Wingman: Enhanced Vision Rule Validates Its Low-Visibility Utility

The FAA’s new rule authorizing the use of an enhanced vision flight system (EVFS) to see required visual references at Category I minimums, validates the utility of this technology, and hints at how it might be adapted to even greater use.  (EVFS is a new FAA term for a system that displays a sensor image of the outside world on a head-up display.)    

The rule allows a pilot to use EVFS to identify specific visual cues at either a CAT I decision height (DH) or minimum descent altitude (MDA) by using enhanced vision, and then continue the approach to an altitude of 100 feet, after which “natural vision” is required to complete the landing.  (The FAA is using the new definition of CAT I, described in its Advisory Circular 120-29A, that includes both precision and non-precision straight-in approaches.)

Essential Attributes of EFVS 

The FAA says that a HUD (head-up display) is an essential part of an EFVS because its display “ is being used as the pilot's primary flight reference during the approach, at least down to 100 feet above the touchdown zone elevation.”  The imagery must be displayed on a HUD, the FAA believes, because this operation depends on the pilot looking forward along the flight path and making an easy transition from the EFVS imagery to an out-the-window view of the runway.  The FAA believes this isn’t possible with a head-down EVS display.  Furthermore, the EFVS image as viewed through the HUD – along with other essential flight symbology – must be conformally aligned with, and scaled, to what the pilot see externally.   

Based on inputs it received, the FAA revised the proposed rule, and now will require an EFVS to display flight path (the intended approach path as shown by lateral and vertical path deviation indications), command guidance, a conformal FPV (flight-path vector), and a flight-path-angle reference cue.  While acknowledging the desirability of energy management, speed error and speed trend cues, the regulators didn’t require them here.  This decision was likely taken because of the FAA's minimalist philosophy, in these matters, of requiring only the equipment necessary to satisfy this particular rule, and nothing else.  Still, most HUD manufacturers will likely include these powerful tools as desirable features for their customers, and because they are “essential features” on CAT-III HUD landing guidance systems.  (More on this later.)

Why SVS Wasn’t Included 

Some might wonder why SVS ((synthetic vision system) wasn’t included in this rule.  The FAA said it excluded the use of SVS for this procedure, even though it has several potential advantages over an EFVS, such as its ability to “see” further in virtually any environmental condition.  In addition, SVS doesn’t use a raster (television) display, which some worry obscures a pilot’s exterior view through the image, but rather only “stroke” (line drawing) technology, which has no obscuration except for such things as runway outlines.

To the FAA, the key difference between SVS and EFVS is that “an EFVS provides an independent real-time view for the pilot,” while an SVS does not.  An SVS has – among other things – a database and a processing component that would compute and “draw”' a forward view.  Importantly, the “drawn” external view is valid only if the SVS data base and navigation processing components are valid, which causes thorny certification issues. Thus, the FAA concluded that a database-derived SVS display is not a real-time source of forward scene information as an EFVS sensor-based image is.

The FAA said, however, that although it can’t presently approve SVS operations below the authorized DH or MDA, using the EFVS rationale, it might in the future as circumstances change.

What Does This Rule Mean In A Practical Sense? 

This rule only allows the pilot to descend below at CAT I minimum altitude, when certain visual cues are recognized while using an EFVS – nothing more.

But many airplanes equipped with EFVS will, at the same time, have HUDs that are qualified – under different rules – to conduct CAT II and III operations to appropriate runways.  This means that these same aircraft may make approaches down to a 50 foot DH and 600 RVR – and in some cases even lower, if an autoland is also being used.

In other words, an EFVS-equipped aircraft isn’t prohibited from being used during Category II and Category III approaches, but if used during these operations, there are “other required equipment, procedures and qualifications,” and the specific EFVS “must first be demonstrated to be suitable during such operations.”  

Wingman interprets this to mean that a user, with an otherwise qualified CAT-III HUD, gets the following by also adding an EFVS: 

1. CAT II and III capability on CAT II and III runways.  (Based on a qualified HUD.)
2. Lower takeoff minimums on certain runways. (Based on qualified HUD, not the enhanced vision image.)
3. Lower minimums on certain, specified CAT-I runways. (Based on qualified HUD, as is already the case.)
4. Lower minimums on many other CAT-I runways. (Based on qualified EFVS.)

Flight Visibility and Visual References 

According to the rule, when using an EFVS to descend below the DH or MDA, one of two requirements must be met or the pilot must execute a go-around:  (1) The approach light system (if installed) must be seen; or (2) Both the threshold and the touchdown zone must be seen.   

The requirement for the pilot to see both the threshold and the touchdown zone has been added to this procedure because of the inability of the EFVS to display color.  But notice what might be an important nuance in this language: It doesn’t specifically say threshold and the touchdown zone lighting.  This could have the practical importance of allowing the use of an EFVS procedure on many runways that don’t have lighting installed.   

After descending past the DH or MDA using EFVS, the pilot can descend below 100 feet if the runway’s red terminating bars or the red side row bars are also distinctly visible and identifiable, without relying on the EFVS image.   

Required Visibility for Commencing Approach 

A significant regulation affecting the utility of an EFVS-equipped airplane is the legality of starting an approach if the reported RVR is less than CAT I minimums.   

Today, part 121, 125, and 135 operators may not initiate an instrument approach procedure unless the reported visibility is equal to or more than the visibility minimums prescribed for that procedure.  This requirement doesn’t apply to Part 91 operators, which includes business jet operators, which, incidentally, are showing a lot of interest in EVFS.  

In its final rule, the FAA discussed this issue, but only in the context of whether or not visibility measured by a transmissometer is a reliable indicator of EFVS performance at or below minimums.  The FAA says that while a transmissometer doesn’t operate in the same portion of the electromagnetic spectrum as the EFVS, “its measurements are just as representative of the visibility conditions at/below 100 feet height above touchdown zone elevation as they are today.”  But then the FAA punts on the question – at least for now – by saying the question is “outside the scope of the NPRM.”  

Leaving aside the rationale of distinguishing between commercial and non-commercial operations on such an issue, one could reasonably ask, if a transmissometer was present, and reporting an RVR of 1200 or greater, why shouldn’t an operator with a certified EFVS be allowed to initiate the approach? 

More Potential

The rule didn’t directly discuss the obvious safety advantages of EFVS, because they are not directly pertinent to the purpose of the rule.  Here are two that Wingman believes are real and substantial: 

• The ability to help prevent CFIT (controlled flight into terrain) accidents. EFVS can go beyond the considerable benefits of TAWS (terrain awareness warning system) in this regard, because of its ability to help the pilot accurately control the aircraft during the last 100 feet before touchdown – especially because of the formidable guidance tools of the modern HUD component.
• The ability to help prevent runway and taxiway incursions and collisions because of the pilot’s ability to now see what would otherwise be obscured.

Other operational advantages of EFVS, that weren’t addressed in the rule, include the possibility of lower visibility takeoffs beyond that allowed by modern HUDs alone, and how EFVS might be used in a WAAS and LAAS environment.     

Wingman believes these improvements could be significant, but will wait for another day.  01-26-2004.

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