time, allowing the reconnaissance aircraft to easily avoid an encounter.

Thus, the term “cruising speed” is not as simple as it might seem at first. Yet, it reveals a lot of fascinating details.