For quite a few flights I have wondered what the exact limits were on the GPS needle for the different types of approaches. More specifically, I was curious about the vertical limits of the glidepath. For example, the vertical glidepath shows a one dot deflection up meaning we are below the glidepath. How far below the glidepath are we? I also wanted to know if the glidepath was angular like the glideslope or linear like it horizontal cousin.
What I discovered may surprise you, may not but in any event just a little deflection on the glidepath could have you land short of the runway. Not good. For completeness of this blog post I will also discuss the horizontal limits of the GPS needle.
First of all, GPS receivers are approved under two different technical standards order (TSO). A GPS without the ability to use a wide area augmentation signal (WAAS) is certified under TSO-c129. A WAAS GPS receiver is certified under TSO-c145. The difference is important as the horizontal and vertical limits of the approach type changes based on which TSO it was certified under.
This is the most used GPS approach type as it can be used by both GPS (TSO-c129) and WAAS (TSO-c145) receivers. Technically, there are no vertical limits defined for this approach minima but Garmin has provided a pseudo-VNAV course that is used for advisory purposes only. When Garmin provides it, the GPS will show LNAV+V instead of LNAV. Even when you see the LNAV+V on the Garmin GPS you can only descend to LNAV minimums.
Starting 30NM from the airport reference point (ARP), the horizontal scaling will change from enroute (ENR) to terminal (TERM) or from ±2 NM to ±1NM. At the capture fix or two miles from the final approach fix the GPS will change from TERM to approach (APP) or ±2SM to ±0.3 SM. Once the GPS goes past and is selected for the missed approach, the scaling increases instantly to ±1 SM.
Not a lot of general aviation pilots are able to use this line of minima. However, with the advent of the WAAS GPS that is beginning to change. It used to be that only corporate GA pilots had the capability to descend on a pseudo VNAV path with a barometric altimeter. This system works through a flight management console (FMC) or through a flight management system (FMS). They are one and the same. There are quite a few companies that make FMS’s for corporate jets. Remember when I said that the scaling is different for a non-WAAS GPS versus a WAAS GPS. It is on this line that it is different.
For a non-WAAS GPS the horizontal scaling is the same as the LNAV horizontal scaling. It makes sense as it is using the same GPS signal for accuracy. The vertical scaling is linear all the way to the ground instead of being angular like on an ILS glideslope. This means that as the airplane approaches the runway the vertical scaling remains the same.
Unfortunately, for us, the vertical scaling is not the same for different FMS’s and you may or may not find this information in the airplane flight manual (AFM) for the airplane you fly. On the couple jet aircraft I fly, the vertical scaling on one is 100’ per dot and the second one is 125’ pert dot deflection. A quick read through AC 90-97 states this exact thing. The manufacturer has discretion to make the vertical scaling whatever they want.
I think it becomes quite obvious that if the airplane is allowed to descend below the GPS derived glidepath it is quite possible to land before the runway threshold while the glidepath is not at full scale deflection. To prove my point I have worked the number for the aircraft I fly both a dot low and full deflection of two dots low.
What’s amazing to me is that a pilot who accepts a one dot deflection low will land the airplane not on the runway but in the approach lighting system for that runway. Oops. On the two corporate aircraft that I fly, the flight director will not sufficiently raise the pitch attitude to center the glidepath needle. It is as though the flight director things the glidepath is angular but in reality the glidepath is linear.
On a WAAS GPS the vertical limits are different than a non-WAAS GPS. Instead of the vertical scaling being linear it is indeed angular like a glideslope on an ILS. At the precision final approach fix (PFAF) which is also at the glidepath intercept angle, the vertical scaling is ± 492 FT for full scale deflection. At this point the vertical scaling reduces angularly at 0.7° to 148 FT where it switches back to a linear fashion. The 0.7° coincides pretty closely to an ILS glideslope signal. Depending on how far away the PFAF is, will determine the distance the vertical scaling hits ± 148 FT. It takes the vertical scaling 4.63 NM to go from 492 FT to 148 FT.
With this in mind we can do the same analysis as before with the limits of the glidepath. Again, we see that a WAAS capable LNAV/VNAV will leave us short of the runway. Though this time full scale deflection will put us in the approach lighting system instead of before it.
An LNAV+V approach from Garmin will fall into this category. Technically, a LNAV+V approach is an LNAV approach and goes to a LNAV minima but the vertical scaling is as described on a WAAS enabled LNAV/VNAV approach. I decided to contact Garmin technical support to determine the vertical limits of the LNAV/VNAV and LNAV+V approaches. They were kind enough to answer my question, which I posted here as well as a picture from RTCA DO-229D that I placed right below.
The horizontal scaling for a WAAS enabled GPS LNAV/VNAV approach is the same as an LPV approach.
This approach is the best a WAAS enabled receiver can get. The approach is not technically classified as a precision approach but an APV approach or approach with vertical guidance.
The angular horizontal scaling for a WAAS LPV approach is like an ILS in that it attempts to maintain 2° horizontal course from the PFAF to a point determined in the approach (but is also past the runway). The horizontal limits will switch from angular to linear at the end of approach (also called the beginning of the runway) to ± 350 FT from each edge of the runway.
From FAA Order 8260.54 shows the design of an LPV approach. In this order, the approach designers have developed a GARP or geometric approach reference point. This point is the start of the 2° splay used for the approach. On a runway that is less than 9,023 FT, the splay is exactly 2°. On longer runways the splay is decreased so that the splay is exactly ± 350 FT at the runway beginning. On these runways the splay is something less than 2° or more accurate than a regular LPV approach.
A LP approach is the same as an LPV approach and needs a WAAS capable receiver. The only difference is that it does not have a vertical glidepath associated with it.
The vertical limits of an LPV approach starts out just like the LNAV/VNAV except at the end of the approach the vertical limits decreases to ± 49 FT. This time the vertical limits will place the aircraft on the runway.
So now you know something that most pilots who use this equipment do not know at all. When trying to figure out this information for the post, I had to search obscure technical manuals that pilots will not normally ever read and contact Garmin technical support. This information was not on the internet anywhere that I could easily find. Hopefully, this information will help pilots who are as curious as I am about the “why” in aviation.