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Leeham News and Analysis
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Bjorn’s Corner: Landing after navigation
By Bjorn Fehrm
6 May 2016, ©. Leeham Co: In a recent Corner, we covered how to navigate a modern airliner with the help of the Flight Management System (FMS). We described how the FMS uses different navigation receivers to fly the aircraft via the autopilot along the flight plan. In the end, we used a special instrument landing system to land at the Nice Airport.
The classical way to land on airports in bad weather has been to use a VHF based Instrument Landing Systems called ILS. Figure 1 shows the large installations of transmitter antennas which are necessary to get such an ILS beam to fly on for landing at a runway.
Figure 1. Antennae to form the lateral ILS localiser beam for a runway. Source: Wikipedia.
The antennae we see are only for the lateral beam, the Localiser. It guides us in the horizontal plane. There is a second transmitter with associated antennae which forms the vertical beam so that we have a two dimensional glide path to land on, with the vertical part called a glide slope.
It’s clear that it’s costly to install such ILS equipment for each runway for an airport. There have not been good alternatives to the classical ILS system until now. Several alternative systems have been proposed based on shorter wave signals to get the size of the antennae down (Microwave Landing Systems, MLS), but these have not caught on outside of military use.
Now there are good alternatives being developed. These are all based on GPS systems, with more or less advanced implementations.
GPS systems
A GPS system, as used in cars and aircraft for underway navigation, is based on the receiver measuring the time it has taken the signal from several satellites to reach the position of the receiver. Standard GPS signals for civil use have an accuracy of around 100m horizontally and vertically.This is not enough for a landing system that could take over from ILS where the accuracy is measured in meters.
Another problem is that the receiver must know if there is something wrong with the satellite signals that form the GPS. It must be able to ignore a faulty satellite. There are GPS receivers for aircraft which use many satellite signals to form an opinion of the GPS signals and if they can be trusted for landing procedures or not.
Non Precision Approach
Landing systems that use a GPS receiver that can monitor the integrity of the satellite signal can take us down to around 400ft above the runway threshold. Such approaches are called Non Precision Approaches (NPA) and they have minima and procedures similar to the procedures that use classical nav aids VOR/DME for guidance.
Figure 2 is taken from a leaflet from Eurocontrol and shows the difference in of a Non Precision Approach (such as GPS based LNAV approach) and a Precision Approach (such as ILS or augmented GPS approaches).
Figure 2. Principle difference between Non Precision and Precision Approach. Source: Eurocontrol.
A Non Precision Approach (NPA) takes us to around 400 feet above the runway threshold by guiding us laterally through Initial Approach Fixes to the Final Approach Fix. From there we descent with a given descent rate to our Minimum Descent Height. Then we fly at that height until we can see the runway. If we have not seen the runway before the Missed Approach Point, we must initiate a missed approach procedure.
This descending and then flying at the Minimum Descent Height is called “dive and drive” and it’s not ideal from a safety perspective as the descent is not controlled by a glide slope. It will be a bit arbitrary where pilots will hit the Minimum Descent Height, dependent on how they do the descent.
There have been several accidents using this type of procedures, the latest widely known was Flight 1354, an Airbus A300-600F freighter from UPS that crashed at Birmingham–Shuttlesworth (AL) USA.
Precision Approach
If a glide slope can be added (i.e. vertical guidance with sufficient accuracy), the procedure goes from non precision to a Precision Approach (PA). The minimum height is then called a decision height and is a point on the continuous descent to the runway threshold. This is a more controlled position, both horizontally and vertically, therefore its minimum height is lower, typically 200ft above the runway threshold. For a very precise ILS system (a CAT IIIb system), the decision height can be less than 50 feet.
If we can get a GPS-based system to have a precision of a few meters instead of 100m, we can use it to guide us both laterally and vertically. We can create a Precision Approach (PA) system which would not need the cumbersome ILS installation for each runway.
In our next Corner we will go through how this is done and how this now revolutionize the way that we will land in bad weather going forward.
consumer GPS receivers are much better than 100m accuracy. More like 5m. Military GPS gets that down to 1m
Hi D,
I got the information from this FAA page and others,
http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/faq/gps/
got to the Question: How do GPS accuracy and integrity compare to that of existing ground-based navigation systems such as VOR/DME?
A. The basic GPS signal is accurate on a worst-case basis to within approximately 100 meters lateral and 140 meters vertical everywhere on earth. GPS, as provided to civil users, appears to be just as accurate as the most accurate service being provided by the VOR/DME, i.e., non-precision approaches. It should be noted that VOR accuracy degrades as you move farther away from the navigation aid. GPS accuracy is space-based, and thus not constrained by ground equipment. The basic GPS signal is not as accurate as the existing ILSs;
I agree that other places describe how you get within 3-5m in normal places. Here it must be worst case sand on any place on earth, its about the 100% safe guidance of an airliner.
I know its history, but lets not forget ADF and Loran!
Having worked Loran on a boat I was amazed. While we ran a safet cours, it was at night, foul weather (and cross check with radar) and it was a stunning early indication of what GPS could do.
US as an interim put Loran across the continental US for air ops and while not approved NPA, it did allow navigation.
ADF was a true challenge but I liked it. Once mastered it was efficient and I enjoyed listening to the radio stations while doing my night flying!
Hi Transworld,
I agree, their are more procedures, among them NDB approach used with an ADF receiver. And yes its the most challenging and therefore fun method to get to a certain Approach FIX, especially when it’s windy, love it. Never done LORAN but VOR/DME is also frequent as procedure for then a circling approach to the runway in use. And let’s not forget the most common, you get approach radar vectored to the point where you see the runway and can continue with a visual approach.
But now we shall describe the method that will replace the most common procedure for airliners, ILS approach. All about how to get GPS’ worst case precision to within fractions of a meter next week.
Ah, the old PAR approach. Was able to practice it once in Florence S.C. (FLO) on a very “slow” Sunday afternoon ten years ago. “You’re four miles from touchdown, on slope, correct ten degrees left.” (Felt like I was in an old 50’s AF movie, like the one with the B47 going in for touchdown in Japan–Yokota?–in a giant rainstorm!)
PAR (Precision Approach Radar) is another exiting instrument lading procedure. We had no VOR/DME/ILS or TACAN on our jet trainers in the Swedish Air Force (not being part of NATO which has TACAN as standard) so PAR was the procedure with the best minima. Required real concentration on what the PAR controller said and precise flying on instruments when the weather was bad.
Bjorn: Was it a conscious move not to adopt TACAN in country? Or was there a tech reason you couldn’t? One of my dads AK bush assignments (power for it and the perimeter monitor sysgtem) was a TACAN site and the maint for it.
Instrument flying was what I was best at.
All my instructors were in awe. They were not with me at age 8 or so off a small island troll fishing in South East Alaska when ones dad has decided that you steered 270 in a small boat amidst waves that had noting to slow them down all the ay to Japan! (I assumed my dad had to steer his WWII LCL(L) R and G variants to an extremely demanding officer)
I took him through a kept patch one day, snagged the two lines (two poles and 7 lures off each pole) he got pretty upset. Well you never said to look up! Good man, bad teacher.
I never got to fly a PAR, we did get to actually go flying in the clouds one day, socked in, instructor asked if I wanted to try it. You betcha Red Rifer (US phrase from a cowboy comic series)
That was truly where you got the feel of instruments and the vertigo. My instructor handled all the coms, I just did the flying. Something I though everyone should be subjected to as its far different than under the hood.
That was at the time when an instrument rating was a separate rating form a commercial license, all we were trained on was instruments, not flying a blind course, arrival, holding patters and all the intricate detail (I did get the instrument a year latter)
Not the greatest instructor (and I was not your typical student) but we worked things out. He got to the point that the standard instrument upsets under the hood was boring routine. He asked if I was ok with getting seriously wild. He never would tell me what he did when I had my eyes closed, but the gyrations were great. Never had a problem recovery from anything he threw at me.
Its always led me to wonder if there are a group of people like me who can absolutely trust the instruments and those who don’t. To me the instruments just were real and the feelings of flying were not. We did it with instruments covered over, pick up the scan, cross check , go to alternate if the primary was gone.
Good stuff. I had areas I was not that great in, ok, but instruments were a delight.
I did have a great sever cross wind for the ADF work the next year. Had it nailed, very satisfying to turn the theory into reality. The instructor said something to the affect of he had never seen anyone keep it within 5 degrees let alone a degree under instruments. I told him early training helps a lot!
Sweden went their own ways in many radio systems. It was deliberate as our main enemy in a conflict was focused on jamming what Nato had. No time bothering with what we had done. I flew our version of TACAN on the Draken, great system that only required one van on the side of the runway. Was a British made system. Instrument flying is all about your initial training and scanning technique and sticking to it, never get lazy and fly on values instead of the artificial horizon.
Amend the TACAN, my dad took care of a VORTAC, primarily we were civilian (FAA) but they had the TACAN part at that installation as it was a primary flight route form Seattle to Anchorage heavily used by the military.
Bjorn:
thank you, interesting Swedish history.
And agreed on the instruments. I am not sure if I was not the seat of your pants type or what, but I believed the instruments (if they cross checked).
With that as the mantra, they never failed to deliver.
It does seem too many pilots missed that.
this one is worth a read – system continues to fail. And by that I mean they don’t find out pilots that don’t react right and get that corrected. There probably should be a separate track to find out if they can ever overcome it or its so hard wired that you would have to train out every possible outcome (at which point you need to do something else)
Its an area I am deeply interested in, is there a human hard wired type that simply never really gets it?
https://www.flightglobal.com/news/articles/united-757-upset-underlines-need-for-startle-train-425202/