Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Confusion; Training / Qualification; Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Human Factors; Procedure
We were given vectors off the STAR arrival into LGA on a 30 degree intercept heading to the Runway 04 localizer course about 15 miles out. We were cleared to intercept the localizer while on a descent to; I think; 4;000 MSL. Once on the localizer we were cleared for the ILS 04 approach; while still in the descent to 4;000.  The First Officer selected approach/land as we were slightly above the glideslope--which we soon captured. We were given another frequency change and the new controller instructed us to cross GRENE at 2700 feet; cleared for the Runway 04 ILS approach. I attempted to put 2;700 into the MCDU for GRENE twice; but the MCDU stated 'NOT ALLOWED.' Since we were already established on the glideslope I was confused why the Controller wanted us to dive below the glideslope to 2;700 feet. The First Officer stated; 'We will meet the restriction at GRENE by staying on the glideslope.' The approach plate depiction wasn't clear if GRENE at 2;700 feet was along the glideslope (we were about 3-4 miles prior to GRENE at this time). We were then instructed to contact Tower. I was distracted with the frequency change but then realized we would not meet the 2;700 foot GRENE restriction; while on the glideslope. (GRENE is way below the extended glideslope.) We immediately configured to add drag and descended below the glide slope to attempt to meet the restriction. I think we missed the restriction by about 700 feet. Remainder of the flight was uneventful. We were cleared for the approach once; and were established on LOC/GS. Then; cleared again for the approach to go below GS to meet GRENE at 2;700. The two clearances can be confusing and a set up for an accident. Although pilots are trained to be flexible; the added requirement to cross GRENE at an altitude below the glideslope you are already established on raises red flags. It would have been better if the Controller stated; 'Previous approach clearance cancelled; cross GRENE at 2;700; cleared for the ILS 04 approach.' We received two clearances for the ILS approach. The first clearance should have been cancelled and the new clearance and altitude restriction given. I think this would be the better way.
Confused by ATC's initial approach clearance to the ILS 4R at LGA; the an air carrier flight crew struggled with the Tower's subsequent clearance to cross GRENE at 2700 (as published); cleared for the ILS to Runway 4R.