Just How "All Right" are "Two Red and Two White?" Published July 30, 2014 By Maj Ryan W. Adams, KYANG 123 GMS/DO TMF Fall 2014 -- If you weren't taught how to use Vertical Glideslope Indicators (VGSI) for landing before your first night flight, I'm betting you heard about it soon thereafter: "Fly two red and two white, and you're alright ..." The point of this article isn't to counter that beginner's level "instruction" or old saying so much as pass along some additional information, backed by mishap findings, and discuss some countermeasures to what is a threat to safe AMC flight operations. BACKGROUND Pilots aren't exposed to the substance of FAA VGSI regulations, which specify that the setting of Precision Approach Path Indicators (PAPI) must allow for two specific outcomes to be achieved. The first, which is assumed, is the lowest possible on-path indication must clear a prescribed obstacle clearance surface (OCS)--hence that old saying. Second, the lowest on-path indication must cross the runway threshold at a height compatible with "the most demanding Height Group aircraft that uses the runway."1 The threshold crossing height (TCH) requirement is intended to ensure an aircraft's wheels pass over the start of the runway with ample clearance. Both requirements must be addressed in VGSI installation. Subsequently, given basic geometry, the latter results in an approach path that may provide significantly more vertical clearance (i.e., margin of safety) over obstacles/terrain further out from the threshold than the former. What are Height Groups and how do I know what mine is (and the desired VGSI TCH for it)? In the United States, aircraft types are classified into one of four Height Groups based on the distance between the cockpit or pilot's eyes and the aircraft wheels when on approach. Table 1 is consolidated from DoD and FAA VGSI publications. 1, 2 Table 1. Classification of USAF aircraft; classification of types in bold currently under review LESSONS (TO BE) LEARNED After a mishap in 2007, the Canadian Transportation Safety Board (TSB) determined that the pilots knowingly deviated from VGSI guidance on final approach to maximize landing distance available.3 The pilots did not recognize they were deviating from a path incompatible (i.e., too low) with their aircraft eye-wheel height (EWH), despite (Canadian) regulatory guidance describing and approach chart symbols depicting VGSI EWH compatibility. 3 The deviation contributed to Controlled Flight Into Terrain (CFIT).3 The TSB came to two conclusions, which seem applicable to all pilots: 1. Pilots must be aware of their aircraft EWH. 3 2. Pilots must ensure they are aware of VGSI compatibility with aircraft EWH. 3 The mishap occurred in day, Visual Meteorological Conditions, and on a runway that had no adverse obstacles or terrain nearby, so the TSB was concerned with the potential for this issue in less ideal conditions/locations where margins of safety are critical.3 They formulated a concise "Caution" to aircraft operators accounting for what could happen in the presence of degraded visual acuity due to darkness and/or visual illusions (e.g., black hole approaches) with obstacles or terrain close to the final approach path.3 The following is published in the Canadian Aeronautical Information Manual (AIM): "CAUTION - Failure to assess the EWH and approach slope indicator system compatibility could result in decreased terrain clearance margins and in some cases, even premature contact with terrain ..." 4 CATCHING UP We know VGSI are usually just one of several visual references used for approach and landing. Normally they aren't mandatory, and sometimes they're out of service or not even installed. Regardless, the information described previously (substituting "Height Group" for "EWH") can help any pilot determine when to rely more on other visual references, in addition to the VGSI. Absent a system of chart symbols like Canada's, Height Group compatibility should be accounted for by referencing VGSI TCH and the substance of Table 1. GOING FURTHER U.S. TERPS procedures impose a unique restriction if there are unlit obstacles or terrain close to an approach's vertical path. This "red flag" cue to aircrew is an annotation in suitability report TERPS reviews or on approach plates such as: "When RWY XX VGSI inop, procedure NA at night." 5, 6 This feature encapsulates two of the conditions the Canadian TSB factored into the "Caution" cited previously: presence of adverse (and unlit) obstacles or terrain close to the final approach path and applicability during times of darkness. Given these conditions and lack of an electronic glide slope, the PAPI is the only aid to help the pilot avoid a CFIT mishap--he/she cannot otherwise see what obstructions to avoid. In these instances, the compatibility of the system with aircraft Height Groups is absolutely critical. REAL (BAD) ODDS The only available instrument approaches to Birmingham, AL (KBHM) Runway 18 (both non-precision) require the PAPI to be operative at night. The PAPI has a TCH of 48 feet, meaning it is compatible with Height Group 3 aircraft and below. Comparing wheel height over the OCS (i.e., possible obstacles or terrain) while flying the current PAPI on-path profile versus one compliant for Height Group 4 aircraft, at a 1.5 nm final, a KC-10's margin of safety is degraded 28 percent - by .2 nm, it's degraded 58 percent. This range is where obstacles/terrain, not visible to the naked eye at night, drive the TERPS VGSI requirement! It's not that a margin of safety doesn't exist, so much as when you are perfectly on-path (as you are all of the time), your room for deviations (which never occur) is negligently non-standard. On short final, if a KC-10 descends into the current PAPI "three red and one white" path (i.e., "slightly below"), the wheel path actually penetrates the OCS! This isn't just a Height Group 4 Mission Design Series (MDS) issue ... this risk exists when any aircraft larger than Height Group 1 operates into runways lacking a precision approach and equipped with VGSI that are mandatory but calibrated for any notably smaller Height Groups. Couldn't pilots of larger Height Group aircraft descend above the PAPI path? This idea alone negates the Precise design and purpose of the PAPI, and begs two questions. How high above path does that aircraft need to be? And how is this indicated visually (i.e., how far into "three or four white") given lack of other visual references? With respect to transitioning from an instrument procedure at low altitude, the idea counters the evolution towards instrument and visual path coincidence (vis-à-vis the stabilized approach concept). These "bad odds" have been mitigated at one U.S. airport. The FAA has prohibited Height Group 4 aircraft from day and night landings on Portland, OR (KPDX) Runway 21 due to VGSI compatibility - despite approaches that don't otherwise require VGSI operation. 7 Outside of imposing similar restrictions and focusing on when this issue is most critical, the lack of other visual references mentioned previously is the risk factor to address. We can mitigate this with Heads Up Display (HUD) flight path angle and vector control (referencing PAPI position and descent angle) or, for non-HUD equipped MDS, Night Vision Device (NVD) approach procedures. THE WAY AHEAD Thanks to the ASAP safety program, where this concern was first raised, all USAF aircrew can expect to see general language addressing this issue. Information similar to the Canadian AIM and referencing some form of Table 1 is expected in a forthcoming re-write of AFMAN 11-217. It's not realistic to think this applies to all MDS that typically don't operate into short runways or austere locations (i.e., fighters/bombers), but proposed courses of action addressing the issue are currently in coordination for AMC consideration. MDS subject matter and safety experts will more fully address specific guidance going forward and include such information in a standardized location within fleet source documents (e.g., 11-2MDS Vol. 3). CONCLUSION The mission oftentimes requires us to operate into locations that weren't originally designed or equipped for AMC-size aircraft, so VGSI compatibility could be a critical issue at night. The good news is this issue has been recognized and vetted, can be mitigated, and is being addressed with the key stakeholder ... you. Cultivate your professional knowledge base and grow stronger by understanding more about all the tools we use in our trade. In this case, I think most would agree that additional general knowledge about VGSI and potential safety restrictions, conservatively applied and aimed at keeping you "out of the weeds," are cheaper to impart than the value of your life and those of our teammates! References and Links 1. Joint Order 6850.2B, Visual Guidance Lighting Systems, August 20, 2010, Federal Aviation Administration. http://www.faa.gov/documentLibrary/media/Order/FINAL FAA Order 6850.2B.pdf. 2. Unified Facilities Criteria 3-535-01, Visual Air Navigation Facilities, 17 November, 2005. Department of Defense. http://www.wbdg.org/ccb/DOD/UFC/ufc_3_535_01.pdf. 3. Aviation Investigation Report A07A0134, November 11, 2007, Transportation Safety Board of Canada. http://www.tsb.gc.ca/eng/rapports-reports/aviation/2007/a07a0134/a07a0134.pdf. 4. TP 14371 - Transport Canada Aeronautical Information Manual, Aerodromes, October 17, 2013, Transport Canada. http://www.tc.gc.ca/media/documents/ca-publications/AGA-AIM-2013-2_ENG-2.pdf. 5. FAA 8260.19E Chg 3, Flight Procedures and Airspace, February 22, 2013, Federal Aviation Administration. http://www.faa.gov/documentLibrary/media/Order/8260.19E.pdf. 6. AFI 11-230, Instrument Procedures, September 27, 2013, USAF. 7. Per telecom with Airfield Management office POC to verify restriction published in Airport/Facilities Directory and duplicated in GDSS2 suitability report.