Enroute controller described a loss of separation event when traffic was issued holding instructions...

Narrative:

Aircraft Y was observed below 3;000 ft on ILS approach into the airport. Aircraft X was also inbound to the airport and advised he was #2 behind aircraft already on approach and to expect approach clearance upon receipt of the first aircraft cancelling IFR. I issued a descent to 4;000 ft and holding instructions to aircraft X at the intersection to hold southwest on the localizer course. Aircraft X advised 'minimum fuel.' he then advised that the aircraft ahead of him was on the ground; trying to cancel on the unicom frequency. I asked aircraft X to tell aircraft Y to come to my frequency and cancel and aircraft X said; 'I'm too busy.' that's when I noticed he was descending through 3;400 ft and had not entered holding. Aircraft Y cancelled IFR on my frequency and I then tried to clear aircraft X for approach; but he did not respond and was level at 3;000 ft flying towards the airport. He then requested vectors to the localizer and started climbing on his own back to 4;000 ft. I issued vectors and he joined the localizer; flew the approach and landed.

Original NASA ASRS Text

Title: Enroute Controller described a loss of separation event when traffic was issued holding instructions awaiting a 'landing assured report'; experienced a fuel issue; and descended below assigned altitude.

Narrative: Aircraft Y was observed below 3;000 FT on ILS approach into the airport. Aircraft X was also inbound to the airport and advised he was #2 behind aircraft already on approach and to expect approach clearance upon receipt of the first aircraft cancelling IFR. I issued a descent to 4;000 FT and holding instructions to Aircraft X at the intersection to hold southwest on the localizer course. Aircraft X advised 'minimum fuel.' He then advised that the aircraft ahead of him was on the ground; trying to cancel on the UNICOM frequency. I asked Aircraft X to tell Aircraft Y to come to my frequency and cancel and Aircraft X said; 'I'm too busy.' That's when I noticed he was descending through 3;400 FT and had not entered holding. Aircraft Y cancelled IFR on my frequency and I then tried to clear Aircraft X for approach; but he did not respond and was level at 3;000 FT flying towards the airport. He then requested vectors to the localizer and started climbing on his own back to 4;000 FT. I issued vectors and he joined the localizer; flew the approach and landed.

Data retrieved from NASA's ASRS site as of July 2013 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	993145
Time
Date	201202
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	Small Transport Low Wing 2 Turbojet Eng
Flight Phase	Cruise
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Transport
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Enroute controller described a loss of separation event when traffic was issued holding instructions awaiting a 'landing assured report'; experienced a fuel issue; and descended below assigned altitude.