Why VOR Checks, but no GS & ILS Accuracy Tests
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My DPE asked me this rhetorical question during the oral exam. The localizer receiver is a separate receiver from the VOR receiver (though it's inside the same box) but the localizer receiver doesn't need an accuracy test. He didn't ask about the glideslope receiver but I guess the same question applies. Other countries (like England) apparently don't require 30 day VOR checks at all.
The signal modulation is different. VOR's use a phase difference between the omni and the directional signal to compute the radial. Both the localizer and the glideslope are just a left signal and right signal (or up and down) that the needle is measuring the balance (sort of an automated version of the old AN stuff).
gismo
Touchdown! Greaser!
While that's true, I suspect that the real reasons that LOC and GS receiver/displays don't require monthly checks are that 1) there's no easy way for a pilot to make such a check, and 2) even if the centering is off (and it is adjustable and can easily become off by more than a dot) the course "width" shrinks to nearly nothing at the threshold so a small angular error won't have much effect in terms of distance when you're close in.
ChrisK
En-Route
Huh. That's a neat mental picture. So if the approach can be thought of as a funnel, then it doesn't matter if the funnel mouth gets slightly wider and the angle slightly shallower - it still directs traffic to the stem!
John Collins
En-Route
I have had an ILS GS failure without a flag indication, where the GS always indicated centered. After this incident, I have checked the operation of the localizer and glideslope on the ground prior to departure anytime it was feasible. At my airport, my hangar is on the opposite side of the runway as the parallel taxiway. I tune in the ILS and note that the localizer is centered in the middle of the runway as I cross over. I also verify that the GS indication has a deflection as I parallel the runway. I know where it should be a fly up indication on the taxiway.
DouglasBader
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That depends on the equipment. In the 747-200, I do a VOR check, as well as a localizer and a glideslope check on every preflight.
Dr. O
Pattern Altitude
The real reason for Part 91 is BECAUSE the regs say so...
gismo
Touchdown! Greaser!
That's a very plausible failure (there are several single failure points with that symptom in most GS setups) but with my instrument skills it's pretty obvious when the needle stays in the center that something is broken.
Seriously though, I don't think it's possible that you could miss such a failure in flight prior to descent after GS intercept and all the preflight testing in the world isn't going to mitigate that same failure mode once you're headed down the glidepath. I'm pretty sure I read at least one report of either an accident or a near accident where the a single GS failed that way (needle stuck in the center after GS intercept) so if you only have one GS receiver/display it's probably a good idea to occasionally drift a dot high on purpose while descending in IMC (I know I've used that as an excuse/explanation on an IPC or two).
DouglasBader
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If you're watching your altitudes on the approach, then a simple cross check of published altitudes at each fix on the inbound course should be enough to establish that you're keeping the glideslope.
It's also a good idea to know what to expect for your rate of descent on the approach; if you crossed the FAF or other fix at the published altitude with your needles centered, and you're flying the rate of descent that you should for the approach gradient and your groundspeed, then chances are you're awfully close. If the instrument doesn't flag and the needle indication doesn't vary, and you continue in that vein, then you're probably in good shape.
It's also a good idea to know what to expect for your rate of descent on the approach; if you crossed the FAF or other fix at the published altitude with your needles centered, and you're flying the rate of descent that you should for the approach gradient and your groundspeed, then chances are you're awfully close. If the instrument doesn't flag and the needle indication doesn't vary, and you continue in that vein, then you're probably in good shape.
John Collins
En-Route
Good advice. In this particular situation we were in a high holding pattern at 6000 feet and waiting for the visibility to come up to minimums. The weather was clear above the field after about 1000 feet above field elevation. I was flying right seat and the student was planning on using the autopilot for the approach. The aircraft was equipped with a single GS receiver. We were given vectors from the holding pattern to the final approach course and had to lose a substantial amount of altitude to get down to the GS intercept altitude of 2500 MSL, so we were descending at 1000 FPM. We were cleared on our last vector to intercept the final approach course and were still using a high rate of descent when we intercepted the localizer. The GS was observed to be in the center and the flag was not visible, so the Approach mode was engaged on the autopilot and it indicated that it had captured the GS and was tracking it.
It took a few moments to recognize that after capture, the descent rate was still around 1000 FPM and way above what would be expected. I told the student to disconnect the autopilot and start a missed approach, but he did not understand and he froze. I took over but at the high rate of descent we had passed thru 2500 MSL. By the time I got control of the airplane from the autopilot, added power and pitched up, got the gear up so we could climb, the approach controller called us with a low altitude alert. I explained we had it under control and were missing the approach as we had some type of equipment failure.
We ended up flying back to our departure airport which was VFR and tried doing the ILS there. We had a similar result, except that we were level at the intercept altitude when the autopilot was engaged. It gave the same indication, but because we were level, it held altitude when it thought it was tracking the GS. Some other points, the flag and receiver seemed to be working correctly because in both ILS situations the localizer was identified and the flag went out of view. What appeared to be not working was the vertical CDI and since the same signals are routed to the autopilot, it confirmed by its actions that it was getting the same input, IOW with an established descent, the autopilot maintained the descent and with level flight established, it maintained level flight, as in both instances, it determined that it was keeping the needle centered and correction wasn't required.
On the ground, we played with the equipment and weren't able to get any deflection on the vertical CDI. In our conversation, the student mentioned that he had the GPS removed for a repair by the avionics shop and it is adjacent to the Nav/Com. Being an ex avionics shop owner, I noted that there was very little space between the two radios and that often one must remove a radio that is immediately below the one you need to remove to eliminate a natural mechanical interference caused by installing the units too close to one another. So I went to my hangar and got my tools and removed the Nav/Com and then re-seated it. I turned everything back on and now the Vertical CDI came to life as even from the hangars, the ILS GS can be received on the ground. I filed a NASA report and got a call to discuss the details with one of the investigators, but never heard from the FAA.
Ever since, I have performed the GS/Localizer test as I taxi to the active at our airport and I have avoided flying in IFR conditions with a single ILS receiver.
DouglasBader
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Good post.
As I read your account, one of the first things that stood out for me was the single ILS receiver. One can do an IFR flight with just one, even without using one at all, but I don't know many professionals who will choose something other than the ILS, given the choice. Two receivers isn't just about redundancy, but about being able to compare one's progress on the approach, one against the other.
Two receivers, however, won't help if one has the same bad signal. I don't have the video presently, but one we've watched numerous times in training has been an account of a south pacific flight that caught a lobe on the ILS, and very nearly flew into the ground. In 2004, a Gulfstream III crashed in Houston while arriving to pick up former President Bush Sr. It was flown by the current Chief Pilot and former Chief Pilot of the company, each high time aviators, each experienced and current in type. Low visibility prevailed as they were turned onto he ILS by vector. They descended into a parking lot, with three dead on board.
I've stopped two pilots in Learjets from following what they thought was an ILS, but instead was a GPS signal, both attempting to descend into mountainous terrain.
It's very important that one observe the altitudes at checkpoints along the approach. Additionally, one should know that based on a generic 3:1 descent into a particular location, where one ought to be. At one mile out, one ought to be about three hundred feet. At ten miles out, one ought to be at 3000'. Five miles, fifteen hundred feet, and so on. If the field is sea level, that's easy. If it's something other than sea level, add those numbers to the field elevation, and there's a rough crosscheck one can do.
Far better than a rough crosscheck is to use the altitudes from step-down fixes along the glideslope, or there fix altitudes, and cross check that one is doing what one's supposed to do. One of our standard call-outs while flying the ILS is to call out the final approach fix "ROBBS checks." The response from each crew member is "Checks." This signifies that the crew member making the call has verified that we're indeed over ROBBS, and that he's verified it by identifying it's position as given on the chart (DME, cross-radial, on the FMS or GPS, etc). He's also verified the correct altitude crossing the fix. The response by each crew member is also "checks."
Our standard calls have long included verifications when passing through a thousand feet. At one thousand above field elevation we called "one thousand," and the airspeed additive ("plus ten," for example, meaning ten knots fast). The flight engineer noted "No flags, descent rate eight hundred." The same calls and checks took place at 500' above the TDZ. The current call-outs only require the calls if something is out of parameters. Otherwise it's only monitored.
Whether one is actually calling out deviations or conditions, or simply noting them, it's important to observe that one is at the appropriate altitude, given one's distance from the runway.
When flying into LAX, a typical approach is ILS 25L, and there are fixes cited on the procedure chart out to 41 DME. This is clearly outside of the ILS limits, but it's very common for crews to pick up the localizer and glideslope out there and follow it down. The FAA published a note a year or two ago about that sort of thing, reminding pilots that the crew still has a responsibility to meet all the minimum crossing altitudes for each published fix along that routing.
When working in mountainous terrain, I usually have a sticky note attached to the panel or control column somewhere that gives me fixes at each mile to the runway, showing what my altitude should be at that point. I can use that as a cross reference for where I ought to be during the descent. I make up the sticky before getting to the terminal area, noting the glide path gradient or the standard.
It was a good lesson for your student, I would imagine, so long as he retained some of what happened during his "frozen" time.
It's interesting to note that you got a low altitude alert by the time you were able to wrest the airplane from the student, disconnect the autopilot, and initiate the go-around. There was a discussion on this board not long ago about touching the runway during a go-around off an ILS, which can and does happen in large transport category aircraft. Perhaps you can see why.
As I read your account, one of the first things that stood out for me was the single ILS receiver. One can do an IFR flight with just one, even without using one at all, but I don't know many professionals who will choose something other than the ILS, given the choice. Two receivers isn't just about redundancy, but about being able to compare one's progress on the approach, one against the other.
Two receivers, however, won't help if one has the same bad signal. I don't have the video presently, but one we've watched numerous times in training has been an account of a south pacific flight that caught a lobe on the ILS, and very nearly flew into the ground. In 2004, a Gulfstream III crashed in Houston while arriving to pick up former President Bush Sr. It was flown by the current Chief Pilot and former Chief Pilot of the company, each high time aviators, each experienced and current in type. Low visibility prevailed as they were turned onto he ILS by vector. They descended into a parking lot, with three dead on board.
I've stopped two pilots in Learjets from following what they thought was an ILS, but instead was a GPS signal, both attempting to descend into mountainous terrain.
It's very important that one observe the altitudes at checkpoints along the approach. Additionally, one should know that based on a generic 3:1 descent into a particular location, where one ought to be. At one mile out, one ought to be about three hundred feet. At ten miles out, one ought to be at 3000'. Five miles, fifteen hundred feet, and so on. If the field is sea level, that's easy. If it's something other than sea level, add those numbers to the field elevation, and there's a rough crosscheck one can do.
Far better than a rough crosscheck is to use the altitudes from step-down fixes along the glideslope, or there fix altitudes, and cross check that one is doing what one's supposed to do. One of our standard call-outs while flying the ILS is to call out the final approach fix "ROBBS checks." The response from each crew member is "Checks." This signifies that the crew member making the call has verified that we're indeed over ROBBS, and that he's verified it by identifying it's position as given on the chart (DME, cross-radial, on the FMS or GPS, etc). He's also verified the correct altitude crossing the fix. The response by each crew member is also "checks."
Our standard calls have long included verifications when passing through a thousand feet. At one thousand above field elevation we called "one thousand," and the airspeed additive ("plus ten," for example, meaning ten knots fast). The flight engineer noted "No flags, descent rate eight hundred." The same calls and checks took place at 500' above the TDZ. The current call-outs only require the calls if something is out of parameters. Otherwise it's only monitored.
Whether one is actually calling out deviations or conditions, or simply noting them, it's important to observe that one is at the appropriate altitude, given one's distance from the runway.
When flying into LAX, a typical approach is ILS 25L, and there are fixes cited on the procedure chart out to 41 DME. This is clearly outside of the ILS limits, but it's very common for crews to pick up the localizer and glideslope out there and follow it down. The FAA published a note a year or two ago about that sort of thing, reminding pilots that the crew still has a responsibility to meet all the minimum crossing altitudes for each published fix along that routing.
When working in mountainous terrain, I usually have a sticky note attached to the panel or control column somewhere that gives me fixes at each mile to the runway, showing what my altitude should be at that point. I can use that as a cross reference for where I ought to be during the descent. I make up the sticky before getting to the terminal area, noting the glide path gradient or the standard.
It was a good lesson for your student, I would imagine, so long as he retained some of what happened during his "frozen" time.
It's interesting to note that you got a low altitude alert by the time you were able to wrest the airplane from the student, disconnect the autopilot, and initiate the go-around. There was a discussion on this board not long ago about touching the runway during a go-around off an ILS, which can and does happen in large transport category aircraft. Perhaps you can see why.
Threefingeredjack
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Threefingeredjack
The above post is an excellent example of CRM and using all available assets to enhance situational awareness. While my wife is not a pilot, she can read charts very well and calling out waypoint altitude checks on approaches is one of her jobs when in the muck. She also looks for the runway while I have my head down. GA pilots could do well by adopting some of the habits Doug outlined above.
One note, and you touched on this some..
high temperature can cause the GS to take you below a published altitude when normally it would match right on. You must still mind the altitude in these situations (usually its not much) but it can keep you out of airspace your not supposed too be in .
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olasek
Pattern Altitude
I do have basic question.
If VOR is off by say 4 degrees per ground testing, how does it influence its accuracy when using it for the LOC tracking. In other words is it possible that LOC needle will be showing say that my tracking is off even though I am precisely in the center of the beam?
If VOR is off by say 4 degrees per ground testing, how does it influence its accuracy when using it for the LOC tracking. In other words is it possible that LOC needle will be showing say that my tracking is off even though I am precisely in the center of the beam?
denverpilot
Tied Down
Kinda depends on where the error is being introduced. Electronics, mechanical, etc.
gismo
Touchdown! Greaser!
With most GA (3-1/4") mechanical CDIs and HSIs the only common aspect of LOC and VOR error is the meter movement itself and that can be checked simply by looking at how close the needle points when all power is off. The LOC demodulator circuitry (which may be in the indicator, the radio, or a separate box) is completely independent from the VOR demodulator and unlike the VOR system, the LOC display doesn't involve any input from the OBS (the thing the knob sets) so that source of error is eliminated as well. The LOC demod is usually adjustable for centering and possible max deflection so there is some potential for an erroneous display even if the meter movement is OK.
DouglasBader
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Extremely cold temperatures will place you lower over a waypoint than you are indicating.
A high temperature will place the aircraft higher, and when following the glideslope, one then indicates a lower altitude than what's published.
The most important thing is to operate safely, of course, and inclusive in that concept is crosschecking for reasonableness while flying the glideslope.
Your exactly right,
The example I had in my head was In this document below dealing with high temps. And step downs..
http://www.faa.gov/other_visit/avia...afety/info/all_infos/media/2011/InFO11009.pdf
olasek
Pattern Altitude
iGismo, great answer, this is what I expected that the error in VOR doesn't have to imply any error with LOC.
azure
Final Approach
And as I understand it, that's why loss of separation can happen when following the glideslope in higher than standard temperatures, since the indicated altitudes are what ATC relies on to effect separation. Also, as I understand it, this can only lead to a pilot deviation if the stepdown fixes are outside the lowest published intercept altitude, and the pilot intercepts the GS without explicit instructions from ATC to do so. Once inside the lightning bolt, any stepdown fixes one finds on the plate only apply to the nonprecision approach with the same lateral guidance (LOC or LNAV).
If I have that wrong, I'm sure someone will correct me.
John Collins
En-Route
And as I understand it, that's why loss of separation can happen when following the glideslope in higher than standard temperatures, since the indicated altitudes are what ATC relies on to effect separation. Also, as I understand it, this can only lead to a pilot deviation if the stepdown fixes are outside the lowest published intercept altitude, and the pilot intercepts the GS without explicit instructions from ATC to do so. Once inside the lightning bolt, any stepdown fixes one finds on the plate only apply to the nonprecision approach with the same lateral guidance (LOC or LNAV).
If I have that wrong, I'm sure someone will correct me.
You are right in most respects including the last sentence.
The only thing incorrect about your post is that ATC can't explicitly permit you to follow the GS and ignore the step down fix minimums outside the lightning bolt. There used to be instructions on some approach charts that were confusing and stated that the GS could be intercepted at various higher alternate altitudes further out on the approach, but these were all eliminated or a NOTAM has been issued that has eliminated them.
azure
Final Approach
John, thanks for the correction/update. I remembered only the discussion of the problem, but was completely out of the loop on the recent changes.