Narrative:

We were trying to beat a line of thunderstorms into pia. Since they were coming from the west and we were approaching from the east; we set up for and were cleared for the ILS 31. Once we were lined up with the runway; we saw that we would not beat the storm and elected to turn off the approach and parallel the line of storms northbound. As my first officer and I were discussing whether to hold and wait; continue several miles northbound and go around to the back side of the storm; or to divert now and wait it out on the ground; approach told us there was a hole he could put us through that would put us on the back side where we could shoot the ILS 13. We elected on this course of action. We were heavily distracted by navigating around and through the thunderstorms. Since we had looked through the notams earlier; we had seen this one:pia 12/021 pia navigation ILS runway 13 gp sfc-1799FT unusable because this is basically the entire second half of the approach; we assumed it meant the glideslope was out of service and instead set up for and briefed the localizer 13. As we were fighting our way through the storms; approach cleared us for the ILS 13. Despite the fact that we were not set up for that approach; we elected to continue on the localizer 13 since we didn't want to get re-vectored through the ts and we could already see the airport environment; nor did we have the time to clarify the NOTAM and clearance with the controller.on the ground; my first officer and I discussed the clearance; and came to the conclusion that we have no idea how we would fly the ILS 13 given that NOTAM. We are not sure if the NOTAM constitutes a change of minimums to the ILS to 1800 feet; or whether we could somehow continue down to the published minimums; since the minimums haven't technically changed. I called our dispatcher and they could not answer that question either. Next; I called pia tower and talked to the supervisor on duty about the NOTAM. He said the glideslope wasn't necessarily 'unusable' below 1800 ft; but that 'something out there' was causing fluctuations below that altitude for some aircraft. He said that they were waiting for flight check to put them on the schedule to get it checked out.I asked him then about what the intent was as far as pilots shooting the approach; whether they wanted us to plan for a normal ILS; an ILS with minimums of 1800 feet; or a localizer approach; and explained that it was confusing whether the NOTAM constituted new minimums or whether it is legal to continue flying the approach down to published ILS minimums since there is no specific mention of them being changed. He said that the intent was to have pilots change the minimums to 1800 feet. He said that in the past several months; the way they have been operating is to advertise the ILS when the ceiling was 2000 feet or higher and visibility >7 nm; and the localizer if they were lower than that. He also mentioned that the approach controller should have cleared us for the localizer approach in our given situation. I expressed how confusing the NOTAM is and asked if they could follow through with either rewriting the NOTAM to be less confusing; issuing an fdc NOTAM to change the minimums; or just to not issue ILS clearances (localizer only) until the situation could be rectified. He promised they would look into it further. I am writing this report to ensure that there is a paper trail associated with this NOTAM; in the hopes that it will get fixed.

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Original NASA ASRS Text

Title: Air carrier Captain reported of a confusing PIA NOTAM advising the Runway 13 ILS glideslope is unusable below 1;800 feet. When asked; ATC noted a glideslope fluctuation as the reason even though the glideslope signal was operable.

Narrative: We were trying to beat a line of thunderstorms into PIA. Since they were coming from the west and we were approaching from the east; we set up for and were cleared for the ILS 31. Once we were lined up with the runway; we saw that we would not beat the storm and elected to turn off the approach and parallel the line of storms northbound. As my FO and I were discussing whether to hold and wait; continue several miles northbound and go around to the back side of the storm; or to divert now and wait it out on the ground; approach told us there was a hole he could put us through that would put us on the back side where we could shoot the ILS 13. We elected on this course of action. We were heavily distracted by navigating around and through the thunderstorms. Since we had looked through the NOTAMs earlier; we had seen this one:PIA 12/021 PIA NAV ILS RWY 13 GP SFC-1799FT UNUSABLE Because this is basically the entire second half of the approach; we assumed it meant the glideslope was out of service and instead set up for and briefed the LOC 13. As we were fighting our way through the storms; approach cleared us for the ILS 13. Despite the fact that we were not set up for that approach; we elected to continue on the LOC 13 since we didn't want to get re-vectored through the TS and we could already see the airport environment; nor did we have the time to clarify the NOTAM and clearance with the controller.On the ground; my FO and I discussed the clearance; and came to the conclusion that we have no idea how we would fly the ILS 13 given that NOTAM. We are not sure if the NOTAM constitutes a change of minimums to the ILS to 1800 feet; or whether we could somehow continue down to the published minimums; since the minimums haven't technically changed. I called our dispatcher and they could not answer that question either. Next; I called PIA tower and talked to the supervisor on duty about the NOTAM. He said the glideslope wasn't necessarily 'unusable' below 1800 ft; but that 'something out there' was causing fluctuations below that altitude for some aircraft. He said that they were waiting for Flight Check to put them on the schedule to get it checked out.I asked him then about what the intent was as far as pilots shooting the approach; whether they wanted us to plan for a normal ILS; an ILS with minimums of 1800 feet; or a LOC approach; and explained that it was confusing whether the NOTAM constituted new minimums or whether it is legal to continue flying the approach down to published ILS minimums since there is no specific mention of them being changed. He said that the intent was to have pilots change the minimums to 1800 feet. He said that in the past several months; the way they have been operating is to advertise the ILS when the ceiling was 2000 feet or higher and visibility >7 nm; and the LOC if they were lower than that. He also mentioned that the approach controller should have cleared us for the LOC approach in our given situation. I expressed how confusing the NOTAM is and asked if they could follow through with either rewriting the NOTAM to be less confusing; issuing an FDC NOTAM to change the minimums; or just to not issue ILS clearances (LOC only) until the situation could be rectified. He promised they would look into it further. I am writing this report to ensure that there is a paper trail associated with this NOTAM; in the hopes that it will get fixed.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.