Visual Descent Point (VDP): What is it Exactly?
Pilots flying a precision instrument approach have both course and glidepath deviation data to guide them the whole way down. But what guides a non-precision approach and how do pilots flying that type of approach know when and where to begin their final descent so they can execute a safe landing? The visual descent point is part of that equation.
What is a Visual Descent Point (VDP)?
The official AIM definition of a Visual Descent Point or VDP is “a defined point on the final approach course of a non-precision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot.”
As a refresher, in the above definition, the MDA is the minimum descent altitude – the lowest altitude to which you can descend on final during a standard instrument approach procedure (SIAP) with no electronic glideslope provided.
During your descent, you must remain at or above the minimum descent altitude until you have the necessary visibility to safely descend and you have acquired one of the approved visual references. You must also be in position to make a “normal” (read: not unsafely steep) descent. The visual descent point is the position from which you can descend from MDA while maintaining a 3-degree glidepath and landing at the touchdown point.
Types of Instrument Approach Procedures
When you are on a final approach and are setting up for landing, you need a controlled, orchestrated means of coordinating both your course and glidepath during the descent so that you land safely at the touchdown point on the runway.
For pilots who are flying under Instrument Flight Rules (IFR), there are three types of terminal instrument procedures (TERPS) or instrument approach procedures (IAPs) that can be used during an instrument landing.
Precision Approach (PA)
A precision approach (PA) is, as one would expect given the name, the most precise. It uses a navigation system that supplies both course and glidepath deviation data. An ILS (Instrument Landing System) is commonly used to guide precision approaches.
Approach with Vertical Guidance (APV)
The second type of TERPS is the approach with vertical guidance (APV). The navigation system used for an APV approach provides both course and glidepath deviation data similar to the PA, but not to such a carefully calibrated standard. Therefore, the vertical guidance supplied by an APV is less precise than that of a PA. Approach with Vertical Guidance is supported by LNAV/VNAV systems.
The final type of TERPS is the non-precision approach (NPA). For this instrument-based approach, the pilot’s navigational system provides feedback on course deviation, but no data on glidepath deviation. The VOR system is commonly used to support and guide non-precision approaches.
A Continuous Descent Final Approach (CDFA) is the newer, preferred method of guiding descent rate on a non-precision approach and CDFAs have been published for most non-precision approaches. Where a CDFA is not available, pilots can land using a step-down approach or the “dive and drive” method of descending to the minimum descent altitude immediately upon reaching the final approach fix (FAF).
In either the CDFA or the dive and drive method, the visual descent point (VDP) is located on the glidepath and plays a key role in guiding the landing process.
What is the purpose of the Visual Descent Point (VDP)?
Making a non-precision approach can be dangerous. The FAA and NTSB have identified unstabilized approaches to be one of the key contributing factors to non-precision approach accidents.
When executing a non-precision approach, some of the potential dangers include:
- Diving too steeply and being unable to pull up to level off
- Hitting an obstacle while descending
- Going below the minimum descent altitude (MDA) too soon
- Landing beyond the safe landing point
By beginning your final descent from the minimum descent altitude (MDA) as you reach the visual descent point and acquire visual reference, you will normally put yourself on a 3-degree glide path to the touchdown point. This is the same glideslope used by most precision approaches. Using the visual descent point to begin your final descent helps you to avoid a too steep or too shallow final descent angle.
How do you find the Visual Descent Point (VDP)?
If published, the Visual Descent Point (VDP) is identified with a “V” on the vertical profile of many approach plates. Pilots can also calculate it manually. If you need to manually determine the location of the VDP for your approach, use the below equation which will give you the distance from the touchdown point to the visual descent point in nautical miles (nm).
Note that the above equation is a “rule of thumb” pilot math estimation which assumes roughly 300 feet per nautical mile for a 3-degree glidepath. The actual trigonometric value is 318 feet, but the easier to work with estimation is only 18 feet or 6% off, and that is sufficiently accurate and conservative for the purposes of calculating VDP. The height above touchdown or HAT, should be indicated on your chart and can be calculated using:
Once you know the distance from the visual descent point to the touchdown zone, you can subtract the distance from the runway threshold to the touchdown zone. This gives you the VDP’s distance from the approach end of the runway.
In most cases, you will use your distance measuring equipment (DME) to alert you to the location of the VDP. If your DME is inoperable, it is recommended to fly the approach as though a VDP was not provided since you have no equipment to locate it.
A visual glide slope indicator (VGSI) can also be used as a visual aid for pilots flying a non-precision instrument approach. Note that if visibility is at or just above minimums, pilots may be unable to view the VGSI when they reach the visual descent point since the VGSI is located past the missed approach point (MAP).
It is important to realize that in many cases, if the VDP is not published, it is because there are terrain or other obstacles that could interfere with a safe descent. In this case, calculate your visual descent point location, but be absolutely certain that you have enough visibility to see any potential hazards before you begin your descent from MDA. If known obstacles are present, the chart will state “visual segment – obstacles” to alert you that you may need to modify your approach to avoid hazards.
What happens if you miss the Visual Descent Point (VDP)?
Remember that to be cleared to descend below the minimum descent altitude, the following three criteria must be met:
- Be in a continuous position to make a normal descent and land on your intended runway
- Have the necessary visibility to safely descend
- Have the runway environment in sight
If all the above criteria are met, you can begin your descent upon arriving at the visual descent point. If not, you must not drop below the MDA until the criteria are met. If the criteria are not met by the time your reach the visual descent point, this is referred to as missing or flying past the VDP.
If you miss the visual descent point, you can technically continue flying at the minimum descent altitude (MDA) until you reach the missed approach point (MAP). If you are still lack the visibility, are out of position, or are unable to pick up at least one of the visual reference points and begin your final descent by the time you reach the MAP, then you must execute a missed approach procedure and do a go around. Never begin your descent from minimum descent altitude (MDA) beyond the missed approach point (MAP). First, it is illegal, and second, your angle will be too steep, or your touchdown point will be shifted too far down the runway for safety.
For added margin, some pilots like to use the visual descent point as a missed approach point. By making the decision to call a missed approach if they do not have enough visibility, are out of position, or cannot see their visual indicators at the VDP, pilots give themselves plenty of time to abort the landing attempt prior to passing the missed approach point.
Ultimately the decision of whether to call a missed approach at the VDP or at the MAP is up to the pilot. If you are unable to descend at the VDP because you do not meet the three necessary criteria, but conditions improve and you meet the criteria prior to arriving at the MAP, it is within pilot discretion to continue landing. Realizing that any descent begun beyond the visual descent point will be steeper than the normal 3-degrees, pilots must take into account their airplane type, configuration, speed, altitude, descent rate, rate of turn, and runway length, then make an appropriate decision. When in doubt, err on the side of caution and call missed.
Can you descend below the minimum descent altitude before reaching the VDP?
There has been much debate between pilots regarding whether you absolutely must wait until you reach the visual descent point before you descend below the MDA or whether waiting until the VDP is merely a guideline and suggestion that pilots are free to disregard if the situation warrants.
The AIM wording in CFR 91.175 c3 regarding when pilots can descend below the MDA confirms that pilots may descend only when they have meet the three criteria for descent below MDA. It does not specify anything about how that relates to the visual descent point.
In their Descent to MDA or DH and Beyond publication, however, while describing when to descend, the FAA specifically states, “Do not descend below MDA before reaching the VDP.”
The visual descent point (VDP) is a reference point used by pilots who are flying a non-precision instrument landing. This point lies at the minimum descent altitude (MDA) and indicates the position from which the pilot may begin a final descent for landing provided they are in position to land, have the necessary visibility, and have acquired visual references with the runway environment. A continuous rate of descent from the VDP to the touchdown point will usually set the pilot up with the same 3-degree glideslope used by pilots flying a precision approach.
If you are not in position, do not have the visibility, and/or you have not acquired visual references by the time you reach the visual descent point, you can continue flying at the minimum descent altitude until you reach the missed approach point (MAP) at which time you must call a missed approach and execute a go-around. Depending on their aircraft, experience level, and conditions, some pilots choose to fly more conservatively and call missed if they are unable to descend upon reaching the visual descent point rather than waiting until they arrive at the missed approach point.
Pilots may descend below the minimum descent altitude at the visual descent point or between the visual descent point and the missed approach point if they are able to do so safely. The FAA instructs pilots to avoid descending below the minimum descent altitude until they have reached the visual descent point. This helps minimize the risk of impacting terrain and obstacles due to an early descent.
A solid understanding of the purpose, location, and use of the visual descent point will help IFR pilots flying a non-precision approach to do so in a safe, smooth manner. Review your instrument approach procedures and other IFR specifics with the Instrument Pilot Handbook. For a clear, straightforward visual presentation of understanding and using the visual descent point, watch Larry Epley’s quick and concise VDP explanation.
Read more about Visual Descent Point (VDP) and other flight training topics in our Flight Training Material: Private Pilot collection.
- PilotMall.com Editor