Constant Angle Non-Precision Approach (CANPA)

The Flight Safety Foundation has done many studies on the inherent dangers of non-precision approaches and the dive and drive technique.  There are too many things that need to come together perfectly for the approach to be successful.  The pilot must have excellent situational awareness during the approach and masterfully control the airplane to level off at the minimum descent altitude (MDA) and then decide to land when the runway is in sight or land. 

The Flight Safety Foundation has published an Approach Landing Accident Reduction (ALAR) article detailing the benefits of a CANPA approach.  First, let me detail the process a pilot must go through while flying a non-precision approach using the dive and drive method.  The dive and drive method consists of descending fairly quickly to the new step down altitude each time it changes.  The pilot has to pull power, descend and try to maintain a constant airspeed while descending.  When the new step down altitude is captured, power needs to be added to ensure the airspeed remains constant.  The more step downs there are on an approach the more of a chance for the pilot to become disorientated on where they are on the approach and descend below the step down altitudes or the minimum descent altitude.  We can see that a CFIT accident risk is much higher than a precision approach (or an approach with vertical guidance).

Here is a classic dive and drive approach into Teterboro, NJ.  The pilot needs to lose 500’ in 5 miles and another 1000’ in the next 4.9 miles.  A total of 1500’ in 9.9 miles.  The dive and drive technique says to get down as quickly as possible to the next step down fix.  The pilot will be constantly adjusting the pitch and power to maintain power altitudes on the approach.  Significant increase in the workload!

 

OK here are some interesting accidents that have involved controlled flight into terrain while on a non-precision approach.  You can do your own search of accident reports at the NTSB website and the ERAU Library Website.

• Instrument rated private pilot practicing instrument approaches into North Central State Airport (KSFZ)
• Korean Airlines Flight 801 into Agana, Guam
• Western Air Lines, Inc.; Flight 470 into Casper, Wyoming
• Eastern Airlines Flight 401 into Miami, FL

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How do we create a constant angle descent?  If you read my previous posts, you will realize that I have completed the math for how many feet we need to lose in one mile while maintaining a 3° glide angle.  Each mile the aircraft must lose 318 feet.  On the table below I have listed the AGL height above TDZE + TCH (touchdown zone elevation and threshold crossing height).

The trick to a constant angle descent is to figure out where the distance between where you want to descend and the runway threshold.  In the example above at AYIYE intersection you are 9.9 NM from the threshold.  If you can stay at 3161’ (3152’ for the 3° glide and 9’ for the TDZE) you can start to descend at AYIYE intersection in a stabilized fashion significantly decreasing the pilot workload.  At ANGLE you will be at 1560’ still stabilized on the descent.

  The other method is to figure out where to start your descent from a given altitude.  In the example above you are at 2000’ at AYIYE.  You can stay at 2000’ till you 6.3 NM from the threshold.  Again, you will cross ANGLE at 1560’ MSL.

The United States has not quite gotten the idea of how powerful a CANPA approach can really be.  I hope they follow the European community in what they are doing to non-precision approaches.  You can search for European Approaches at European AIS Database and the United Kingdom NATS Website.

A couple things to notice about this VOR23R approach into Manchester, England.  On the top of the planview you will notice a DME distance from  I-NN and an altitude eerily similar to the CANPA chart above.  The idea is to to hit each altitude as you pass the DME distance.  If the aircraft does this they will be on a stabilized constant angle descent.  Notice that each mile, the pilot loses 320’ or the aircraft is descending on a 3° glide.  Some european countries are going one step farther and making each of these non-precision approach have a decision altitude (DA) instead of a minimum descent altitude (MDA).  I also think this is a good idea and the recommended procedure is to add 50’ to the MDA value.  For the Teterboro approach above, if you don’t have the required runway insight at 550’ MSL it is time to go missed.  For this approach into Manchester, at 690’ MSL without the runway insight means a missed approach.

Visual Descent Point (VDP) Calculations

In the previous post, I talked a lot about the theory behind VDPs; why they exist, what the purpose is and the reason why we should figure out the VDP for every nonprecision approach we fly.  In this post, I am going to discuss some methods to figure out your VDP point and provide an analysis of the data.

There are basically two methods to calculating the VDP point.  The first method is the HAT / 10 method where we get the number of seconds to subtract from the time box on the approach for our speed.  The second method is to calculate the distance the VDP point is from the threshold and figure out a way to identify that point either by a DME distance, GPS along track distance, crossing VOR, etc.  Obviously, the first method is the easiest in the airplane to calculate unless or course you have a GPS then the second method is more accurate.

Let’s take a look at an NDB approach into Salina, KS.

The first method is 469’ / 10 = 47 sec. The time for 120 kts groundspeed is 2:51.  Subtracting  47 sec  get us a VDP time of 2:04 sec.  The second method is 469 / 300 = 1.56 NM or 4.14 NM from the FAF.

As you cross the FAF, start the timer and 2:04 sec later either see the runway or perform a missed approach. Just a side note, TERPS do not account for obstacle clearance with the missed approach turn is initiated before the missed approach point. So if you decide to go missed climb on course and pass the MAP before turning.

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There are inherent errors with the two methods described above but are close enough for government work. I am going to explain these errors so you can make a better decision for the aircraft speeds you actually fly.

The first method is only accurate for aircraft that have a groundspeed of 113kts. If there is a headwind or a tailwind the time will obviously be different.  In the picture listed below, I have created a formula that will give you a divisor for any groundspeed you wish.  The most common ground speeds are listed below and you can see that a divisor or 10 is really only good for speeds between 110 kts and 120 kts.  For slower aircraft the divisor is lower meaning there is more time after the VDP point to the threshold than for faster aircraft.  In afterthought, that makes perfect sense… it passes the reasonableness test.  Since we are talking knots which is NM / hour, I needed to covert knots to feet per second hence the 6076.1 / 3600 in the equation.  Notice the equation does not include time or distance.  This means that these variables are factored out of the final equation.

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Have you ever wondered what your glide angle is if you descend before or after the VDP point? I think the analysis will convince the pilot to treat the VDP point as a decision point on any non-precision approach. This simple change in behavior will increase the safety of the flight. The descent angle to the landing threshold is hugely dependent on the ground speed of the aircraft is flying and the distance the VDP is from the threshold.. A slower aircraft has more leeway on descending at the VDP than a faster aircraft. In the example below, the VDP is 1NM away from the threshold and is 318’ high.  The chart below shows that a faster aircraft needs to descend on time at the VDP point in order to land in the touchdown zone of the runway otherwise non normal maneuvering may be required. 

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A put together one last example of how we can quickly put together something we can take into the aircraft for VDP calculations.   This spreadsheet requires information typically found right on the approach plate.  It will calculate the effects the wind has on the groundspeed and will adjust the time accordingly in the third table.  The VDP times are adjusted as well.  You may need to look in the A/FD for some of the information.  For the Salina Rwy 17 approach I have calculated the VDP information to the right.  Notice that a slower speed causes the VDP time to increase while a faster time causes the VDP time to decrease.

I used a glide slope of 3.00° because the A/FD shows the VGSI set to that glide angle.  The A/FD also lists the TCH as 52’ AGL.

Excel File: VDP.xlsx

Visual Descent Point (VDP) Theory

There is quite a bit of information on visual descent points but not a lot of information concerning an analysis of visual descent points and what happens to the descent angle when you descend earlier or later than the VDP point.  First a little background on the why the visual descent point was created.

Every year when the annual Nall report is released landing phase are among the highest accidents recorded.  The National Transportation Safety Board (NTSB) has recommended updating visual descent points or requiring Part 121 and 135 operators to use a VDP five times over the past 30 years.  One of the most famous accidents happened on September 8th, 1989 when a USAir flight collided with and severed four transmission cables.  The flight performed a missed approach and landed uneventfully right after that.  The crew did not understand the importance of a VDP, even though they had referred to the VDP several times in the approach brief.

The National Transportation Safety Board determines that the probable cause of this incident was the flightcrew’s failure to adequately prepare for and execute a nonprecision approach and their subsequent premature descent below minimum descent altitude. Contributing to the cause of the incident was the inadequate and deficient services provided to the flightcrew by air traffic control personnel.

The safety issues raised in this report include:

• Identification of potentially confusing features near runways on instrument approach charts.
• FAA oversight of air traffic control quality assurance.
• FAA training of and guidance to operations’ inspectors.
• Application of visual descent points to training in and execution of nonprecision instrument approaches, and incorporation of requirements for visual descent points in FAR Part 135 operations.
• Communication of weather information between air traffic control and the National Weather Service.
• Revision of minimum safe altitude warning inhibit areas.

There are many good reasons why a pilot would want to make a decision to land or go around at the VDP point.  The Flight Safety Foundation has a really good article on constant angle non precision approaches (CANPA) that perfectly fit in with the use of a visual descent point.  A CANPA approach allows the pilot to fly a stabilized approach with a decision to go missed at the VDP point.  I am a huge fan of CANPA and VDP procedures.

The graphic on the left is from the FSF article “Constant-angle Nonprecision Approach”.  The FAA publishes Safety Alert for Operators (SAFO) articles on safety information and recommended actions.  One such article “Constant Angle of Descents Techniques for Nonprecision Approaches” states this.

The operator should develop procedures that address approach profile techniques using a stabilized constant angle of descent from the FAF to arrive at the published MDA prior to the published visual descent point (VDP). If the approach does not have a published VDP, the flightcrew may determine a point along the course between the FAF and Missed Approach Point (MAP) that would be appropriate for a VDP. With the runway environment in sight, and at the VDP or established on glidepath by means of a visual landing aid, the flightcrew may begin a normal descent from MDA to the landing runway.

How do we calculate a Visual Descent Point?  Well most of the work has already been completed for us by the FAA and is typically defined by a DME distance.  In the Tulsa, OK plan view we can see a dark “V” indicating the visual descent point at 6.5 DME from TUL.  If you are at 6.5 DME and don’t have the required runway information in sight I would recommend a go-around even though the missed approach point (at the threshold)  is 1.1 NM later.

Let’s take a look at how the VDP is calculated on the approach plate and under what circumstances a VDP is not placed on the approach.  A look through FAA Order 8260.3B U.S. TERPS procedures has some really interesting things.

The VDP is a defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference is established.

 

VISUAL DESCENT POINT (VDP) (applicable to straight-in procedures only).

(1) For Non-RNAV SIAP’s, mark the VDP location with a DME fix. The DME must be collocated with the facility providing final approach course guidance (USN/USA/USAF NA). If DME is not available, do not establish a VDP. Maximum fix error is ± 0.5 NM.

(2) For RNAV SIAP’s, mark the VDP location with an along track distance (ATD) fix to the MAP.  Maximum fix error is ± 0.5 NM.

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When dual minimums are published, use the lowest minimum descent altitude (MDA) to calculate the VDP distance.

PUBLISH A VDP FOR ALL STRAIGHT-IN NONPRECISION APPROACHES except as follows:

• Do not publish a VDP associated with an MDA based on part-time or full time remote altimeter settings.
• Do not publish a VDP located prior to a stepdown fix.
• If the VDP is between the MAP and the runway, do not publish a VDP.
• If DME is not available, do not establish a VDP

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For runways served by a VGSI, using the VGSI TCH, establish the distance from threshold to a point where the lowest published VGSI glidepath angle reaches an altitude equal to the MDA. Use the following formula:   (VGSI – visual glide slope indicator)

VDP Distance = (MDA – (TCH + TDZE)) / TAN(VGSI Angle)

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For runways NOT served by a VGSI, using an appropriate TCH from table 19A, establish the distance from LTP to a point where the greater of a 3 degree or the final segment descent angle reaches the MDA. Use the following formula:

VDP Distance = (MDA – (TCH + TDZE)) / TAN(VGSI Angle) where VGSI angle >= 3 degree

 

It becomes apparent that the VDP glide angle should be 3° or more.  Using this information we can apply an analysis of Salina VOR 35 approach.  Notice the black V at 5.5 DME from SLN.  This is the VDP point.  Applying some of the information we know now we can figure out how they got this value.  The glide slope from SLN VORTAC to MAP is 2.29° meaning the VDP glide slope will be the VGSI slope from the A/FD.  The page for Salina show a PAPI on the left and a 3.00° glide angle crossing the threshold at 52’ AGL.  The TDZE for RWY 17 is 1246’ MSL.  We can use this information to calculate the VDP point.  Using the formula above we get a value of 1.45 NM or with the .5 NM maximum error.

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A couple other things are interesting from the FAA TERPS manual.  They are talking about the visual portion of the approach of the portion that is below DA / MDA.  The FAA is concerned of any obstacles with those last few feet.

Straight in visual segment.

• Length.  The visual area begins 200 feet from the threshold at threshold elevation, and extends to the DA point for precision procedures or to the VDP location (even if one is not published) for non precision procedures.
• If any obstacle penetrates the 20:1 surface take ONE of the following actions:

1. Adjust the obstacle height below the surface or remove the penetrating obstacles.
2. Do not publish a VDP, limit minimum visibility to 1 mile, and take action to have the penetrating obstacles marked and lighted
3. Do not publish a VDP, limit minimum visibility to 1 mile, and publish a note denying the approach (both straight-in and circling) to the affected runway at night

It looks to me that the FAA uses the VDP point (or point where the glideslope intercepts DA) as the area to evaluate for obstacles in the visual segment.  It becomes more important to calculate the VDP point so we can ensure ourselves that we aren’t going to hit anything as we descend below MDA.  This is also the reason why the AIM states that if we can identify the VDP point, we should not descend below MDA till reaching the VDP point.  It would appear that we shouldn’t descend below MDA (even on approaches without a published VDP) till we reach the VDP point.

In the next post, I am going to cover how to calculate your VDP either by time or by using the method discussed above.  I am also going to share insight on glide angles for different aircraft speeds if we descend earlier or later than the VDP point.