Alright, let’s kick things off with the most requested (but not so bite-sized) topic by far: Performance Based Navigation (PBN). Out of all the topics I am planning to cover, you all pointed me at one of the chunkiest one straight away. It’s not going to be a quick one, but at the same time it will give you a lot of very transparent explanations and I will keep things as simple as possible and cut through all the confusion to get PBN crystal clear in your brain!
Why is PBN such an important topic nowadays? Because we are currently part of a global transition to PBN, it offers a lot of benefits to both fixed wing and rotary IFR traffic such as more efficient routes, more capacity and an increase in safety. It is especially exciting for the helicopter industry, as it will allow for instrument approaches to elevated (hospital) helipads and low level IFR routes. Unfortunately it can also be confusing to people as it combines a lot of different variables that each require their own explanation first. That’s why today we are going to look at:
- The PBN definition
- What is RNAV?
- What are PBN performance requirements?
- What is a PBN navigation specification?
- What is RAIM and SBAS?
- The PBN approach
- The PBN approach plate
If you can stick with me all the way to the end (even if it’s with breaks in-between), it will be well worth your time. Ready? Here we go!
THE PBN DEFINITION
So let’s zoom out and look at what PBN actually is. I will cut it down to just 1 sentence:
PBN is area navigation (RNAV) based on specific performance requirements for aircraft flying on a route or approach, or inside controlled airspace.
So basically, to keep things simple:
PBN = RNAV + SPECIFIC PERFORMANCE REQUIREMENTS
Now, you might ask “what exactly is RNAV?” and “what do you mean with specific performance requirements?” Let’s break these down, but keep that little formula in mind for now, and remember that PBN is not a ‘thing’, it’s a concept or framework to standardise IFR navigation accuracy and procedures globally.
WHAT IS RNAV?
So PBN builds on top of Area Navigation (RNAV), which refers to a method of navigation where aircraft do not rely on overflying ground based NAVAIDS (beacons) anymore, which used to be the norm for flights under IFR. I could spend ages going into little details, but the easiest way to understand this is by looking at my doodle below:
On the left you can see the conventional way of IFR navigation: aircraft are required to directly fly over ground based NAVAIDS along the route, this can lead to inefficient and silly looking routes. The aircraft’s position is only determined by using signals from these beacons, which can be really really old as well!
On the right side however, the aircraft does not have to fly directly over those beacons, resulting in a much more efficient route, this is called area navigation or RNAV. The aircraft’s position can be determined using onboard calculations that utilise “Fixes”. A Fix is essentially a waypoint defined by a name, a latitude and longitude and can be setup anywhere on earth by either using something called Global Navigation Satellite System (GNSS) or even those exact same ground based NAVAIDS. This instantly allows for much more flexible and efficient routes. Got it? Excellent, let’s move on.
THE SPECIFIC PERFORMANCE REQUIREMENTS
Now that RNAV is out of the way, let’s go over how RNAV links into PBN. We already mentioned that PBN is RNAV plus specific performance requirements. The PBN concept essentially tells us pilots: please go ahead and use RNAV for route X or approach X, BUT ONLY IF you can comply with Y. The reason for this is to globally standardise the added efficiency and safety RNAV can bring to the table, and to make sure pilots fly with appropriate equipment and during appropriate circumstances, for whatever procedure they are about to fly.
This “Y” consists of 3 main pillars that make up PBN:
- Pillar 1 is the navigation specification: what is our required navigation performance (how accurate do we need to navigate)?
- Pillar 2 is the navigation application: what kind of procedure can actually be flown?
- Finally, pillar 3 is the navigation infrastructure: are we using space based NAVAIDS (satellites), ground based NAVAIDS (beacons) or even other systems to aid our navigation?
The navigation specification determines what both our actual and required navigation performance is or should be, how do we “measure” this? By looking at these factors for aircraft equipment:
So, if we want to fly a specific PBN procedure, we need to verify that the available equipment, training and other required tools at our disposal match up with what is actually required for procedure X. These requirements are defined as a certain amount of accuracy, integrity, availability and continuity, depending on the selected procedure.
Let’s have a look at what those together actually look like in real life. The main thing to remember here is that there are different levels of “navigation performance”. The question we need to ask ourselves every time we want to fly a PBN route or procedure is: Does my navigation performance meet the navigation performance requirement for this specific procedure? If yes: yay! Let’s get on with it. If no: abort or pick a procedure with a ‘lower level’ of required navigation performance. These ‘levels’ or ‘specifications’ will be explained and then laid out below.
WHAT IS A NAVIGATION SPECIFICATION?
So now we need to know what these ‘specifications’ are. To start, when we want to fly a PBN procedure we can choose between 2 options: a procedure with an RNAV or RNP specification. RNP stands for required navigation performance and the only way this is different to RNAV is that it requires onboard performance monitoring and alerting. Don’t worry, that is just a fancy way of saying the aircraft will start shouting at you when it thinks it cannot provide you with the required navigation performance. Flying an aircraft with equipment that can only fly RNAV procedures, simply cannot check its own accuracy.
Now, these two options each have their own individual specifications which are very relevant to us. Why? Because we need to comply with this specification if we want to fly the route or procedure that we are looking to use. Both options can be split up into oceanic or remote routes (airways), and enroute and terminal procedures (i.e SIDs, STARs, approaches). Please remember though that only RNP can be used for the approach phase. So to summarise:
- Aircraft equipment can cross-check itself? -> RNP and RNAV procedures can be flown.
- Aircraft equipment can’t cross-check itself? -> Only RNAV procedures can be flown, and also means no PBN approaches!
Have a look below at the overview on what specification belongs to each group:
I know all the acronyms and numbers can be a little overwhelming, but don’t worry, we will look at each one individually in a little bit. The first thing to understand is what those little numbers mean. When you see something like “RNAV 10”, the 10 means that the aircraft flying that specific procedure or route must stay accurately on track with a 10 mile accuracy 95% of the flight time. So the smaller the number, the more accurate your aircraft equipment needs to be to fly said procedure, got it?
So where do those numbers actually come from? Are they trying to make life harder than it already is? No you silly, they are based on some actual calculations and definitions, which I will simplify to make you not want to scream and run away. Let’s say we, as the regulator, want to create a route from A to B. We figure out what the best location on earth is for the route and start with something called the “desired path“.
Then, unfortunately, when we create and publish the actual route on a chart, there will be a little error between the desired path and the defined path on the chart, to be used by pilots, due to technical inaccuracies (life isn’t perfect!). The difference between the 2 is called the Path Definition Error (PDE). So now we have:
Now that we have our path, the next inaccuracy will be due to the fact that the aircraft will have to be controlled by either us, or the autopilot (AP), which is going to present something called the Flight Technical Error (FTE). Neither us or the AP will be able to perfectly stay on track at all times (not even you, you egomaniac), so the FTE is the difference between our estimated position and the defined path.
Are we done yet? Almost, there is one final (and most relevant for us) error called the Navigation System Error (NSE). This is the difference between where the aircraft thinks it is (estimated position), and where we actually are, our true position.
The NSE, PDE and FTE together make up the Total System Error (TSE) and is found as a value in Nautical Miles for each procedure. So long story short (you deserved it): the numbers behind RNAV and RNP refer to the difference between our desired path and our true position (the white line below) and is called the Total System Error, otherwise known as the Lateral Navigation Accuracy (ANOTHER term, I know!).
So based on all these details so far, ask yourself if you could tell someone what the difference is between RNP 1 and RNAV 1 as well as the difference between RNAV 10 and RNAV 5. Answers are below (but don’t peek if you’re still thinking about it!).
RNP 1 vs RNAV 1: Both have the same accuracy requirement but RNP 1 requires a nav system onboard that can check itself for accuracy and integrity.
RNAV 10 vs RNAV 5: RNAV 10 requires the navigation to be accurate by 10 miles for 95% of the flight time, RNAV 10 is therefore only half as ‘strict’. Neither require systems that can check itself for accuracy or integrity.
Did you get it right? Next, we are going to look at the 2 major solutions that enable PBN, called RAIM and SBAS. Without these, none of this would be possible!
WHAT IS RAIM AND SBAS?
“Wow, calm down with the new acronyms!” I know, but we have to look into these before our final segment can be discussed. These are all systems that allow our navigation systems to be cross-checked for accuracy and integrity, which is what we need for some of these PBN approaches, remember?
RAIM stands for Receiver Autonomous Integrity Monitoring, a fancy term for the aircraft equipment’s ability to cross-check the GPS signal’s integrity that it receives. Basic RAIM does require 5 satellites though, and a 6th one if the systems wants to isolate a faulty satellite. What if you lose signal to any of these required satellites? Then you no longer have RAIM capability and you need to re-evaluate your options! This is why you should check RAIM coverage before flying a procedure that requires RAIM. One way to do this before flying is using the Eurocontrol RAIM prediction tool, you can find this at: https://augur.eurocontrol.int/status/
The other system that enables PBN approaches is SBAS. With RAIM, all the processing is done onboard the aircraft, this is not the case with SBAS. SBAS stands for Satellite Based Augmentation System, it uses the GNSS satellite constellation, geostationary satellites and ground stations to determine a very precise position of the aircraft. Here in Europe it is called EGNOS, but there is coverage in a lot of different areas around the world as well. Have a look at my doodle below which covers EGNOS. There are 4 elements to this system:
- 3 geostationary satellites (they maintain their position in respect to earth’s surface)
- GNSS satellites
- The aircraft
- SBAS ground stations
The aircraft receives the GNSS signal to determine it’s location (during an approach for instance). This signal is also received by the SBAS ground station, which gets processed by any of the mission control centres to detect any errors and potential compromises in integrity. This signal then gets sent to the geostationary satellites, which link that info back to the aircraft, which will receive a very accurate signal that is cross-checked by the ground station. Not too complicated right?
So to summarise: RAIM: aircraft receiver checks signal integrity using satellites, the processing is all done onboard the aircraft -> less accurate. SBAS: ground stations check signal integrity using and send it to the aircraft -> more accurate. This is a little oversimplified but we can elaborate on these in a future article.
THE NAVIGATION SPECIFICATION LEVELS
Now that we have gone through all these behind the scenes technicalities and you’re still awake, we’ve now come to the part where we can start to actually understand what each requirement means, and summarise everything using those 3 pillars that form PBN (remember them?). Each level or specification will have their own use and required infrastructure. (Note that both RNP and RNAV specifications can both utilise GNSS, not just RNP!)
So we have RNAV 10 to RNAV 1, and RNP 4 to RNP 0.3. All the RNAV specifications can utilise both ground and space based NAVAIDS. The RNP specifications all require the use of space based NAVAIDS in the form of GNSS and/or SBAS, but also BaroVNAV (the aircraft’s barometer for altitude readings). RNP APCH is the specification we need to fly a PBN approach, with the RNP AR APCH needing prior authorisation in the form of special training (AR means Authorisation Required).
RNP 0.3 is a specification mainly utilised by helicopters and will be used in the future for low level IFR routes, which is already being used in certain places in Europe for routes which transition into hospital helipad approaches, this technique is also referred to as Points in Space (PinS) and will hopefully allow loads of HEMS operations to operate under IFR in the future!
The main takeaway here is that you will need to comply with an RNP specification if you want to fly a PBN approach. RNAV equipment by itself is simply not allowed to be used by us during an approach. Why? You now know why: let’s say the onboard systems are not as accurate as they need to be, or we lose crucial signals while we fly the approach, in cloud, focussed on the instruments. Who is going to tell us we need consider to go around? That’s right, no one!
Remember that some of these procedures could reduce minima even further compared to a conventional ILS or MLS approach. You REALLY want a system that can tell you to consider going around if it thinks it lost its required accuracy or integrity, makes sense right?
THE PBN APPROACH PHASE
Ok, so the approach phase is probably the most exciting bit of PBN, as it allows for highly accurate IFR approaches without the need for ANY ground based equipment at the landing location! Welcome to the 21st century – let’s find out what our options are. To start we need to understand the difference between an angular and a linear operation.
As shown below, a linear operation means that the maximum tolerable distance away from the approach centreline stays the same as you fly along the approach, this operation requires RAIM. An angular operation however, has a reducing (angular) band. This means the maximum tolerable distance away from the centreline reduces as you get closer – i.e more accurate flying required, comparable to how an ILS works. This requires SBAS.
Keep that concept in mind. Now, we can choose between 2D (non precision) or 3D (precision) approaches. That just means: does the approach have lateral guidance only, like a localiser? (2D) Or does it give us lateral and vertical guidance (3D), like a an ILS? Let’s focus on the 2D approaches first.
There are two 2D PBN approaches (lateral guidance only): the LNAV approach and the Localiser Performance (LP) approach. The main difference here is that the LNAV approach is linear while the LP approach is angular and SBAS supported (more accurate). Note in the picture below which requires RAIM and SBAS and remember that SBAS gave us better accuracy!
Then, for the 3D approaches we have LNAV/VNAV (linear), and LPV (angular) which requires, you guessed it, SBAS! LNAV/VNAV however, requires RAIM and uses GNSS for lateral guidance and BaroVNAV for vertical guidance (onboard altimeter). Angular approaches, while more accurate, do require SBAS which is both a benefit but also an extra limitation for us.
If SBAS is not available for any reason, you can not continue the angular type of approach, even if you have already started it! A gotcha here is that you might think, ok I can do an LP approach (2D) instead of LPV (3D) now right? Nope! LP requires SBAS just like LPV, so your only options are LNAV/VNAV and LNAV. This is why a system that can check itself is so important, it will tell you when integrity is lost, so we can make a decision during the approach and after the go around, using the information the equipment provided us with.
Well, that is a lot of acronyms and terms, but hopefully this has given you a good idea of the structure of all the PBN options. Just remember: No SBAS -> no LP or LPV approaches for us! There is one final part left to revise some more practical things:
THE PBN APPROACH PLATE
You have made it really far at this point, good job, we’re almost done! Let’s have a look at what kind of shenanigans we can expect on a PBN approach plate. Have a look at the Exeter RWY 26 PBN plate from the UK AIP:
First of all you want to check, just like with any chart, that the correct plate is selected by looking at the runway as well as the type of the approach plate. Normally you would check if it’s a VOR DME or ILS chart etc, but with PBN, all you will now see in the top right below the runway, is “RNP APCH” (this used to be “RNAV”). This was the navigation specification we needed to fly an approach, remember?
Now, on the Exeter plate here, you can see EGNOS CH followed by a number. This is the channel number of the LPV approach, which is the channel the geostationary satellites transmit the signal with enhanced accuracy and integrity on, for that approach. Your FMS will have this channel number already linked to the approach, so you can use it to load it up that way. If you have a plate without a channel number, it just means LPV is not available for that airport! Also, if you use GCAP plates, you cannot fly LPV either as these plates do not include any channel numbers!
The section below will show the different Initial Approach Fixes (IAF’s) which can be used to start the approach section. These will usually look like a T (called a T-bar) or a Y (called a Y-bar). These are used as entry points into the procedure. The exeter one has a Y-bar approach setup:
You can start the approach procedure from any of these IAP’s but have an extra look at their capture regions. For the Y bar approaches like this one, the top and bottom IAP’s (LETSI and BATSU) have a 180° capture region, while the middle one (NEXAN) can only be captured from within 140° (2 x 70°). If you have a T bar approach setup, then all 3 capture regions are 180°.
At the bottom of the chart, you will still find the information you are used to such as the recommended profile, the groundspeed vs vertical speed scale and the missed approach procedure. Pay extra attention to finding your approach minima though as each procedure has their own values (LPV, LNAV etc). Helicopters are all CAT A (or CAT H), so if you are used to GCAP plates, just look at column A. For the fixed wing pilots, you already know what to do!
So there you have it, PBN in a (relatively) bite-sized way! The article is a little longer than I was aiming for, but I blame you all for that, as everyone requested PBN as the first “Pilots who ask why” article! I hope this has added value to your PBN knowledge and got rid of some of that brain fog. I will cover individual PBN topics in the future to elaborate on more details, but there will be loads of other items first to keep things fresh. If you have ANY questions whatsoever, or future requests: just send them in. I read and reply to every single message.
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