Despite the hundreds of data elements displayed in a modern cockpit, one piece of information is missing: the position of the airplane with respect to its intended vertical path. Pilots have to put that "picture" together in their minds. Why not display it for them?
Such a display might go far to combat the pernicious persistence of a certain class of crash. In fact, this type of accident occurs with such frequency, it has acquired its own acronym: CFIT, for controlled flight into terrain.
The grim toll for 1999 was some three dozen accidents due to CFIT in which the body count totaled 220 dead. Airplanes smashed into mountains, ridges, flat ground and water. With more than 30 events in 1999, virtually no progress was made to reduce CFIT crashes over 1998, when the toll was 29 events. Fortunately, no large jets have plowed into terra firma recently. Maybe that’s because more of the big jets are being equipped with ground proximity warning systems (GPWS). Fully 80% of the CFIT events in 1999 involved airplanes that were not GPWS equipped.
Yet the fatal 1997 crash of a Korean Air Lines 747 at Guam was not prevented by the alerts from its GPWS. The crew took the alarm to heart too late to pull out of a collision with Nimitz Hill, which lay along the approach path to the runway. To be sure, a lot of things went wrong in that fatal approach. The captain and crew erred, no doubt. At the same time, the post-mortem indicates the crew was not fully aware of the descending airplane’s flight path with respect to the rising ground.
Indeed, the crew had penetrated below the 2,000-foot and the 1,440-foot altitude "floors" on the step-down approach. Both Capt. Park-Yong Chul and First Officer Song-Kyo Ho had lost completely their situational awareness. The penalty for loss of situational awareness in the vertical plane tends to be far greater than in the horizontal plane. Thus it was suggested at the National Transportation Safety Board’s (NTSB’s) Nov. 6 hearing that a profile view of terrain on the approach chart might have helped prevent the crash of KAL Flight 801. These charts typically show a plan (top) view and a side (profile) view, but surrounding terrain is only shown in the plan view.
As Capt. Paul Misencik, a member of the NTSB’s professional staff, remarked at the hearing, "The profile view can convey a sense that the earth is flat." His remark generated considerable discussion among NTSB members about adding terrain to the profile view on the approach plate. The concern was that such a modification could add potentially confusing clutter.
"There may be a time when they’ll misuse the added information," Misencik conceded, but hastened to add, "There also will be times when they’ll desperately need the information."
Gregory Feith, the NTSB’s investigator-in-charge for the KAL Flight 801 accident, seconded Misencik’s view. Feith believes that if high terrain had been shown on the approach profile for the runway at Guam International Airport, "The crew would have known the VOR was on top of a hill, not on a flat area near the runway."
Indeed, the manufacturer of the enhanced ground proximity warning system (EGPWS) technology now being deployed in the campaign against CFIT believes that a vertical situation display is the next logical step. In development over the last three years and available in about two years, the vertical situation monitor currently in design by AlliedSignal Corp. could dramatically improve situational awareness.
Tentatively dubbed the Independent Vertical Situation Monitor, this new display is seen as a complement to the company’s EGPWS display, which provides a "top down" view of surrounding terrain, akin to a map, in which terrain higher than the airplane is shown in yellow or, if more than 2,000 feet higher, in red.
Don Batemen, AlliedSignal’s chief safety systems engineer, believes the new display will help reduce CFIT accidents. "The utility of a vertical situation display, especially near the runway where most CFIT accident occur, has never been exploited. We’re going to do that," he declared.
Briefly, the vertical profile will occupy about a 2-inch (5.1-cm) high band across the bottom of the current EGPWS display (see illustration). It will show the airplane with respect to a programmed descent profile, whose waypoints appear as stars in the upper or map display and as triangles in the profile view. A trend vector sticking out from the "nose" of the airplane symbol, in the form of a dashed line, shows where the airplane will be flying in the absence of any further control inputs. The length of the trend vector varies from 10 to 30 seconds, depending upon whether the airplane is on final or initial approach.
The minimum descent altitude (MDA) is shown as a horizontal dotted line. More importantly, a profile of the terrain is shown. To avoid confusion, terrain above the airplane is not color-coded in yellow or red. Ground higher than the airplane is self-evident in the vertical profile, so the color-coding incorporated into the top down or map display is not needed. Engineers designing the display are acutely conscious of the need to eliminate clutter.
In their hearing, NTSB members also were concerned about how wide a band of terrain should be represented in any profile of the ground below. That concern, too, is being addressed with an unseen subtlety in the design of AlliedSignal’s electronic terrain profile. The terrain "slice," as it were, varies in width.
On approach, the slice represents a band about one-quarter mile wide, which corresponds to the cell size of the digitized terrain database that is integral to the EGPWS technology. During cruise, the terrain slice represents a band about 4 nautical miles wide, corresponding to the typical airway width en route.
Bateman and Howard Glover, an avionics engineer working on the project, listed the vertical situation display’s benefits:
The trend vector of the aircraft’s projected flight path will show immediately if one is too high on the approach. "If there is no way the approach can be completed, the go-around can be initiated much earlier," Glover said.
Altimeter errors become self-evident. The altimeter reading is duplicated on the vertical situation display, while the terrain and the airport height above sea level are related to the terrain database. "You don’t have to read numbers to see a problem," said Bateman.
According to AlliedSignal officials, low-cost retrofit into existing aircraft is a major design goal. Accordingly, the Independent Vertical Situation Monitor will not require additional line replaceable units.
The display would not have been possible without the integration of a digital terrain database with a programmed flight path and inputs from on-board sensors. In this respect, the march of technology just may help force a retreat in CFIT crashes.
For additional information, contact AlliedSignal at 425-885-8465.