Date: 201702
Local time of day: 0001-0600
Location: TWF.Tower, ID
Flight conditions: Marginal
Aircraft: Regional Jet 200 ER/LR (CRJ200)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class D TWF
ATC / advisory: Tower TWF
Reporter organization: Government
Reporter function: Ground; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Second reporter function: Approach
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Contributing factors: Airspace Structure; Human Factors; Manuals; Procedure
Primary problem: Airspace Structure

Narrative: While I was working local control/ground control combined; I overheard the non-radar approach controller approve a request from the ZLC sector 31 controller to have Aircraft X descending to 9;000 feet direct GABBY from the southeast for an ILS approach. The IFR flight progress strip clearly showed the projected inbound radial of Aircraft X to be in a 9500 foot Minimum IFR Altitude (MIA).When the Aircraft X came over to approach's frequency; the approach controller issued a clearance to Aircraft X to 'cross GABBY at or above 9'000 feet cleared for ILS approach.' the weather was such that there were low ceilings to the southeast and it was obvious the aircraft was did not have visual contact with the ground. There is a misnomer in this facility that 'if it works for the center then it must be ok for us' or; 'if the center makes a particular request (usually altitude related) then it must be ok since the center asked for it.' I've had this same situation happen before this controller and other controllers and I advised them to speak to competent authority to get the authorizing IFR aircraft below the MIA issue resolved. Their response is 'I talked to other controllers or see other controllers do the same thing.'Our letter of agreement with ZLC clearly shows what altitudes aircraft will be assigned over coordination fixes. The one referenced above says aircraft will be descending to 11;000 feet or instructed to cross GABBY at 11;000 feet. This should be adhered to as it causes confusion when a lower altitude is requested that is below our MIA'S; but the center says that that altitude request meets their MIA'S. Have the appropriate center officials compare their MIA charts with TWF'S MIA charts to identify/correct discrepancies.


Second narrative: During this event; I was working as approach. ZLC called and coordinated Aircraft X direct GABBY down to 9000 feet for the ILS Runway 26 approach. Our Letter of Agreement with ZLC states that GABBY should be coordinated at 11;000 feet. Our MIA in that area is 9500 feet. I asked if 9000 feet was good and they said yes; that works for them. I cleared Aircraft X to cross GABBY at or above 9000 feet on the ILS; and they were at or above 9000 feet until the transfer of control point. I was then told an hour and a half later by a controller that we cannot use that altitude even if ZLC says it works for them. I later called the Center Area and talked to the controller that coordinated 9000 feet. He said that 9000 feet works for their Minimum Descent Altitude.I would share what information each facility has and what their minimums may be for certain airspace; or airways; so that we can make sure that they are not descending below prescribed altitudes.


Synopsis: TWF Approach controllers approved an aircraft entering their airspace at an altitude below their Minimum Vectoring Altitude.