P80 controller chronicles readback hearback error that caused aircraft to descend below MVA.

Attributes
ACN	642024
Time
Date	200412
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : hio.airport
State Reference	OR
Altitude	msl single value : 2800
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : p80.tracon
Operator	general aviation : corporate
Make Model Name	Learjet 31
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller radar : 21 controller time certified in position1 : 12
ASRS Report	642024
Person 2
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Events
Anomaly	non adherence : required legal separation non adherence : published procedure non adherence : clearance non adherence other other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	faa : investigated
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I vectored a lear 31 to the final approach course for the ILS runway 12 at hio. The aircraft was at 5000 ft MSL. When I issued the clearance to 'descend and maintain three thousand five hundred.' the pilot readback 'down to twenty-five.' it was a very fast readback and I missed the 'twenty' portion of the readback. The MVA in the area is 3500 ft. I noticed the aircraft going through 3300 ft and at 3200 ft, I again issued the clearance 'maintain three thousand five hundred.' the pilot responded 'roger, down to twenty-five.' again, very fast, but this time I caught the readback. I again issued 'climb and maintain three thousand five hundred.' the pilot readback 'up to thirty-five now, but I read back twenty-five.' the act descended as low as 2800 ft before climbing back up. The pilot's insistence on short, group form readbacks, ie, 'down to twenty-five, up to thirty-five,' was a major contributor to this error. I doubt I would have missed a proper readback of 'two thousand five hundred,' or 'three thousand five hundred.'

Original NASA ASRS Text

Title: P80 CTLR CHRONICLES READBACK HEARBACK ERROR THAT CAUSED ACFT TO DSND BELOW MVA.

Narrative: I VECTORED A LEAR 31 TO THE FINAL APCH COURSE FOR THE ILS RWY 12 AT HIO. THE ACFT WAS AT 5000 FT MSL. WHEN I ISSUED THE CLRNC TO 'DSND AND MAINTAIN THREE THOUSAND FIVE HUNDRED.' THE PLT READBACK 'DOWN TO TWENTY-FIVE.' IT WAS A VERY FAST READBACK AND I MISSED THE 'TWENTY' PORTION OF THE READBACK. THE MVA IN THE AREA IS 3500 FT. I NOTICED THE ACFT GOING THROUGH 3300 FT AND AT 3200 FT, I AGAIN ISSUED THE CLRNC 'MAINTAIN THREE THOUSAND FIVE HUNDRED.' THE PLT RESPONDED 'ROGER, DOWN TO TWENTY-FIVE.' AGAIN, VERY FAST, BUT THIS TIME I CAUGHT THE READBACK. I AGAIN ISSUED 'CLB AND MAINTAIN THREE THOUSAND FIVE HUNDRED.' THE PLT READBACK 'UP TO THIRTY-FIVE NOW, BUT I READ BACK TWENTY-FIVE.' THE ACT DSNDED AS LOW AS 2800 FT BEFORE CLBING BACK UP. THE PLT'S INSISTENCE ON SHORT, GROUP FORM READBACKS, IE, 'DOWN TO TWENTY-FIVE, UP TO THIRTY-FIVE,' WAS A MAJOR CONTRIBUTOR TO THIS ERROR. I DOUBT I WOULD HAVE MISSED A PROPER READBACK OF 'TWO THOUSAND FIVE HUNDRED,' OR 'THREE THOUSAND FIVE HUNDRED.'

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.