Approach Angle

Approach Angle (Vertical Descent Angle, Glideslope Angle, Descent Angle)

The approach angle—also referred to as the vertical descent angle (VDA), glideslope angle, or descent angle—is the defined angle between an aircraft’s final approach path and the horizontal plane of the runway. This angle is fundamental to both instrument and visual landing procedures, ensuring that aircraft follow a safe, predictable, and standardized flight path to the runway threshold. The prevailing standard for the approach angle is 3 degrees, as set out by the International Civil Aviation Organization (ICAO) and the Federal Aviation Administration (FAA) , balancing safety, performance, comfort, and airport infrastructure.

Where Is the Approach Angle Used?

• Instrument Approach Procedures: Published on approach charts for ILS, GLS, LPV, and RNAV approaches; programmed into avionics like FMS and autopilots.
• Visual Approaches: Visual aids such as PAPI and VASI are calibrated to display the standard angle.
• Flight Planning: Used to compute descent profiles and crossing altitudes during approach.
• Airport Design: Influences runway layout, obstacle clearance zones, and lighting system placement.
• Pilot Briefings: An essential element in approach and landing checklists.

Key Points:

• Dictates the aircraft’s descent profile on final approach.
• Ensures obstacle clearance and compliance with safety margins.
• Critical to stabilized approach concepts and regulatory adherence.

Standard Approach Angles: ICAO and FAA Norms

The 3-Degree Standard

A 3-degree approach angle is the international norm for precision approaches. Both ICAO PANS-OPS (Doc 8168) , ICAO Annex 14 , and FAA TERPS specify this angle for most instrument and visual procedures.

Why 3 Degrees?

• Obstacle Clearance: Maintains a safe buffer over terrain and obstacles.
• Aircraft Performance: Matches aerodynamic descent characteristics for most commercial jets and turboprops.
• Passenger Comfort: Minimizes abrupt altitude changes.
• Runway Use: Ensures touch down near the threshold, maximizing available stopping distance.
• Training and Regulation: Standardization simplifies pilot training and procedural design.

Mathematical Conversion

• A 3° approach yields a descent rate of approximately 318 feet per nautical mile (ft/NM).
• The horizontal to vertical ratio is about 19:1.

Sources:

• FAA Instrument Procedures Handbook, Ch. 4
• ICAO Annex 14, Volume I

Calculation Methods and Practical Rules

1. 60:1 Rule

A rule of thumb for descent planning:

• Each 1 degree of approach = 100 ft/NM of descent.
• For 3 degrees: 3 × 100 = 300 ft/NM (actual is 318 ft/NM).

Example:
At 10 NM, 3,000 ft above the runway = 300 ft/NM ≈ 3°.

2. Descent Rate Formula (VSI Calculation)

[ \text{Descent Rate (ft/min)} = \text{Groundspeed (kts)} \times 5 ]

Example:
140 knots groundspeed → 140 × 5 = 700 ft/min

3. Rule of Three

To plan top of descent:

• Multiply altitude to lose (in thousands of feet) by 3 = NM needed at 3°.

Example:
8,000 ft to lose → 8 × 3 = 24 NM before runway.

4. VNAV and FMS Automation

Modern avionics calculate and display the required approach angle and vertical path in real time, accounting for wind, speed, and altitude constraints.

Operational Factors Impacting Approach Angle

Aircraft Performance

• Aerodynamic Optimization: Aircraft are designed for controlled approaches at 3°.
• Configuration Management: Flap and gear extension allows for steeper approaches if needed.
• Weight and Speed: Higher landing weights alter speed and descent rate, but not the angle.
• Engine Considerations: Low power on descent must be managed for engine health.

Environmental and Procedural

• Weather: Wind, turbulence, and wind shear can alter required descent rates.
• Field Elevation: High or hot airports require careful descent planning.
• Runway Slope: Downhill or contaminated runways may require adjustments.
• Obstacles: Terrain or manmade objects may require non-standard angles.
• Approach Type: Precision approaches provide vertical guidance; non-precision do not.

Approach Types and Associated Guidance

Flight Planning and Approach Briefing

Standard Practice:

• Review approach angle and threshold crossing height (TCH) on the approach chart.
• Brief descent rate, speed, configuration, and any restrictions.

Example (Boeing 737, ILS 27):

• “Approach angle 3°, TCH 50 ft, target descent 700 fpm at 140 kts.”

Regulatory Context and International Standards

U.S. FAA Regulations

• FAA Order 8260.3 (TERPS): 3° standard for instrument approaches.
• Instrument Procedures Handbook: Explains descent angles and approach minimums.
• AIM: Details stabilized approach procedures.

ICAO Standards

• Annex 14: Prescribes approach slope for runways.
• PANS-OPS (Doc 8168): Criteria for approach procedure design.

Safety, Passenger Comfort, and Airport Engineering

Safety

• Obstacle Clearance: Ensures safe separation from terrain and structures.
• Stabilized Approaches: Reduces risk of unstable or unsafe landings.
• Predictability: Enables reliable use of automation and consistent training.

Passenger Comfort

• Pressure Changes: Minimizes rapid fluctuations, reducing discomfort.
• Smoothness: Predictable descent improves overall passenger experience.

Airport Engineering

• Runway Length: Standard angle ensures efficient runway usage.
• Lighting Systems: ILS and PAPI/VASI calibrated for the standard angle.
• Obstacle Clearance Surfaces: Designed using the standard approach slope.

Edge Cases and Special Scenarios

Steeper Approaches

• London City Airport (EGLC): 5.5° approach; requires special aircraft/crew approval.
• Mountainous Airports: (e.g., Innsbruck, Paro) may require steeper or curved approaches.
• Noise Abatement: Some airports use steeper angles to reduce noise impact.
• Short Field/STOL: Steep approaches for runways shorter than 1,200 m.

Shallower Approaches

• Obstacle-Free Airports: May allow <3° only if safety margins are preserved, but rare.
• Caution: Shallower approaches mean less obstacle clearance and are discouraged for instrument procedures.

Any deviation from 3° requires regulatory approval and special training.

Example Approaches: Use Cases

Example 1: Standard ILS Approach

• Procedure: ILS RWY 27, Glideslope 3.00°
• Aircraft: Boeing 737
• Groundspeed: 140 knots
• VSI: 700 ft/min
• Brief: “Intercepting glideslope, 3-degree path, threshold crossing at 50 ft, target descent rate 700 fpm.”

Example 2: RNAV (GPS) Baro-VNAV Approach

• Procedure: RNAV (GPS) Y RWY 12, VDA 3.10°
• Aircraft: Airbus A320
• FMS: Enter angle, cross-check VNAV path.

Example 3: Steep Approach (London City)

• Procedure: ILS RWY 09, Glideslope 5.5°
• Aircraft: Embraer 190 (certified)
• Notes: VSI up to 1,400 ft/min, special crew/aircraft approval.

Summary Table: Common Approach Angle Scenarios

Procedural Checklist: Computing and Executing Approach Angle

1. Reference Approach Chart: Confirm published angle and threshold crossing height.
2. Determine Configuration: Review aircraft weight, flap/gear settings, and approach speed.
3. Calculate VSI: Use the formula: VSI = Groundspeed × 5 (for 3°).
4. Monitor Instruments: Track ILS glideslope, FMS VNAV, or visual slope indicators.
5. Adjust for Conditions: Compensate for wind, turbulence, or runway slope.
6. Brief Crew: Ensure approach angle and any special considerations are reviewed.
7. Cross-Check: Use altimeter and DME to verify position and glidepath.

Regulatory and Industry Reference Links

• FAA Instrument Procedures Handbook, Chapter 4
• AIM – Arrival and Approach Procedures
• ICAO Annex 14 – Aerodrome Design and Operations
• Procedures for Air Navigation Services (PANS-OPS)
• Instrument approach procedure – Wikipedia