Two Rules of Thumb Every Pilot Should Know for the Three-Degree Glide Path

Most pilots react to the PAPI lights or glideslope needle rather than anticipating what the three-degree standard actually requires. Two rules of thumb eliminate that reactive flying: 300 feet of descent per nautical mile and groundspeed multiplied by five for target descent rate in feet per minute. Together, they build the mental model that separates pilots who fly the approach from those who get flown by it.

Why Does the Three-Degree Standard Matter?

The three-degree glide path is the universal benchmark in both IFR and VFR flying. The ILS glideslope, the VASI, and the PAPI are all calibrated to three degrees. Airport designers, approach engineers, and the FAA adopted this standard because it delivers a safe, stable, obstacle-clearing path to the runway environment.

The problem is that most pilots have no feel for what three degrees demands from their airplane in terms of vertical speed. They react to the needle or the lights instead of anticipating the numbers. That reactive flying is a direct path to unstabilized approaches - one of the most persistent causes of accidents in both general aviation and commercial operations.

How Do You Calculate Your Descent Rate on Final?

Rule 1: Multiply your groundspeed by five to get your target descent rate in feet per minute.

A longer version of the same math - divide groundspeed by two, then multiply by ten - gives the identical result and can be easier to work through in two steps when you’re task-saturated on approach.

This matters most when hand-flying in IMC. You know your groundspeed from the ATIS broadcast. You can factor in wind correction before you ever get to the approach. Calculate the descent rate on the ground, establish it when you intercept the glideslope from below, and let the needle confirm what you already computed. That is the difference between flying a number and chasing a needle.

A point on ILS geometry: when you are eight to ten miles out on a long vector, the glideslope needle is measuring angular deviation over a large distance. A small needle deflection at that range represents a significant altitude error. Pilots who don’t understand this geometry overcorrect early and then chase corrections all the way to minimums. Establishing the correct descent rate from the start - based on groundspeed, before the needle has moved - eliminates that problem entirely.

How High Should You Be at Each Distance from the Runway?

Rule 2: On a three-degree glide path, you descend approximately 300 feet for every nautical mile traveled.

That translates directly into a memorizable altitude profile above the runway threshold:

• 6 miles → 1,800 feet
• 5 miles → 1,500 feet
• 4 miles → 1,200 feet
• 3 miles → 900 feet
• 2 miles → 600 feet
• 1 mile → 300 feet
• Threshold → approximately zero AGL

All of these figures are above field (threshold) elevation, not above mean sea level. At an airport like Denver International, which sits at approximately 5,400 feet MSL, the math works identically - you’re thinking about height above the runway environment, not what the altimeter reads in absolute terms.

How Do You Use These Rules Before the Flight?

Rule 2 becomes a preflight tool on instrument approaches. Pull up the approach plate and check the final approach fix altitude against its distance from the runway threshold. The charted altitudes should be consistent with a 300-feet-per-mile profile - and for any properly constructed instrument approach, they will be.

If the ATIS reports a broken ceiling at 1,000 feet, you now have a precise mental picture of where you’ll break out. At 1,800 feet above field elevation on a six-mile final, and 900 feet on a three-mile final, a 1,000-foot ceiling means you should emerge from the clouds somewhere between three and four miles from the threshold. That is not guesswork. That is geometry.

This mental model eliminates the surprises that erode situational awareness in the terminal environment. When a pilot says they didn’t expect to still be in the clouds at a given point, what they’re describing is a missing mental picture of where the glide path was going to intersect the cloud bases. These two rules build that picture before engine start.

How Do These Rules Apply to Visual Approaches?

The PAPI and VASI are calibrated to the same three-degree standard, so both rules apply directly to visual approaches - and they change how you use those lights.

• PAPI: two red and two white means you’re on the glide path; four whites means high; four reds means dangerously low - go around immediately
• VASI (two-bar): red over white means you’re on the path

For a Cessna 172 flying a visual approach at a groundspeed of approximately 70 knots, Rule 1 gives a target descent rate of around 350 fpm. Rule 2 says that at four miles out, you should be 1,200 feet above the runway.

If you’re rolling out on final from a mile and a half at pattern altitude - say 800 feet AGL - you need to already be in a descent. Trying to lose 800 feet in a mile and a half from a level attitude will push your approach angle well above three degrees. The PAPI will show two whites, then three, then four. You’ll have a stabilized approach problem. The fix is to begin the descent to intercept the glide path before you’re on final, not after.

What Is a Stabilized Approach and Why Does It Matter?

The Flight Safety Foundation, whose standards are endorsed by most airline operations specifications, defines the stabilized approach gates as:

• By 1,000 feet AGL on an instrument approach: correct path, correct speed, correct configuration and power setting
• By 500 feet AGL on a visual approach in VMC: the same criteria

If you are not stabilized at those gates, the correct action is a go-around. These thresholds are not arbitrary - they come from accident data. High-energy unstabilized approaches kill people in general aviation aircraft and commercial jets alike.

Understanding the three-degree math lets you build a stabilized approach from the outside in. You arrive at the initial approach fix already knowing your required altitude, your descent rate, and what the PAPI should show. The instruments confirm; they don’t lead.

How Do These Rules Apply to Non-Precision and RNAV Approaches?

Non-precision approaches - VOR, localizer, and GPS/RNAV without vertical guidance - provide step-down fixes rather than a continuous glidepath. You descend to a minimum descent altitude, level off, and search for the runway environment visually.

The strong trend of the past decade has been toward RNAV approaches with advisory vertical guidance, known as LPV approaches. An LPV approach delivers glidepath guidance from the GPS constellation to minimums that in many cases rival a full ILS - built on the same three-degree standard, with the same math.

The FAA has actively pushed pilots to fly LPV approaches when their aircraft has the capability and the approach is published. Continuous vertical guidance eliminates the most dangerous segment of a non-precision approach: the descent from MDA toward the runway environment with no electronic path to follow. If vertical guidance is available, the FAA’s Instrument Flying Handbook and your local flight service station say the same thing - use it.

Putting It Together: A Practical Scenario

You’re flying into an unfamiliar airport under VFR. No ILS. The runway has a PAPI, and ATC sequences you for a five-mile final. Your groundspeed is 90 knots.

• Rule 2: Five miles out, your target altitude is 1,500 feet above field elevation
• Rule 1: At 90 knots, your target descent rate is 450 fpm

Establish 450 fpm, set power and pitch to hold approach speed, then check the PAPI. Two red and two white. You’re on the path - not because you reacted to the lights, but because you flew a number you already knew.

Key Takeaways

• Rule 1 - Descent rate: Multiply groundspeed by five for target fpm on a three-degree glide path (90 knots = 450 fpm; 120 knots = 600 fpm)
• Rule 2 - Altitude profile: Descend 300 feet per nautical mile; six miles out means 1,800 feet above field elevation
• Both rules reference height above field elevation, not MSL - the altimeter reading is irrelevant to the geometry
• The PAPI and VASI confirm a path you’ve already calculated; they are verification, not primary guidance
• Stabilized approach standards require correct path, speed, and configuration by 1,000 feet AGL on instrument approaches and 500 feet AGL on visual approaches; if not met, go around