Learn & Understand

Where the 3-Degree Glideslope Comes From, and Why You Go Around

The companion calculator converts groundspeed and glidepath angle into a descent rate, and notes the handy groundspeed-times-five rule for a 3-degree path. But why is it almost always 3 degrees? Where does that glidepath physically come from, and why does flying it badly trigger one of aviation's most important safety maneuvers? The descent rate is a number; the stabilized approach is the discipline it serves.

Why 3 Degrees Became Nearly Universal

The standard instrument glidepath is a 3-degree descent, and the choice is a compromise between competing pressures.

Three degrees threads that needle: steep enough to clear normal obstacles and keep the aircraft comfortably above the ground on final, shallow enough that the descent rate stays manageable and the flare is gentle. A few airports with challenging terrain use steeper approaches, but 3 degrees is the default the whole system is built around - which is exactly why the groundspeed-times-five rule of thumb works so widely.

Where the Glideslope Physically Comes From

On an ILS approach, the 3-degree path is not imaginary - it is a radio beam. A glideslope antenna array beside the runway broadcasts overlapping signals that define a narrow inclined plane in the sky; the aircraft's receiver senses whether it is above or below that plane and drives the needle accordingly. Visually, light systems like PAPI or VASI do the same job with color: a row of lights beside the runway shows red or white depending on whether you are high or low, so "red over white, you're all right" reproduces the glidepath for the eye. Instrument or visual, the aircraft is being guided down one specific inclined surface anchored to the touchdown zone.

The Descent Rate Is a Health Check on the Approach

Because the glidepath angle is fixed, the required descent rate is set entirely by groundspeed - which is why the calculator ties them together. That makes descent rate a diagnostic. If holding the glidepath demands an abnormally high sink rate, it usually means the groundspeed is too high (often a tailwind), and a high sink rate close to the ground is a classic ingredient in accidents. The descent rate is not just a target; it is a warning light for an approach going wrong.

The Stabilized Approach and the Go-Around

Modern safety culture crystallizes this into the stabilized approach concept: by a defined gate (commonly around 1,000 feet above the ground in instrument conditions), the aircraft must be on the correct path and speed, in the landing configuration, with a normal descent rate - or the crew executes a go-around. A go-around is not an emergency or a failure; it is the pre-briefed, professional response to an unstable approach, and treating it as routine is one of the strongest defenses against approach-and-landing accidents. An excessive descent rate near the ground is one of the exact triggers that mandates it.

Reading the Descent Rate as a Signal

Use the calculator's descent-rate figure to know what a normal, on-path sink rate should look like at your groundspeed - and then treat any large deviation from it on a real approach as information. If the number the situation demands is far higher than the number the calculator predicts, the approach is telling you it is not stabilized, and the safest, most professional answer may be to go around and simply fly it again.