Sliced Bread
Performance Based Navigation

Sliced Bread
Performance Based Navigation

Performance based navigation (PBN) provides for designing and implementing automated flight paths. It will facilitate improved access to airspace and runways, enhanced safety and reduced costs. 

Using a machine invented by Otto Rohwedder, a Missouri-based jeweler, the first automatically sliced commercial loaves of bread were produced by the Chillicothe Baking Company on July 6, 1928. The popular idiom “the greatest thing since sliced bread” means that something is “the best and most useful innovation or development invented for a long time.” The first use of the expression is commonly attributed to Red Skelton, who used it to describe TV in 1952. NextGen is about halfway through a multi-year implementation plan, and the FAA plans to introduce technologies, procedures and policies through 2025 and beyond. This modernization of our air transportation system is “the most useful innovation or development invented for a long time.” It’s like sliced bread for pilots.    

You gotta be very careful

if you don’t know where

you’re going, because

you might not get there.

Yogi Berra

The introduction of the VOR (Very High Frequency Omnidirectional Range) was air navigation’s previous sliced bread. No longer did we need VMC or scud-running in order to use lighted airways or ground markers. A VOR ground station uses a phased array antenna to transmit a directional signal that rotates clockwise 30 times a second. They’re assigned radio channels between 108.0 MHz and 117.95 MHz with the first 4 MHz shared with the ILS band. We started with relatively inexpensive “Low” VORs
followed by the more useful but costly “High” VORs. 

A worldwide land-based network of victor airways below 18,000 feet, and jet routes at and above 18,000 feet, was created. From the mid-1940s to the turn of the 21st century, VOR and Distance Measuring Equipment (DME) were the predominant navigational aids. “Area Navigation” began with LORAN-C in 1957 and OMEGA in 1971. The first aviation-specific Area Navigation systems (circa 1968, i.e., Narco CLC-60, King KNS-80 and Collins ANS-31A) used existing VORs and “electronically moved” the ground-based transmitters to align them into the desired course. In 2000, there were about 3,000 VORs around the world, including 1,033 in the U.S. By 2013, they had been reduced to 967 and by 2020, to just under 900. Even though reliance on the above ground-based systems limits both the availability and accuracy of routes, the FAA and aviation alphabet groups recognize the necessity of maintaining multiple forms of navigation as a backup, and a Minimum Operational Network (MON) of VORs will remain operational indefinitely. The minimum number of VORs is expected to be 589 stations.  

Necessity is the

mother of invention.


A historic increase in airline travel has been the major impetus for a new (and overdue) approach to air navigation. The advent of Flight Management Systems and computers that use multiple navigation sensors, like VOR and DME, allowed the electronic calculation of routes between points without flying over them. Computers also removed the need for pilots to calculate fixes and program RNAV units in order to “electronically move” ground stations. Position and route accuracy then increased geometrically with the advent of a satellite-based global positioning system. 

As GPS was added to existing navigation units or as a stand-alone capability, Required Navigation Performance (RNP) was initiated. RNP describes how aircraft may fly an RNAV route or procedure using either ground-based or satellite-based navigation, as long as the required performance (RNP) specified in the procedure can be achieved. A required navigational performance is specified when position accuracy is essential for navigation accuracy, separation, and in many cases, obstacle clearance. RNP requires navigation systems to not only monitor performance compliance but to alert us if the aircraft flies outside of those parameters – it’s at the heart of PBN. GA has not only ridden on the coattails of the resulting capability, utility and safety but very often has led the way.

You’re right…we didn’t

have those things when

we were young. We

invented them.

Ronald Reagan

Over the years we’ve all seen changes in our airspace system: TCA’s (terminal control areas – now class B), TFR’s (that aren’t so temporary after all), RVSM (Reduced Vertical Separation Minimums), LAHSO (Land
And Hold Short), PRM (Precision Runway Monitoring), and now with PBN, enroute RNAV, descend via and climb via RNAV arrivals and RNAV instrument approaches. These procedures have been developed for a variety of reasons, some of them to increase the efficiency of our airspace system and airports, and others to allow more aircraft to use the same airspace and airports simultaneously. There is no doubt the procedures are more stable, predictable and accurate than ever before. 

And for general aviation, GPS/WAAS has opened thousands of “little”
airports to us through the use of GPS approaches, especially when they have LPV minimums. GA continues to dominate the U.S. airspace and airport system, with three out of four takeoffs and landings conducted by GA. And out of some 19,000 GA airports, 4,074 have a GPS/WAAS augmented LPV approach with 2,876 of those to runways with no ILS – and over 1,050 of those have minimums to 200’ HAT. The significance of this capability is astonishing to anyone who grew up flying ADF, VOR and circling approaches.

Except It’s Your Butt

Single-pilot IMC in high-performance aircraft is a safe endeavor because of our training, equipment, and the reliability of avionics, powerplants and airframes. And because of the decreased workload these new technologies and reliability provide, including GPS, WAAS, LNAV, VNAV, ADS-B and real-time weather, our inflight workload has been reduced. But as our airspace becomes more saturated, we have seen some of their workload shifted to the pilot to lessen the workload of controllers. A potential fly-in-the-ointment in this shift is the current tendency of controllers to use the word “except” when issuing a clearance to fly a three-dimensional (plus velocity) procedure. 

We are normally prepared for the descent and instrument approach but sometimes get behind.  ATC, weather, traffic, mechanical and passenger issues are common distractions. You will have reviewed the arrival and approach as published, including related NOTAMS and you are prepared to fly them – as published. It’s less of an issue in a multi-pilot cockpit, but when you’re by yourself and faced with late notice, unplanned, unpublished changes, we can easily get behind.

A vision, without a plan,
is just a hallucination.

Will Rogers

Controllers sometimes modify procedures by using the word “except” to change how we fly departures and descend via arrivals – even instrument approaches. For example, when they tell you to cross a fix on the approach “at” 5,000 when it’s published as “at or above” 5,000 feet, that is an unpublished modification. The same is true when they tell you to fly 250 kts to fix A, 210 kts to fix B and then 180 to the marker. How often can ATC modify published procedures, routes, altitudes or speeds before they cease to be “published” procedures? How do we protect ourselves from violating short notice changes? Shall we add a note to our flight plans in the remarks section: PAPO (Procedures As Published Only). Or do we simply respond with “unable” when we’re asked to comply with too many modified, inconsistent or unpredictable arrival or approach procedures? Due to COVID, our airspace system has been in “casual” mode for over a year, and the number of modifications has decreased. But as travel resumes, we can expect congestion and some confusion.

Pandemic Headwinds

The General Aviation Manufacturers Association’s state of the industry
media event on February 24 marked the first time the annual announcement of shipment and billing statistics was conducted virtually. During the event, GAMA released figures showing aircraft deliveries for the year valued at $22.8 billion, down from $27.8 billion in 2019. The 2,399 airplanes shipped in 2020 marked a 9.7-percent decline from 2019, with $20 billion in total billings, down 14.8 percent. Piston airplane deliveries (1,312) remained nearly stable from 2019 (1,324). However, turboprops took a 15.6-percent hit with 443 shipments, down from 525, and the 644 business jet deliveries skidded 20.4 percent from 809 in 2019. 

“As expected, in 2020, the COVID-19 pandemic negatively impacted general aviation and stifled the industry’s growth. While we continue to face headwinds globally, all signs point to strong demand for our products and services that are unfortunately being constrained by pandemic induced supply chain limitations and a vast array of disjointed barriers to air travel across national borders,” GAMA President Pete Bunce said. This was seen as a bright spot in the business jet segment, however, since the decline was less than what had been predicted. As we fly out the backside of the pandemic, we will see airspace and airport congestion increase as it supports the coming resumption in travel from massive pent-up demand.

Listen Up and Ask

As all aviation begins a post-COVID recovery and we become more comfortable with PBN and the associated policies, procedures, regulations and avionics, we will be less inclined to need memorization and math to fly our machines. Perhaps the E6-B and plotter have finally gone the way of unsliced bread, and this will decrease our workload. But while PBN may be like sliced bread for pilots, we should be prepared for the coming congestion and modifications to procedures. 

Until ATC gets a handle on how often they can modify published procedures, we must be ready for changes in airspeed, altitude and direction – often using different phraseology; unless we respond with “unable.” If you don’t hear the word “resume” sometime after the word “except” in the modification of a speed, an altitude or a fix, then you are released from only “parts” of the procedure. But which parts? Listen up and query the controller if the clearance can be interpreted in more than one way. If we get confused or behind during the arrival and approach, the rolling snowball of dung can get smelly and unmanageable very quickly. And remember, it’s your behind if you
get behind.  

About the Author

Leave a Reply