For an IAN Approach to be recognized and work in a Boeing 737, does the Approach G/P need to be exactly 3° or can it vary?
The G/P can deviate from the typical 3 degrees, but not much.
For any non-precision approach (NPA) flown with VNAV or Integrated Approach Navigation (IAN), the navigation database needs to contain a stored glide path (GP) angle or a suitable waypoint (runway or MAP):
VNAV should be used only for approaches that have one of the following features:
- a published GP angle on the LEGS page for the final approach segment
- an RWxx waypoint coincident with the approach end of the runway
- a missed approach waypoint before the approach end of the runway, (e.g., MXxx).
The FMC obtains the GP angle displayed on the LEGS page from the navigation database. [...] A GP angle is coded in the navigation database for nearly all straight-in approach procedures.
This GP angle is normally defined by the regulatory authority responsible for the approach procedure and provides a continuous descent at a constant flight path angle for a final approach path that complies with minimum altitudes at intermediate step-down fixes. The typical GP angle is approximately 3.00°, but can vary from 2.75° to 3.77°.
(Boeing 737 NG FCTM 5.45 - Approach and Missed Approach, emphasis mine)
As you can see, the GP angle can vary between 2.75 and 3.77 degrees. For IAN, this G/P information is passed on to the AFDS by the FMC. The list of IAN restrictions does not limit the GP angle:
IAN Requirements and Restrictions
- airplanes must be equipped with FMC U10.5 or later and IAN FMA displays
- dual or single engine approaches are authorized
- waypoints in the navigation database from the FAF onward may not be modified
- raw data monitoring is required during localizer based approaches. During FMC based non-ILS approaches, raw data monitoring is recommended when available in accordance with the techniques described in the Non-ILS approach section in this chapter
- QFE operation is not authorized
- cold temperature altitude corrections are not permitted
- RNP appropriate for the approach must be used
- the autopilot is required until suitable visual reference is established when performing an approach requiring an RNP of 0.15 or lower.
(Boeing 737 NG FCTM 5.60 - Approach and Missed Approach)
The FCTM does however says the following about when an approach can be flown with IAN:
For FMC based approaches, a proper series of legs/waypoints describing the approach route including an appropriate vertical path or glide path (GP) angle must appear on the LEGS page. A GP angle displayed on the LEGS page means the vertical path complies with final approach step-down altitudes (minimum altitude constraints). A glide path angle suitable for an IAN approach is one that approximates 3° and crosses the runway threshold at approximately 50 feet.
(Boeing 737 NG FCTM 5.61 - Approach and Missed Approach, emphasis mine)
It does not say if the full range from 2.75 to 3.77 degrees is considered approximately 3.00°, but it certainly does not have to be exactly 3.