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Glideslope 101

We don't hear the word glideslope mentioned as often as you did in the past; today's buzzwords are GPS, WAAS, MFD, TCAD and the list goes on and on.Ground Glideslope Station There's a couple of reasons we don't include glideslope in our buzzwords nowadays; one is the system works great and with modern radios such as the King KX-155 there's seldom a glideslope problem related with the equipment or the ground station. Another reason we don't talk about glideslope is because it's boring. You can't "crank in" a glideslope frequency, listen to the ident or turn the knob on the CDI and make the needle move. You may find this interesting; while you must check your VOR's for accuracy every 30 days (FAR 91.171) for IFR operation, there is no requirement for part 91 operators (GA guys) to ever have the glideslope system tested for accuracy. Imagine, an instrument that can get you within 200ft of the ground in most cases but it never has to be checked to verify proper operation. Why not you ask? To be honest I haven't a clue but I personally believe the system should be tested anytime you are in the avionics shop for your bi-annual transponder test. While the glideslope may not have the flashing lights and pizzazz such as the panel-mounted GPS or MFD, it's still the only legal electronic vertical navigation today in the general aviation world. By the way, the word glideslope is not mentioned in Mr. Webster's dictionary; as far as I could determine, it's exclusively an aviation term.

The Glideslope System. The glideslope signal is radiated via an antenna array located in the general area of the approach runway, a vertical tower about 20' tall. The signal is radiated to produce two intersecting lobes, one on top of the other. The upper lobe is modulated with a 90-hertz and the bottom is modulated with 150-hertz. These lobes are transmitted from the end of the approach runway outward at an angle between 2.5 3.3 degrees. There's a big difference in flying a 2.5 versus a 3.3 degree glideslope, discuss this with your instructor. When the aircraft glideslope receiver is receiving equal amounts of 90 and 150-hertz signal, the glide path is defined and the glideslope needle is centered. Based on this format, the glideslope provides the pilot with vertical guidance to an altitude (MAP) determined by the FAA.

If the aircraft is above the glide path then the receiver is receiving more of 90-hertz signal than 150 hertz and glideslope needle will be below the center position within the indicator; alerting the pilot that he/she is above the glideslope and needs to increase the descent rate. If the aircraft is below glide path, the receiver is now receiving more 150-hertz signal than 90-hertz signal and needle will move toward the top of the indicator telling the pilot to fly up.On the Glideslope I know you keep a centered glideslope needle throughout the approach but I wanted to tell you what you would see just in case you fly with someone who flies off the glide path... From what you just read, one can easily determine that the glideslope system is a simple system.

A glideslope is always paired with a localizer; both the localizer and glideslope needles and flags are located within the same indicator. There is a glideslope flag also. If a problem occurrs within the aircraft glideslope receiver or the radiated signal from the ground becomes a problem, the warning flag will be in view. Anytime there is a flag in view you MUST assume there is a problem and not rely on the signal. In other words if you have a glideslope flag in view, DO NOT use the information on the indicator even if it appears to be displaying what you think is proper information. There are procedures that must be followed if a glideslope flag is in view. Please consult your favorite CFII for details. Another thing, NEVER use a glideslope data on a back course, again talk to an instructor as to why not.

How do you "Tune In" a glideslope frequency you ask? Hmm, there's not an extra set of knobs on the navigation receiver to tune the glideslope, nor is there a glideslope frequency on the approach plate.Cessna Glideslope Antenna OK, grab a cold glass of milk and some Oreo cookies and lets discuss the process. Localizer and glideslope frequencies are what we call "Paired" frequencies. When you tune in the ILS frequency called out on your approach plate you are in fact tuning the glideslope, you just don't know what that frequency is. Because the frequencies are paired, when you tune to the ILS frequency of 108.90 MHz, the navigation receiver automatically tunes the glideslope receiver to 329.30MHz. Our VOR and localizer frequencies are in what we call the VHF (Very High Frequency) band; glideslope frequencies are in the UHF (Ultra-High Frequency) band. If you look at a paired frequency chart; you would notice that the glideslope frequency doesn't necessarily increase as you tune in a higher ILS frequency. The reason for this is the glideslope frequency selected is paired with that particular localizer frequency so not to cause interference. In other words the UHF and VHF frequency are paired so not to cause any type of interference that would distort the other. By the way, there 40 ILS frequencies available. Normally you be able to receive a glideslope signal 10 miles out or more. Be advised you may not receive a valid glideslope signal unless your aircraft is on the localizer and heading for the proper runway, more on this later. Another point to keep in mind is the closer you get to the runway, the more sensitive the needle is which increases the pilot workload to keep it centered. High performance autopilots such as the S-Tec System 55X and King KFC-200 will capture and track the glideslope; they do a pretty good job of keeping the needle centered; much better than Mike Busch but not as good as yours truly.

In the old days, glideslope receivers were large, remote boxes that weighed several pounds and took a lot of space. Modern radios such as UPSAT,Glideslope Splitter Garmin and Narco have the glideslope built inside the panel-mounted box; it weighs just a few ounces. Some of the older equipment had the glideslope receiver built inside the indicator such as the King KI-214, normally these are problem children and should be avoided by any serious instrument pilot. With few exceptions a single glideslope receiver can drive several indicators along with the autopilot.

The glideslope antenna. One of the advantages of using UHF frequencies for glideslope operation is only a small antenna and ground plane is required. As a rule of thumb, the higher the frequency, the smaller the antenna can to be. For instance, note how long your VHF COM antenna is versus the size of the GPS antenna. Piper, Mooney, Beechcraft for most parts used the standard navigation antenna located at the top of the vertical stabilizer for the glideslope antenna. This antenna picks up both the VHF Nav channels along with the paired glideslope. Near the radio stack is what we call a splitter, which takes the UHF signal and sends it to the glideslope and also takes the VHF signal and sends it to the navigation receiver. Notice that I didn't mention Cessna with the aircraft manufacturers listed above. Cessna installed the glideslope antenna in the wing on some 210's, which cost a bundle to replace when it failed but didn't work any better than the antenna on the vertical stabilizer. Then they placed the antenna in the windscreen, thinking it would have an unrestricted shot at the glideslope ground array. This system performs poorly under some conditions; Cessna even stated in the POH to avoid some RPM ranges to "avoid oscillations of the glideslope deviation pointer caused by propeller interference". If I submitted a flight manual supplement to the FAA with a note such as mentioned above; the FAA would be all over me. All Cessna aircraft with ARC radios used a remote glideslope receiver; here's another weird thing Cessna did. The mounted the glideslope receiver in the tail of the aircraft on many of their models. This is how it worked: The antenna is located on the vertical stabilizer and a cable runs from the top of the stabilizer to the splitter, which is located under the instrument panel.Mounted in the Leading Edge of the Wing From the splitter a cable runs back to the glideslope receiver in the tail of the aircraft. In layman's terms, that's a lot of loss due to the long cable runs. Cessna finally started using the standard navigation antenna for glideslope like most manufacturers have done for years. Large twin-engine aircraft normally have the glideslope antenna behind the radome. Jets normally have a segregated glideslope antenna some where near the front of the aircraft. The UPSAT SL30 uses regular VHF input and takes the glideslope signal from that input inside their box, thus no splitter is required.

Glideslope has been around for decades; it works great and gets us lower on the approach than any other product out there today. How long will glideslope be around you ask? Different folks give us different answers; I'd be willing to bet it still be our main source of vertical navigation well into the 21st century. While the glideslope isn't high-tech it is a valuable tool to the instrument pilot.

This article by Tom Rogers originally
appeared on AvionicsWest.com and
is republished here with permission.
Note: This article was first
published before WAAS precision
approaches were widely available.
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