Glideslope Check

[your_dreamguy]

Hello,

In our company procedures, the captain is required to check their altitude readout crossing the outer marker when conducting an ILS. This is done to make sure that we do not have a false glideslope readout. However, our airline does not give an error tolerance. Does anyone know if the FARs or AIM give an error tolerance? EX: An ILS plate says when crossing the OM your altitude should read 1480 feet. (Within plus or minus) how many feet of 1480 should the captain's altitude readout be to continue the approach? Also, if the altitude readout is false...would you leveloff, continue you to the MAP and then execute the published missed approach? I am assuming the MAP would be the localizer missed approach point.

 

[Mercy98]

In previous lives where the altitude was required to be checked at the FAF when beginning the descent to dh/mda, it was checked in both msl and agl as the final reference for accuracy and the "am I at the correct point and altitude" gut check for making the descent to terra firma. It never happened that it was off by such a wide enough margin (whether it was either mechanically or human induced) that a missed appch was necessary, but if that were the case, then a level-off at the current altitude or higher would make perfect sence...continue to the MAP and then begin the missed. I agree that if on a precision appch then the MAP is at the localizer MAP.

I do remember some limits regarding barometric altimeters of 75', but your company should have its own limits regarding altimeter errors, both individually and cross-checked against the opposite altimeter and the stand-by. It should be listed for both on the ground and in-flight errors.

 

[viper548]

I don't know for sure, but I'd say within 100 ft or so. That will cover you in case of a wrong altimeter setting too. From what I understand false glideslopes found in multiples of the true glideslope, so a typical 3 degree glideslope would have false one and 6 and 9 degrees. If you're on a 6 degree glidepath, you're gonna be really high at the marker

 

[FoxHunter]

Hello,

In our company procedures, the captain is required to check their altitude readout crossing the outer marker when conducting an ILS. This is done to make sure that we do not have a false glideslope readout. However, our airline does not give an error tolerance. Does anyone know if the FARs or AIM give an error tolerance? EX: An ILS plate says when crossing the OM your altitude should read 1480 feet. (Within plus or minus) how many feet of 1480 should the captain's altitude readout be to continue the approach? Also, if the altitude readout is false...would you leveloff, continue you to the MAP and then execute the published missed approach? I am assuming the MAP would be the localizer missed approach point.

What is the temp on the ground? Of it is cold it will read high at the OM, if it is warm it will read low.

 

[radarlove]

This sounds like a flight instructor coffee klatch.

Let's sit around and argue regulations on a saturday night, yeah

 

[Sky Jet]

This is not such a moot point Radar Love, we lost an aircraft at one of my previous carriers that would have been saved if we would have had a similiar procedure at that time.

Jet

 

[Cardinal]

Our operation doesn't specify a tolerance, it's just a sniff check. The thing about these "standard calls" is that they never happen at exactly the same time. Noticing the marker a little late means you'll be lower than the published altitude when somebody gets around to making the call. What are you going to do, go missed because the PNF totally blew the call, despite the fact that you're on glideslope at normal rate of descent?

IMHO a specified tolerance would not be helpful.

 

[Carl_Spackler]

What is the procedure for an ILS that doesn't have an OM? Seems OM's are getting more and more scarce.

 

[Mercy98]

For any ILS, the FAF is the GSIA. That altitude is listed on the plate. The OM was the confirmation of that point, but the OM wasn't always at the exact point of the gsia, so it's really irrelevant for an ils. In days prior to the fms it was mainly used to mark the point for descending when making a LOC appch.

 

[avpro91]

Whether the altitude on your altimeter is within 10 (or whatever) feet of the alt. on the plate is irrelivant for this call. Theres no set value and no FAR that pertains to this. This is just a company procedural call. Not all airlines practice this. This is simply a situational awarness check. It's purpose is to send up a big red flag if what you read on your altimeter is greatly different from that of the published altitude on the plate. If your at the FAF at the altitude on the chart then your doing ok, if not then you better figure out why and fix it quick. If that results in a go-around; so be it.

 

[GuppyWN]

I think there are 10 posters prior to me that need to get laid.

Gup

 

[canyonblue]

I think there are 10 posters prior to me that need to get laid.

Gup

I agree, I'd rather be talking about a "panty check" in BUR. :beer:

 

[Jmoney]

Multiples of 3....

False glideslopes exist at multiples of 3..... So at 6 degrees and again at 9, etc. you can find a false glideslope.....

Obviously, your altitude would have to be way off-100 ft or so won't matter.

-Why does anyone care? If you are 1000 ft off, you have got bigger problems than a false glideslope.

 

[Wing Twist]

Cold temperature altimetry errors are corrected to zero at the altimeter setting source which is the airport elevation. The error applies to altitudes above this reference point and becomes greater as height increases. The correction is important in all phases of an approach, not just the final approach fix.

Refencing TBL 7-2-3 in the AIM shows that at -20C a 200' DH or 500' MDH result is a deviation of 30' and 70' below minima respectively. A small error but a deviation nonetheless. The outer marker/DME fix check altitude is your last chance to detect such errors.

The problem gets more serious further back on the approach. Say for example you are conducting an approach with a number of step downs. Working backwards on the approach with platform altitudes of 1500', 2000', and 4000' the deviations at -20C will be 210', 280' and 570', potentially leaving only 430' of terrain clearance.

Furthermore, VNAV approaches are particularly vulnerable to such errors. Thus the limitations place on them.​

Reference AIM 5-4-22​

(f) Cold Temperature Limitations. A
minimum temperature limitation is published on
procedures which authorize Baro−VNAV operation.
This temperature represents the airport temperature
below which use of the Baro−VNAV is not authorized
to the LNAV/VNAV minimums. An example
limitation will read: Baro−VNAV NA below
−20C(−4F)." This information will be found in the
upper left hand box of the pilot briefing.​

All that being said most operations within the continental US won't be affected by extreme low temparatures. The exception is Alaska. Anchorage and Fairbanks (a common alternate for our operation) suffer from some inhumanely cold temperatures during a better part of the winter season. Altimetry errors can lead to reduced safety margins in operations that are already operating close to or at the limit.

The following is the best explanation I've found regarding ATC assigned altitudes and adjustments. It can be found in Jeppesen - Canada - Air Traffic Control page CA 9-9.

Quote:

1. IFR altitudes may be either accepted or refused.(applies anytime,anywhere). Refusal in this case is based on the pilot's assessment of temperature effect on obstruction clearance.

2. IFR assigned altitudes accepted by a pilot shall not be adjusted to compensate for cold temperatures, ie. if a pilot accepts "maintain 3000" an altitude correction shall not be applied to 3000.

3. Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by pilots.

4. When corrections are applied to a published final approach fix crossing altitude, procedure turn or missed approach altitude, pilots should advise ATC how much of a correction is to be applied.

The page incorporates the table, ATC and Pilot responsibilities and a practical example of using the table