What is an obstacle identification surface?
Why would a pilot intentionally penetrate it during an ILS approach?
Obstacle Identification Surfaces apply to departures and the portion of an non-precision IAP beyond the Visual Descent Point. If you are in the clouds, you should never penetrate the OIS on departure.
On arrival, the VDP assures obstacle clearance with a normal approach to landing. ILS only approaches do not have a VDP depicted so there is no OIS to penetrate. (The glide slope takes you right to the touchdown zone.) ILS/LOC (and Localizer only) approaches do have a VDP for the non-precision version of the approach. You could penetrate the OIC on a localizer approach and if that happens, visibility minimums are increased as explained in the last paragraph from the AIM below.
AC 150-5300-13-c12 Airport Design has a good visualization of the departure OIC. (As an aside, it has lots of charts and graphs showing how departures and approaches are designed.)
AIM 5−2−8. Instrument Departure Procedures (DP) − Obstacle Departure Procedures (ODP) and Standard Instrument Departures (SID)
…if an obstacle penetrates what is called the 40:1 obstacle identification surface, then the procedure designer chooses whether to:
Establish a steeper than normal climb gradient; or
Establish a steeper than normal climb gradient with an alternative that increases takeoff minima to allow the pilot to visually remain clear of the obstacle( s); or
Design and publish a specific departure route; or
A combination or all of the above.
The 40:1 obstacle identification surface (OIS) begins at the departure end of runway (DER) and slopes upward at 152 FPNM until reaching the minimum IFR altitude or entering the en route structure. This assessment area is limited to 25 NM from the airport in nonmountainous areas and 46 NM in designated mountainous areas. Beyond this distance, the pilot is responsible for obstacle clearance if not operating on a published route, if below (having not reached) the MEA or MOCA of a published route, or an ATC assigned altitude. See FIG 5−2−1. (Ref 14 CFR 91.177 for further information on en route altitudes.)
5−4−5. Instrument Approach Procedure (IAP) Charts
h. The Visual Descent Point (VDP), identified by the symbol (V), is a defined point on the final approach course of a nonprecision straight−in approach procedure from which a stabilized visual descent from the MDA to the runway touchdown point may be commenced. The pilot should not descend below the MDA prior to reaching the VDP.
i. A visual segment obstruction evaluation is accomplished during procedure design on all IAPs. Obstacles (both lighted and unlighted) are allowed to penetrate the visual segment obstacle identification surfaces. Identified obstacle penetrations may cause restrictions to instrument approach operations which may include an increased approach visibility requirement, not publishing a VDP, and/or prohibiting night instrument operations to the runway. There is no implicit obstacle protection from the MDA/DA to the touchdown point. Accordingly, it is the responsibility of the pilot to visually acquire and avoid obstacles below the MDA/DA during transition to landing.
Edit: This AIM is consistent with what the Instrument Procedures Handbook says about non-precision approaches but not consistent with what the handbook says about the potential for obstacles on ILS approaches.
...if an ILS is configured with approach lights or a nonprecision approach is configured with either MALSR, SSALR,or ALSF-1 lighting configurations and the procedure has a published visibility of 3/4 SM or greater, a penetration of the final approach OIS may exist. Also, pilots will be unable to determine whether there are penetrations of the final approach OIS if a nonprecision procedure does not have approach lights, or is configured with ODALS,MALS, or SSALS/SALS lighting since the minimum published visibility will be 3/4SM or greater.
As a rule of thumb, approaches with published visibilities of 3/4SM or more should be regarded as having final approach OIS penetrations and care must be taken to avoid any obstacles in the visual segment. Approaches with published visibilities of 1/2 SM or less are free of OIS penetrations...