Confused by ATC's initial approach clearance to the ILS 4R at lga; the an air carrier flight crew st...

Narrative:

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 localizer/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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

Attributes
ACN	1169068
Time
Date	201404
Local Time Of Day	0601-1200
Place
Locale Reference	N90.TRACON
State Reference	NY
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Undershoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

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.