April 8, 2011
Operators with aircraft equipped with an FAA Approved Enhanced Flight Visual System (EFVS) must make it known to their pilots it is imperative to understand how to use the system safely. Two key elements of that understanding are:
- When can a pilot use EFVS to descend below Minimum Descent Altitude (MDA) or Decision Height (DH), and
- When the pilot decides to do so, what is the relationship between the final approach segment vertical descent angle (VDA) and any installed Visual Glide Slope Indicator (VGSI)
To descend below DA or MDA on an approach to a runway with straight in minima using an approved EFVS, the enhanced flight visibility observed by the EFVS cannot be less than the visibility prescribed in the Instrument Approach Procedure (IAP) being flown, and the visual references required by the regulations (91.175) must be distinctly visible and identifiable to the pilot using the EFVS. This permits a pilot to use an EFVS to do the following when the runway environment may not be visible to a pilot relying on natural vision alone:
- Determine that the enhanced flight visibility is not less than the visibility prescribed by the IAP being used,
- Identify the required visual references,
- Verify proper runway alignment, and
- Descend from DA or MDA down to 100 feet above the Touch Down Zone Elevation (TDZE) of the runway of intended landing.
To descend below 100 feet above the TDZE of the runway of intended landing, the pilot must be able to see the visual references required by §91.175(l)(4) with his or her natural vision.
The next issue is how the pilot be can be assured they will not hit something that is not visible using EFVS when below MDA or DH. Published approaches with vertical guidance (e.g., instrument landing systems (ILSs) or microwave landing systems (MLSs)) or computed vertical paths (e.g., LNAV/VNAV, LPV, etc.) provide the highest level of safety for both obstacle clearance and controlled rate of descent. If vertical guidance is not provided by the aircraft’s navigation systems, the use of the Heads Up display’s (HUD’s) Flight Path Vector (FPV) and Flight Path Angle (FPA) reference cue set to the published vertical descent angle on the instrument procedure, VGSI angle, or where no angle is provided, an angle determined by the pilot to safely clear all obstacles, is recommended.
The presence of a vertical descent angle (VDA) does not guarantee obstacle protection in the visual segment and does not change any of the requirements for flying a non-precision approach. The published VDA is for information only, and is strictly advisory in nature. There is no implicit additional obstacle protection below the MDA. Approaches without visual descent points (VDPs) have not been assessed for terrain or obstacle clearance below the MDA, and may not provide a clear vertical path to the runway at the normally expected descent angle. Therefore, pilots must be especially vigilant when descending below the MDA at locations without VDPs. This does not necessarily prevent flying the normal angle; it only means that obstacle clearance in the visual segment could be less, and greater care should be exercised in looking for and avoiding obstacles in the visual segment.
VDPs provide pilots with a reference for the optimal location to begin descent from the MDA, based on the designed VDA for the approach procedure, assuming the required visual references can be seen. Additional protection for the visual segment below the MDA is provided if a VDP is published and descent below the MDA is started at or after the VDP.
Protection is also provided if a VGSI is installed and the aircraft remains within the lateral limits and vertical guidance provided by the VGSI from the MDA. In some cases, the VDA and VGSI angles are not coincident. When this is the case, a chart note will indicate that they are not coincident. Use of VGSI systems can aid the pilot in determining if the aircraft is in a position to make a descent from the MDA. However, when the visibility is close to minimums, the VGSI may not be visible at the start descent point for a normal glidepath, due to its location down the runway. In addition, as of this posting EFVSs are not capable of displaying color; therefore, glidepath information from the VGSI may not be usable when using EFVS alone. The use of the HUD’s FPV and FPA reference cue set to the VGSI angle will help overcome this shortcoming. In the case when the VDA and VGSI angles are not coincident, it is important for the pilot to know and fly the VGSI angle. The VDA and VGSI may not be coincident in two ways, the angle itself (3.0° versus 4.0°) or the location of where the angle crosses the threshold (3.0° and a Threshold Crossing Height (TCH) of 50 feet versus 3.0° and a TCH of 73 feet).
Through the FAA’s Aeronautical Charting Forum’s Charting Group, in which NBAA’s Access Committee Members are active participants, the FAA’s Aero Nav Services has recently begun publishing notes that call the attention to and explain inconsistencies between VGSI settings and final approach descent angles.
- For Precision approaches – VGSI and (ILS/MLS/TLS/RNAV as appropriate) glidepath not coincident
- For non-Precision Approaches – VGSI and descent angles not coincident
When used correctly, EFVS is a tool which greatly enhances safety and efficiency. Pilots determine if EFVS can be used below MDA or DH, then determine, if there is a VGSI, is the angle coincident to the depicted angle with the final approach segment vertical angle. If it is not, then the pilot must fly the VGSI angle noted on the chart, to ensure obstacle clearance.
FAA Advisory Circular 90-106 Enhanced Flight Vision Systems
Code of Federal Regulations Title 14 Part 91.175 Takeoff and Landing Under IFR
US DOT FAA Order 8260.19E Flight Procedures and Airspace
Aeronautical Charting Forum – Charting Group – Item 09-02-222