AEROSPACE TECHNOLOGIES
Your browser is not supported.
For the best experience, please access this site using the latest version of the following browsers:
By closing this window you acknowledge that your experience on this website may be degraded.
VNAV Vertical Deviation (VDEV) Pointer Jump
VNAV Vertical Deviation (VDEV) Pointer Jump
Information found below is applicable to Epic and NZ FMS platforms. Figures provided, for illustration purposes, are from Gulfstream Symmetry. Aircraft symbology may differ by platform.
____________________________________________________________________________
Honeywell often gets inquiries from customers and operators about Vertical Deviation (VDEV) pointers, and questions about why they are “jumping”. What is happening? It’s not the actual deviation that is changing but rather the scale that is changing. When the aircraft is more than 2 miles from the Final Approach Fix (FAF), the deviation scale is 250 feet per dot and then once at 2 miles from the Final Approach Fix (FAF), it changes to 75 feet per dot. When that happens any needle (or pointer) that isn’t centered at that transition will appear to “jump”.
____________________________________________________________________________
VNAV Vertical Deviation Scale
As shown in the example images below, the Vertical Deviation Scale consists of four dots along with a pointer that indicates the current vertical deviation and the source of the vertical deviation data. The scale and markings are identical regardless of the driving source. The pointer is color coded and has the source identifier contained inside it, for example “P” for the VGP P-pointer. For VNAV-related functions, it can be marked and color coded as follows:
- V for VNAV from an FMS; Figure 1
- P for VGP; Figure 2
Figure 1. Example of VNAV Pointer
Figure 2. Example of VGP Pointer
VNAV Vertical Deviation Pointer Jump
The VNAV Vertical Deviation (VDEV) pointer on the Vertical Deviation Indicator (VDI) is designed to move smoothly without a jump under normal conditions. The pointer movement is rate-limited to 50 ft/sec. For example, if during descent you change the barometric setting from STD (standard) to the local altimeter setting at the transition level, the pointer will move smoothly.
However, there are two situations where the pointer can jump – both are by design!
Situation #1 can occur as a result of modifying the flight plan during a descent, which causes the lateral path and the descent path to be reconstructed. In some systems, FMS will also reconstruct the path without any pilot actions if the aircraft is flying significantly slower or faster than planned. In this situation, the pointer can jump.
Situation #2 is when the VDI scale switches from 250 ft to 75 ft per dot (500 ft/150 ft, full scale) at 2NM from the Final Approach Fix (FAF) and is usually the pointer “jump” which may cause confusion or question among pilots and will be the focus of further discussion here.
As illustrated in Figure 3 below, this usually happens while following a descent path which is shallower than the VGP glidepath. When the aircraft is within five (5) NM of the FAF or the FAF becomes the active waypoint, the VGP pointer appears at the top of the VDI scale and begins to slowly move down. If the VDI scale change occurs prior to the VGP pointer being centered, the VGP pointer will appear to “jump”.
Figure 3. VDEV Pointer “Jump” During Scale Change
(for illustration purposes)
Erik Tobler is a Honeywell Flight Technical Services Technical Pilot. He can be reached via email at FTS@Honeywell.com.
