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.