AIM

4/3/14

1−1−35

Navigation Aids

vertical off flag appears, the pilot may elect to use the
LNAV minima if the rules under which the flight is
operating allow changing the type of approach being flown
after commencing the procedure. If the lateral integrity
limit is exceeded on an LP approach, a missed approach
will be necessary since there is no way to reset the lateral
alarm limit while the approach is active.

3.

Another additional feature of WAAS receiv-

ers is the ability to exclude a bad GPS signal and

continue operating normally. This is normally

accomplished by the WAAS correction information.

Outside WAAS coverage or when WAAS is not

available, it is accomplished through a receiver

algorithm called FDE. In most cases this operation

will be invisible to the pilot since the receiver will

continue to operate with other available satellites

after excluding the “bad” signal. This capability

increases the reliability of navigation.

4.

Both lateral and vertical scaling for the

LNAV/VNAV and LPV approach procedures are

different than the linear scaling of basic GPS. When

the complete published procedure is flown, +/−1 NM

linear scaling is provided until two (2) NM prior to the

FAF, where the sensitivity increases to be similar to

the angular scaling of an ILS. There are two differ-

ences in the WAAS scaling and ILS: 1) on long final

approach segments, the initial scaling will be

+/−0.3 NM to achieve equivalent performance to

GPS (and better than ILS, which is less sensitive far

from the runway); 2) close to the runway threshold,

the scaling changes to linear instead of continuing to

become more sensitive. The width of the final

approach course is tailored so that the total width is

usually 700 feet at the runway threshold. Since the

origin point of the lateral splay for the angular portion

of the final is not fixed due to antenna placement like

localizer, the splay angle can remain fixed, making a

consistent width of final for aircraft being vectored

onto the final approach course on different length

runways. When the complete published procedure is

not flown, and instead the aircraft needs to capture the

extended final approach course similar to ILS, the

vector to final (VTF) mode is used. Under VTF the

scaling is linear at +/−1 NM until the point where the

ILS angular splay reaches a width of +/−1 NM

regardless of the distance from the FAWP.

5.

The WAAS scaling is also different than GPS

TSO−C129 in the initial portion of the missed

approach. Two differences occur here. First, the

scaling abruptly changes from the approach scaling to

the missed approach scaling, at approximately the

departure end of the runway or when the pilot

requests missed approach guidance rather than

ramping as GPS does. Second, when the first leg of

the missed approach is a Track to Fix (TF) leg aligned

within 3 degrees of the inbound course, the receiver

will change to 0.3 NM linear sensitivity until the turn

initiation point for the first waypoint in the missed

approach procedure, at which time it will abruptly

change to terminal (+/−1 NM) sensitivity. This allows

the elimination of close in obstacles in the early part

of the missed approach that may cause the DA to be

raised.

6.

A new method has been added for selecting

the final approach segment of an instrument

approach. Along with the current method used by

most receivers using menus where the pilot selects the

airport, the runway, the specific approach procedure

and finally the IAF, there is also a channel number

selection method. The pilot enters a unique 5−digit

number provided on the approach chart, and the

receiver recalls the matching final approach segment

from the aircraft database. A list of information

including the available IAFs is displayed and the pilot

selects the appropriate IAF. The pilot should confirm

that the correct final approach segment was loaded by

cross checking the Approach ID, which is also

provided on the approach chart.

7.

The Along−Track Distance (ATD) during the

final approach segment of an LNAV procedure (with

a minimum descent altitude) will be to the MAWP. On

LNAV/VNAV and LPV approaches to a decision

altitude, there is no missed approach waypoint so the

along−track distance is displayed to a point normally

located at the runway threshold. In most cases the

MAWP for the LNAV approach is located on the

runway threshold at the centerline, so these distances

will be the same. This distance will always vary

slightly from any ILS DME that may be present, since

the ILS DME is located further down the runway.

Initiation of the missed approach on the LNAV/

VNAV and LPV approaches is still based on reaching

the decision altitude without any of the items listed in

14 CFR Section 91.175 being visible, and must not be

delayed until the ATD reaches zero. The WAAS

receiver, unlike a GPS receiver, will automatically

sequence past the MAWP if the missed approach

procedure has been designed for RNAV. The pilot