Visual Descent Point (VDP) Calculator

A specialized tool to calculate the Visual Descent Point (VDP) for non-precision instrument approaches using either time or DME.

What is a Visual Descent Point (VDP)?

The Visual Descent Point (VDP) is a crucial, defined point on the final approach course of a non-precision, straight-in instrument approach. From this point, a pilot can begin a normal, stabilized descent from the Minimum Descent Altitude (MDA) to the runway’s touchdown point, provided they have the required visual references of the runway environment. Its purpose is to prevent pilots from descending too early (risking obstacle collision) or too late (risking an unstabilized approach or a runway overrun). Essentially, if you hit the VDP and can’t see the runway, you should not descend and must prepare to execute a missed approach at the Missed Approach Point (MAP).

The VDP Formula and Explanation (Using Time)

While many approach charts publish a VDP based on DME (Distance Measuring Equipment), not all do. For approaches where timing is the primary method of identifying fixes, or if DME is unavailable, pilots must calculate their own VDP. A widely accepted and reliable rule-of-thumb is used for this calculation.

The core principle is to determine how many seconds it takes to travel from the VDP to the MAP. This time is then subtracted from the total time from the FAF to the MAP.

Time from VDP to MAP (seconds) = HAT / 10

VDP (as time from FAF) = Time from FAF to MAP – (HAT / 10)

This formula is based on a standard 3-degree glideslope, where for every 100 feet of altitude loss, it takes approximately 10 seconds of travel time at typical approach speeds. This makes the math simple and effective in the cockpit. This VDP calculator helps you quickly perform this essential “calculate vdp using time” task.

Variables Table

Variables used in the VDP time calculation.

Variable	Meaning	Unit	Typical Range
HAT	Height Above Touchdown	Feet	250 – 1000
Time to MAP	Time from Final Approach Fix to Missed Approach Point	Seconds	120 – 300
VDP Time	Time from Final Approach Fix to Visual Descent Point	Seconds	90 – 270

Practical Examples

Example 1: Standard Non-Precision Approach

A pilot is flying an approach with a HAT of 520 feet. The approach chart indicates the time from the FAF to the MAP is 3 minutes (180 seconds).

• Inputs:
  • HAT: 520 ft
  • Time from FAF to MAP: 180 seconds
• Calculation:
  • Time from VDP to MAP = 520 / 10 = 52 seconds.
  • VDP (as time from FAF) = 180 – 52 = 128 seconds.
• Result: The pilot should expect to reach the VDP and look for the runway at 128 seconds after passing the FAF.

Example 2: Higher Altitude Approach

Consider an approach into an airport at a higher elevation, where the HAT is 750 feet. The timed portion from FAF to MAP is 4 minutes (240 seconds) due to groundspeed.

• Inputs:
  • HAT: 750 ft
  • Time from FAF to MAP: 240 seconds
• Calculation:
  • Time from VDP to MAP = 750 / 10 = 75 seconds.
  • VDP (as time from FAF) = 240 – 75 = 165 seconds.
• Result: The VDP is reached at 165 seconds after the FAF. Having this calculated beforehand is critical for situational awareness.

How to Use This VDP Calculator

Using this tool to calculate VDP using time is straightforward and enhances flight safety.

1. Enter HAT: Find the “Height Above Touchdown” (HAT) or “Height Above Touchdown Zone Elevation” (HATD) on your non-precision approach chart. Enter this value in feet into the first field.
2. Enter Time to MAP: Find the time from the Final Approach Fix (FAF) to the Missed Approach Point (MAP) for your groundspeed on the chart. Enter this value in seconds.
3. Interpret Results: The calculator instantly provides the VDP as the number of seconds from the FAF. This is your cue to decide if a stabilized descent is possible. The intermediate result shows the calculated time buffer between the VDP and the MAP.

Key Factors That Affect VDP Calculation

• Accuracy of HAT: Using the correct HAT from the approach plate is the most critical factor. An incorrect value will lead to a miscalculated VDP.
• Groundspeed: Your actual groundspeed determines the accuracy of the timed leg from the FAF. The time you enter should correspond to your calculated groundspeed.
• Wind: A strong headwind or tailwind will alter your groundspeed, affecting the time it takes to reach the MAP and, consequently, the VDP. You must use the correct time from the chart based on your actual groundspeed.
• Non-Standard Glideslope: The 10% rule assumes a standard 3-degree glideslope. If the approach is designed with a steeper Vertical Descent Angle (VDA), the VDP would occur earlier.
• Pilot Reaction Time: The calculation provides a precise point in time. Pilots must anticipate this point to effectively scan for the runway environment without delay.
• Published VDA vs. Rule of Thumb: If a Vertical Descent Angle (VDA) is published, a more precise calculation can be made, though the “divide by 10” rule is a highly effective and accepted substitute for timed approaches.

Frequently Asked Questions (FAQ)

1. Why isn’t a VDP published on every approach chart?

A VDP may be omitted if an obstacle penetrates the 20:1 visual surface area on a straight-in approach. In such cases, pilots must be extra cautious and understand that a “normal” descent from the MDA might not be possible.

2. What do I do if I reach the calculated VDP and don’t see the runway?

You must not descend. Continue at the MDA until you reach the Missed Approach Point (MAP). If the runway environment is not in sight at the MAP, you must execute a missed approach.

3. Can I descend before the VDP if I see the runway?

While technically possible, descending before the VDP will result in a shallower, and potentially unstabilized, approach. The VDP is designed to put you on a stable 3-degree glideslope.

4. Is this time-based VDP calculation an official FAA method?

It is a widely taught and accepted rule of thumb for pilot-calculated VDPs when one is not published. The FAA acknowledges that pilots can create their own VDP.

5. How does this differ from a DME-based VDP?

A DME-based VDP is a distance from the runway or a NAVAID. A time-based VDP is a point in time after passing the FAF. Both serve the same purpose: identifying the start of the normal descent path. A common DME rule of thumb is HAT/300 to find the VDP’s distance in nautical miles from the threshold.

6. What if my approach speed changes?

You must recalculate your groundspeed and use the corresponding time from the FAF-to-MAP table on your approach chart. This calculator’s accuracy depends on the correct time input.

7. Is the VDP the same as the MAP?

No. The VDP always occurs before the MAP. The VDP is the “decision” point to start a normal descent, while the MAP is the “final” point where you must decide to land or go missed.

8. Does this calculator work for circling approaches?

No. VDPs are only for straight-in non-precision approaches. Circling approaches have their own specific minimums and procedures.