How to read a flight's path: altitude, speed and the step climb

An altitude graph has four parts: a steep climb, a long flat cruise, a slow descent and a flat stretch or two near the end. Anything else on it has a reason. Most of the time the reason is fuel, wind or traffic, and most of the things that look odd are routine.

The staircase in cruise

A long-haul graph often climbs in steps: 31,000 feet for two hours, then 33,000, then 35,000. That’s a step climb, and it’s the plane getting lighter.

A heavy plane can’t reach its best altitude right after take-off. As it burns fuel, the best altitude rises, and the most efficient flight would be a very slow climb the whole way. Air traffic control works in levels, so airlines climb in steps as they get permission.

Those steps tend to be 2,000 or 4,000 feet because of the direction rule. Levels are stacked 1,000 feet apart, with eastbound flights on odd thousands and westbound on even, as the usual pattern above 29,000 feet. So a plane heading east from 35,000 goes to 37,000, not 36,000.

Reading a descent

The start of the descent is called top of descent, and on a graph it looks like a gentle corner. A jet comes down far earlier than most passengers expect. The rough pilot’s rule is 3 miles forward for every 1,000 feet down, so from 35,000 feet that’s about 105 miles out.

The final approach is steeper in appearance than it is. The usual glide path is 3 degrees, about 320 feet of height for each nautical mile.

Flat steps in the descent are normal. They’re usually a controller holding the plane at a level to keep it clear of other traffic, and they get more common near busy airports. A tiny climb in the descent is unusual but not alarming. A real climb on final approach is a go-around: the crew has decided to land on the next try, and it’s a maneuver they train for.

Spotting a hold

On the map, a hold is a racetrack: two straight legs joined by two turns, usually repeated. The standard pattern turns right, with an inbound leg of one minute at or below 14,000 feet and a minute and a half above that. On the altitude graph it’s a flat line where a descent should be, sometimes stepping down a level at a time.

Holds mean the airport can’t take you yet, often because of weather or a runway change. A tracker’s map often shows it before the airline’s estimate moves.

Long S-turns or a wide loop on the way in aren’t holds. Controllers lengthen the path to space planes on approach, and it’s common at hubs.

What the speed line tells you

Trackers usually show ground speed, which is the plane’s speed over the Earth. That’s why the same jet can read far faster eastbound than westbound on the same day, at the same power. Its speed through the air has barely changed. Wind accounts for most of the difference.

A sudden jump in ground speed at cruise usually means a change in wind or a turn relative to it. Speed falls steadily through the descent. Near the airport, there’s a step down as the plane slows for flaps. Don’t read these as anything the crew did in a hurry.

Where the graph lies

The altitude in a broadcast is pressure altitude, set to a standard reference, not true height above the ground. The FAA’s flight levels are defined that way, because every plane at a level uses the same reference. That means a flight showing 35,000 feet isn’t at exactly 35,000 feet of real height. It also means a graph near a mountain airport says little about clearance from the ground.

Gaps and straight lines usually mean coverage, not behaviour. An assumed path has no information in it. If a graph looks flat for an hour over the ocean, it may be a guess.

A sudden spike to zero or to some absurd height is a bad data point. A plane doesn’t do that. Ride bumps don’t show up on these graphs either: turbulence moves a plane by far less than the scale of the chart, so a flat line tells you nothing about how the ride felt.

Terms used here are in the flight tracking word list. Once the line reaches the runway, the wait isn’t over, and this is what’s left. If you’re watching someone’s flight from the ground, why the tracker and airline can disagree will save you a phone call.

To see what a step in your own flight was, Play the flight in Skycap replays the whole trip with the route, the ride and the view. What it shows in the air