cu·mu·luo-/gran-ite/
/’kyoomy lo/’gran t/
nounpilot slang:
mountains obscured by clouds
One of many hilarious flying-related Far Side cartoons is the classic goat in the clouds. We all hope to never see such a thing in the clouds, or any other solid object for that matter. Most often encountered during departures, arrival and approach, or if situational awareness is lost, CFIT – Controlled Flight Into Terrain – is a serious subject. Pilots can avoid CFIT, and any resident goats, by installing a TAWS: Terrain Avoidance and Warning System commonly known by Honeywell’s trademark name EGPWS (Expanded Ground Proximity Warning System). GPWS and EGPWS are essentially the same system except that EGPWS includes a terrain database which adds a “predictive” function to the traditional GPWS.
Studies were conducted after a rash of CFIT accidents in the late 1960s that concluded some type of automatic warning system, alerting pilots to the proximity of the ground, could have prevented the accidents. As a result of the studies and recommendations by the NTSB, the FAA (in 1974) and ICAO (in 1979) required all large turbine and turbojet aircraft to install GPWS. In 2002, the rule was amended to include all jets and all aircraft with a MTOW greater than 12,500 lbs. or 5,700 kg. There is a further distinction in the type of TAWS required (Class A or B, and with or without a visual display) based on the number of passenger seats configured and whether the airplane is in commercial air transport or GA. For TAWS neophytes and those like me who have trouble remembering the rules for who needs a TAWS or can’t remember the 11 or so alerts and warnings presented by TAWS, I offer this brief dissertation on terrain avoidance and warning systems.
Who Needs ‘Em
There are three categories of TAWS: Class A, B and C. Class C is simply a Class B custom modified for use in smaller GA airplanes with no mandated requirement to have a TAWS. Class A is at the top of the food chain in both cost and capability, Class C is at the bottom. The Class A TAWS must have both an aural and visual output to the crew. Class B has the option of having a display or not having a display. Class C has no display. The “who needs ‘em” question is answered by the FAR “Part” under which you operate, the type of motor propelling your vehicle, your MTOW, and the number of “configured” passenger seats in your plane. Here’s a summary:
Part 91.223 – Piston powered – no TAWS mandated. Turbine powered with less than six passenger seats and less than 12,500 lbs. (5,700 kg) no TAWS mandated. Turbine powered with six or more passenger seats regardless of MTOW, Class B TAWS, no display mandated, which infers Class C.
Part 135.154 – Piston powered over 12,500 (5,700kg) and 10 or more passenger seats, Class B with no display mandated. Turbine powered with six to nine passenger seats, Class B with no display mandated, Turbine powered with ten or more passenger seats, Class A with display required.
Part 121 – Class A with display mandated for all operators. For a Class A TAWS, you need additional equipment to provide input to the system: Gear and flap position, radar/radio altitude, CADC, GPS, Glide Slope and SAT (static air temperature), for example. Here are the alerts and warnings provided by Class A and Class B TAWS (neither system warns of even-toed ungulates):
CFIT happens most often when flying non-precision approaches, now officially called “non-ILS.” Continuous Descent Final Approach (CDFA) is one of the initiatives to address the problem in this phase of flight. In industry parlance, the previous method of flying a non-precision approach such as VOR, NDB, LOC-BC or GPS/RNAV approach was called “Dive-and-Drive.” The phrase describes the technique of diving from each fix on the approach, and then from the FAF to the MDA, and then once level, driving to the MAP. This means we’re cruising at a very low altitude, relatively slow and often several miles from the runway.
We would then calculate a VDP, Visual Descent Point, from which to descend from MDA to the runway. This type of approach descent differs greatly in stability, consistency, predictability and pilot workload from a constant rate of descent approach such as on an ILS GS. Most non-precision (non-ILS) approaches now incorporate some form of a CDFA. Combining a TAWS with a CDFA goes a long way in helping us avoid CFIT on arrival. And a Class A TAWS will help to ensure proper landing configuration as well.
Climb, Climb Now!
We should all be practicing a handful of the alert and warning modes from our TAWS each time we go to the sim for recurrent training. At the airlines and my new Part 135 gig, wind shear and terrain alerts are two of the many “first-look” maneuvers included in a list of mandatory events that we perform every time at the sim. First-look means we are not forewarned about when, where or how often the alerts will occur – just like in real life. The requirement is both a test and a strengthening of the mental memory muscles to ensure instinctive compliance with the warnings. While I’ve never experienced an actual terrain warning, I once experienced a TCAS RA (resolution advisory – a traffic warning in which you must react) at the airlines.
We were in IMC and the TCAS display was being shared with the weather radar display. The display scale was at 40 nm due to convective activity making the TCAS target aircraft appear at the very bottom where the radar display comes to a point. At DFW, all departures have a top altitude of 10k and all arrivals have a bottom altitude of 11k – a 1,000-foot difference. If the departing and arriving aircraft have a climb rate and descent rate of over 1,300 fpm or so and are converging, the TCAS of one or both aircraft will compute an incursion into the safety bubble. This was the case during our arrival, and the TCAS instructed an immediate climb of over 2,000 fpm instead of our existing 1,200 fpm descent. Autopilot off, autothrottles off, initiate rapid but smooth compliance. Shortly after beginning the maneuver, a break in the clouds allowed us to spot the other aircraft. And as you would hope, it wasn’t even close.
Immediately and Instinctively
Even if the TAWS operates as it should, action initiated by the crew is what makes the airplane avoid the rocks, not the avionics. We are the ones flying the plane after all. The ones who are supposed to know exactly where we are always, and the first ones to the scene of the impact if we fail to do so. In the past, I’ve discussed the phenomenon of time compression during an intense event: How our memory works and, more relevant to this article, how we experience a mental delay when we’re surprised or shocked by an event. Unlike piston-engine failure training in which we verify, identify and feather, and the well-known axiom to “Wind The Clock” to slow us down so as to not make a bad situation worse through our haste, it’s critical that we are trained and conditioned to react immediately and instinctively to both TCAS and TAWS warnings.
The escape maneuver or procedure for your plane likely includes disengaging some or all of the automation then hand-flying a prescribed maneuver. Once into the maneuver, it probably has you check your configuration for such things as spoilers and power settings, and then to monitor aircraft performance to ensure terrain separation followed by a return to normal performance after an event. Don’t think about it, don’t question it, don’t analyze a false warning, don’t look around to figure out what it sees – and don’t delay. Be smooth but be quick. You can get on the radio after you have a successful recovery in progress, tell ATC or other aircraft what you are doing. And don’t worry, following a TCAS RA or a TAWS warning gives you a get-out-of-jail-free card for violating any clearance or regulation.
Pilots Who Stare at Goats
Based on Jon Ronson’s non-fiction bestseller of the same name, the movie “Men Who Stare at Goats” (2009) is a tale based on the U.S. Army’s foray into psychic research for use as a weapon. During an allegedly true incident, a psychic operative stared a goat to death. Staring at a TAWS warning is like staring at the goat in a cloud bank, believing it must be wrong. Even if you think you’re a psychic, the goat in the cloud won’t fall over, and the rocks won’t move out of your way. This is one of the few times in aviation you are encouraged to act quickly and instinctively. When the TAWS gives you an alert or warning, check your configuration and execute an escape maneuver if needed. Don’t wait until you see a goat in the clouds.
