2
$\begingroup$

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?

$\endgroup$
1
  • $\begingroup$ I assume you meant 3° (the typical G/P angle), not 3%? $\endgroup$ – Bianfable Sep 11 '20 at 10:28
1
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.