Is there a real difference or benefit in using Integrated Approach Navigation (IAN) instead of LNAV/VNAV for a non-precision approach (NPA)?
Both give you lateral and vertical guidance during an NPA. Is IAN more precise?
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A Non Precision Approach (NPA) flown with Integrated Approach Navigation (IAN) is not more precise than the same approach flown with LNAV/VNAV. The advantage of IAN is that the instrumentation and procedures are very similar to ILS (Cat I), which is the most common approach type flown for most airline pilots. Having a common set of indications on the PFD and procedures to follow improves flight safety.
The Boeing 737 Flight Crew Training Manual (FCTM) says:
IAN approach techniques are similar to ILS approach techniques [...]
All IAN approaches provide the functions, indications and alerting features similar to an ILS approach. Although non-ILS approaches using LNAV and VNAV can still be performed, IAN is normally used in place of LNAV and VNAV because of improved approach displays, alerts and standardized procedures.
(Boeing 737 NG FCTM 5.60 - Approach and Missed Approach, emphasis mine)
The article about IAN on flaps2approach goes into a bit more detail:
Advantages of Using IAN
There are multiple advantages to an airline using IAN, the foremost being flight safety. Unstable approaches contribute to many aircraft accidents, and flight crews strive to always establish a stabilised approach profile for all instrument and visual approaches.
The Global Position System has enabled stabilized approaches at many airports and advanced features such as IAN take advantage of this technology to provide consistent, intuitive displays that support stabilized approaches.
18 Approaches Types to 1
Through the use of IAN, the number of approach types has been reduced from 18 to 1, greatly simplifying the approach procedure and minimizing the amount of time an airline needs to train pilots in numerous approach types. Time is money and utilizing advanced technology such as IAN can increase airline productivity.
Additional Data - Increased Awareness
The distance to runway threshold, approach guidance information, and vertical and lateral deviation markers are displayed when IAN is in range of a designated runway. Whether IAN is used or not, this information provides additional guidance when executing an approach.
For example, when executing a VOR approach, this information has been displayed on the Navigation Display (ND) as the distance to the actual NAVAID (VOR) - which may or may not be aligned with the threshold of the runway. IAN will by default, display the lateral and vertical deviation, and distance to the runway threshold, allowing for greater precision during a non-automated approach.
These are but a few of the advantages to using the Integrated Approach Navigation system.
(flaps2approach.com - Integrated Approach Navigation (IAN) - Review and Procedures)