B737 Undifferentiated or Other Model
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6.0
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Procedure; Human Factors; Chart Or Publication; Airspace Structure
I cleared Aircraft X for the ILS approach with 'maintain four thousand till intercepting the glideslope; cleared ILS ... approach'. Aircraft Y was going to the parallel runway at 3;000 feet. Aircraft X joined the final and immediately descended out of 4;000 feet. Final monitor corrected them and climbed them back up to 4;000 feet until the glideslope descended out of 4;000 feet again. Aircraft X descended out of 4000 feet next to Aircraft Y before Aircraft Y was established. We have requested 4;000 foot intersection points on the finals for a long time and this is the exact reason why. Even though the pilot knew exactly what I meant; he still descended to 3;000 feet even though the glideslope was way above their altitude. Technically; 'maintain an altitude until intercepting the glideslope' isn't approved phraseology; so having a crossing fix at 4;000 feet on every final would be a very nice option.
I was Pilot Flying with autopilot on. On base leg vectors to ILS at 4;000 ft approach control cleared us to turn to heading 190 and maintain 4;000 ft until glide slope intercept; cleared for the approach. I heard maintain 4;000 feet until established; cleared for the approach. After established on localizer I reset the altitude and started down to 3;000 foot  crossing altitude. The captain was talking on the radio and by the time he realized I was descending; we were already at 3;500 feet. ATC queried us and we climbed back to 4;000 feet. I'm not aware of any traffic conflicts. The controller gave a non standard clearance and I heard the standard clearance. Pay closer attention to actual clearance not what I expected to hear. ATC could use standard clearance or make a greater emphasis when it's not standard.
An air carrier crew and ATC controller reported that a descent was begun following localizer capture rather than after intercepting the glideslope as assigned.