Narrative:

We were being vectored from the north for an ILS 28R at pit. While descending to 5000', we were cleared 'turn right to 250 degree, cleared ILS 28R, maintain 5000' until established.' I assumed 'until established' meant that we could descend to the final G/south intercept altitude after established on the localizer (if not, the phrase 'maintain 5000 until established' is redundant because the ILS procedure is to maintain 5000 until kerrs. After leveling at 4000' and passing kerrs (IAF), the controller questioned our descent below 5000' prior to kerrs. A contributing factor was the note (1) on the approach plate (21-5), which although applicable after passing kerrs, is printed outside kerrs under the holding pattern. This note allows descent to 3000' if authority/authorized by ATC. Another distraction was a request from the controller to increase airspeed to 210 KTS until the marker, which we accomplished by increasing our rate of descent to 4000'. After landing I called approach control on the phone as requested. We discussed the clearance terminology and the other contributing factors. The controller stated that the terminology he used is to prevent aircraft which intercept inside kerrs from descending below 5000' until established on the localizer so there won't be a conflict with runway 28L traffic if either aircraft overshoot. However, we intercepted well outside kerrs and did not overshoot. Our clearance should more appropriately have been 'turn right to 250 degree, cleared ILS 28R,' which would have eliminated the insinuation that we could descend below 5000' after localizer intercept. The controller then acknowledged that this is a recognized area of confusion and that the error occurs frequently. He also stated that no conflict with other traffic occurred during our incident. Supplemental information from acn 170239. When the captain was descending I thought he was following the G/south down, so I didn't verify my DME with altitude after getting the clearance for the approach. The approach controller had also said that there was no conflict with any other traffic. Also after receiving the approach clearance I reset the altitude alert to an erroneous altitude. Setting this was to prevent a distracting altitude warn during the approach. The latter along with the above mentioned reasons caused the confusion and resulted in the deviation. Supplemental information from acn 170241. As I looked at the flight instruments I saw we were leveling at 4000' MSL, 2 dots below G/south. I also noted that the altitude alerter had been spun up and read 13800'. I had an uneasy feeling at that point, with the aircraft 2 dots low and altitude alerter out of the picture. I believe that the altitude alert was prematurely removed from the picture while I was involved in turning on engine and wing anti-ice and setting thrust. In my opinion, the altitude alert should have been left reading 5000' until we intercepted the G/south. The captain admits to misinterpreting our clearance as clearance to descend to 4000' after intercepting the localizer. I felt uneasy, but I was not assertive enough at that moment and I should have said something.

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Original NASA ASRS Text

Title: ACR LGT FLC DESCENDS BELOW CLRNC ALT ON APCH AT PIT.

Narrative: WE WERE BEING VECTORED FROM THE N FOR AN ILS 28R AT PIT. WHILE DSNDING TO 5000', WE WERE CLRED 'TURN R TO 250 DEG, CLRED ILS 28R, MAINTAIN 5000' UNTIL ESTABLISHED.' I ASSUMED 'UNTIL ESTABLISHED' MEANT THAT WE COULD DSND TO THE FINAL G/S INTERCEPT ALT AFTER ESTABLISHED ON THE LOC (IF NOT, THE PHRASE 'MAINTAIN 5000 UNTIL ESTABLISHED' IS REDUNDANT BECAUSE THE ILS PROC IS TO MAINTAIN 5000 UNTIL KERRS. AFTER LEVELING AT 4000' AND PASSING KERRS (IAF), THE CTLR QUESTIONED OUR DSNT BELOW 5000' PRIOR TO KERRS. A CONTRIBUTING FACTOR WAS THE NOTE (1) ON THE APCH PLATE (21-5), WHICH ALTHOUGH APPLICABLE AFTER PASSING KERRS, IS PRINTED OUTSIDE KERRS UNDER THE HOLDING PATTERN. THIS NOTE ALLOWS DSNT TO 3000' IF AUTH BY ATC. ANOTHER DISTR WAS A REQUEST FROM THE CTLR TO INCREASE AIRSPD TO 210 KTS UNTIL THE MARKER, WHICH WE ACCOMPLISHED BY INCREASING OUR RATE OF DSNT TO 4000'. AFTER LNDG I CALLED APCH CTL ON THE PHONE AS REQUESTED. WE DISCUSSED THE CLRNC TERMINOLOGY AND THE OTHER CONTRIBUTING FACTORS. THE CTLR STATED THAT THE TERMINOLOGY HE USED IS TO PREVENT ACFT WHICH INTERCEPT INSIDE KERRS FROM DSNDING BELOW 5000' UNTIL ESTABLISHED ON THE LOC SO THERE WON'T BE A CONFLICT WITH RWY 28L TFC IF EITHER ACFT OVERSHOOT. HOWEVER, WE INTERCEPTED WELL OUTSIDE KERRS AND DID NOT OVERSHOOT. OUR CLRNC SHOULD MORE APPROPRIATELY HAVE BEEN 'TURN R TO 250 DEG, CLRED ILS 28R,' WHICH WOULD HAVE ELIMINATED THE INSINUATION THAT WE COULD DSND BELOW 5000' AFTER LOC INTERCEPT. THE CTLR THEN ACKNOWLEDGED THAT THIS IS A RECOGNIZED AREA OF CONFUSION AND THAT THE ERROR OCCURS FREQUENTLY. HE ALSO STATED THAT NO CONFLICT WITH OTHER TFC OCCURRED DURING OUR INCIDENT. SUPPLEMENTAL INFO FROM ACN 170239. WHEN THE CAPT WAS DSNDING I THOUGHT HE WAS FOLLOWING THE G/S DOWN, SO I DIDN'T VERIFY MY DME WITH ALT AFTER GETTING THE CLRNC FOR THE APCH. THE APCH CTLR HAD ALSO SAID THAT THERE WAS NO CONFLICT WITH ANY OTHER TFC. ALSO AFTER RECEIVING THE APCH CLRNC I RESET THE ALT ALERT TO AN ERRONEOUS ALT. SETTING THIS WAS TO PREVENT A DISTRACTING ALT WARN DURING THE APCH. THE LATTER ALONG WITH THE ABOVE MENTIONED REASONS CAUSED THE CONFUSION AND RESULTED IN THE DEV. SUPPLEMENTAL INFO FROM ACN 170241. AS I LOOKED AT THE FLT INSTRUMENTS I SAW WE WERE LEVELING AT 4000' MSL, 2 DOTS BELOW G/S. I ALSO NOTED THAT THE ALT ALERTER HAD BEEN SPUN UP AND READ 13800'. I HAD AN UNEASY FEELING AT THAT POINT, WITH THE ACFT 2 DOTS LOW AND ALT ALERTER OUT OF THE PICTURE. I BELIEVE THAT THE ALT ALERT WAS PREMATURELY REMOVED FROM THE PICTURE WHILE I WAS INVOLVED IN TURNING ON ENG AND WING ANTI-ICE AND SETTING THRUST. IN MY OPINION, THE ALT ALERT SHOULD HAVE BEEN LEFT READING 5000' UNTIL WE INTERCEPTED THE G/S. THE CAPT ADMITS TO MISINTERPRETING OUR CLRNC AS CLRNC TO DSND TO 4000' AFTER INTERCEPTING THE LOC. I FELT UNEASY, BUT I WAS NOT ASSERTIVE ENOUGH AT THAT MOMENT AND I SHOULD HAVE SAID SOMETHING.

Data retrieved from NASA's ASRS site as of July 2007 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.