Flight International evaluates the world's first enhanced ground proximity warning system for helicopters
Peter Gray/MORRISTOWN, NEW JERSEY
Aside from war, no single factor has led to more aircraft accidents and fatalities than controlled flight into terrain (CFIT). It is by far the aviation industry's number one killer.
Honeywell has been monitoring both fixed- and rotary-wing aircraft accidents over several decades and has produced the first Enhanced Ground Proximity Warning System (EGPWS) for helicopters.
Early ground proximity warning systems (GPWS) used just the radar altimeter (radalt) to warn the pilot when the ground beneath the aircraft was too close. A radalt only looks down, not ahead or around the aircraft. In 1996 Allied-Signal, now Honeywell, produced an enhanced GPWS to overcome these limitations.
The EGPWS has a terrain clearance floor function to provide protection even when an aircraft is in a landing configuration. There is also a "look ahead" capability using a terrain database to provide pilots with better situational awareness of terrain and other obstructions, and to give more warning time to carry out corrective manoeuvres. Using aircraft position and the internal database, terrain above the aircraft altitude is presented on the system display.
Industry first
Honeywell and Sikorsky conducted flight tests last year using an S-76 to certificate an EGPWS for helicopters - the industry's first. Tests were witnessed by the US Federal Aviation Administration, NASA, the National Rotorcraft Technology Centers, Rotorcraft Industry Technology Association, the US Air Force and Navy, industry representatives and pilots. Flight International evaluated the system the day before FAA certification.
As helicopters often fly lower than their fixed-wing counterparts, extra features have been incorporated in the EGPWS. These include a higher resolution terrain database for both on- and off-heliport operations, a tail strike warning, autorotation height awareness call-outs, and bank angle and undercarriage position alerts that depend on the presence of a load on the belly hook.
In normal mode, the system provides a 30s look-ahead terrain and obstacle caution with a 250ft (76m) terrain clearance floor minimum. In low-level flight, in order not to be bothered with all the normal alerts, the pilot can select an enhanced low-altitude mode which reduces the look-ahead alert to 18s and the terrain clearance floor to 200ft minimum.
Other features include "peaks" and "geometric altitude" capabilities. "Peaks" mode provides a digital elevation display of the highest and lowest terrain. It also shows a unique representation of sea level water and its corresponding shoreline. "Geometric altitude" uses GPS, radalt, air data computer and terrain database information to compute the aircraft's true altitude, no matter what the pilots may have set on their own altimeters (altimeter mis-sets account for 25% of all CFIT accidents). This also eliminates altitude errors due to other factors, such as extreme temperatures and ambient air pressure variations.
Essentially, the EGPWS looks ahead and around and calculates if there is the likelihood of collision with a solid object, be it ground or sea, mountain or other obstruction. High power transmission lines and high-tension cables are a collision risk for helicopters and will be added regionally to the database as information becomes available. The system can have up to 11 inputs, depending on the level of equipment fitted). These are:
28v dc; airspeed, altitude, ambient air temperature; radar altitude; position; attitude; heading; glideslope; undercarriage position; engine torque; weight on wheels; load on the belly hook. Surface mappedHoneywell says it has mapped about 99% of the world's surface, including cables and every obstruction above 30m (98ft) in the USA and other countries and at all known airports and heliports. At the moment, wires are treated like any other obstruction, but the plan is to change the warning to a specific wire alert.
The company constantly updates its terrain databases and offers the revisions free to purchasers. The company has spent over $23 million on database development and spends a further $875,000 annually on updates.
Terrain information is displayed in the cockpit similarly to weather radar. At a glance, the pilot can immediately determine if the surrounding terrain is above or below the aircraft's altitude and if it presents a potential threat. Terrain information can be presented on the weather radar, electronic flight instrument or multi-function display, or any other compatible screen. Or it can have its own dedicated display which, in turn, will accept other information such as weather radar.
There are four system variants, depending on what the buyer needs and can afford. The system installed in the S-76 which I examined was Honeywell's Mark XXII EGPWS, which has all the functions, accepts all the inputs and provides all the cautions, warnings and alerts yet weighs a mere 1.6kg (3.5lb). It costs $46,500. The next version (the Mark XX) is smaller and lighter, but has less information and fewer functions. It costs less than $15,000 and is suitable for helicopters with no radalt. Another version, the Mark XXI, will cost under $30,000 and is designed for helicopters with radalt but no vertical gyro and other sensors. The fourth version is aimed at aircraft without a display.
Sikorsky standard
Sikorsky will install the equipment across its helicopter product line and will also offer the system for retrofit. EGPWS became available mid-January and Honeywell has already received several orders.
Flight International was invited to evaluate the world's first helicopter EGPWS at Honeywell's base facility at Morristown airport in New Jersey. After a presentation by Andrew Cindric, GPWS/EGPWS business development manager, I was seated in the S-76's left-hand co-pilot's seat. Cindric, Sikorsky representative John Studer and test engineer Dennis Martin sat in the back. Cindric could see my section of the cockpit, including the terrain display and describe what was happening. We were all on intercom. Honeywell's Edward Ferguson was our pilot. The FAA was coming to certificate the equipment the next day, so our aircraft was fully operational with all the EGPWS modes and functions working and available. We planned to do a dummy FAA run to demonstrate all modes. The flight was carefully planned to expose us to all the hazards which the system has been designed to detect and warn about.
The S-76 was well-equipped. I had a full set of blind flying equipment on my side, although the aircraft was classified as experimental and cleared for day visual meteorological conditions only. The foot warmers and a hook for hanging my coat were also appreciated.
The terrain display screen fitted neatly below the attitude director indicator. Ferguson removed some of the other information from the screen to leave me with an easy to interpret terrain display. Beneath the screen were buttons allowing me to select/deselect such functions as range scale - from 4.6km (2.5nm) to 185km - glideslope, audio inhibit, terrain and low altitude modes.
Ferguson started the engines and rotor and we took off. After take-off he turned at what the EGPWS considered an excessive angle of bank and we got an audio "bank angle, bank angle" warning from a digitised, male voice. Some aviation medical experts say a female voice grabs the pilot's attention better. The buyer can delete this mode if they wish.
After climbing a little, Ferguson deliberately allowed the aircraft to drift downhill. This has been the cause of several ditchings when helicopters have been coming off an offshore platform in poor weather or at night. The algorithms immediately compared various parameters, such as the percentage of altitude lost against time, and calculated that, if our descent continued, we would hit the ground. So we got a "don't sink, don't sink" warning.
We then climbed and headed north towards the mountains. The terrain presentation was excellent and easy to interpret. Black and green represent terrain safely beneath us. Yellow means danger, but indicates that the system calculates you can climb over it. Red, a real attention grabber, shows areas you cannot climb over, and should turn away from. The lower you get, the darker the colours become.
Easy to interpret
A digital read-out, the so-called "peaks" mode, indicates the highest obstacle presently in view on the screen. A red 020 was displayed, which meant the highest peak was 2,000ft above sea level. Beneath the red 020 was a green 008, which meant that the average contour around the black/green area was 800ft above sea level and therefore was at least 250ft below us. I could easily see the black/green area on my screen (present position is represented by a small helicopter diagram at the bottom of the screen).
When we entered the mountainous area, it was easy to follow the black area, avoiding the high ground (yellow and red). The EGPWS is not intended to be a navigation aid, but it will help when visibility is difficult - a big potential killer. The system will also see around corners, so cul-de-sacs (another potential killer) can be quickly identified and avoided.
We next headed for a ridge crossing our track at right angles. On top of the ridge and directly ahead was a 30m tower, tall enough to appear on EGPWS database. Looking down at the display, I adjusted range so that the tower was easily identified as a yellow area amongst the green. Honeywell puts a 600ft² block around such objects, on the assumption that towers have 30° guide wires all around. This is one of the EGPWS enhancements introduced for helicopter operations.
As we got to within 30s of the tower, we heard "caution obstacle", then "warning obstacle" as we got even closer. Ferguson turned away and the warning immediately ceased. He deliberately rolled out too early, however, and the system, sensing that we were still vulnerable, brought back the warning. Smart.
We continued towards the mountains. Ferguson turned off his terrain display, switching it to weather radar. I adjusted my range scale to beyond 10km. He then flew towards steeply rising ground, which the algorithms calculated that we would hit. With 30s notice, both our displays reverted automatically to the 4.6km range scale and we got "caution terrain", followed by "warning terrain". This automatic reconfiguration will occur even if the pilot has switched off other facilities. The system will also warn automatically of any other obstruction. Pilots do not need to change their screen manually to display terrain or change the range.
With the terrain display system and GPS switched off, we still received adequate warning when approaching high ground using the 30-year-old system of radalt and air data information only. One difference with the helicopter EGPWS is that, when we switched off the GPS, the system alerts the pilot with "be alert, terrain inoperative". Fixed-wing aircraft have only an on/off light. As we approached rising terrain we got "terrain, terrain, pull up".
If the EGPWS sees that your rate of climb is the same or better than the gradient of rising ground below, it keeps quiet. If the rate of descent deteriorates to an extent that the algorithms see danger, cautions are activated. As soon as the pilot flies the correct solution to the potential problem, the cautions stop.
Into the mountains
We entered the mountains and approached and landed at an unattended airfield surrounded by high ground. The visual presentation of the surrounding area was excellent. We got a countdown of our height above the runway. This is another customer option. You can select whatever heights you prefer or delete the facility altogether. On an instrument approach, decision height can be selected and this will be called out. On any flight, the pilot would hope to see only greens and blacks on the screen and, apart from the height countdown, hear nothing else. The system is passive, waiting and monitoring noiselessly unless the pilot exposes the aircraft to potential danger.
We continued our flight through the mountains, below the level of surrounding peaks. Again the presentation was excellent, allowing us to spot excessively high ground and follow the safe, black/green areas. This would be an excellent situational awareness aid for a pilot unfamiliar with his surroundings when lost or on an emergency medical or search-and-rescue mission. It should help to eliminate the too-frequent crash where the pilot experienced rising ground in a narrowing valley with deteriorating visibility and/or descending cloud.
Low-altitude mode was then selected. We were allowed to get much closer (18s) and lower (200ft) before being alerted to a potential hazard. We approached a ridge below the top and got "terrain, pull up" as well as lots of yellow on the screen. Although we were still approaching the ridge, the warning stopped when the algorithms calculated our rate of climb was sufficient to clear it. The system is designed not to give unnecessary or redundant warnings.
On our return to Morristown, Ferguson autorotated to the airfield with the undercarriage up. Because we were using less than 7.5% torque (this parameter will depend on the type of aircraft and is adjustable), the system knew we were in autorotation and therefore assumed the manoeuvre was intentional. So it kept quiet.
However, when we passed through 400ft, the system came on the air with "too low, gear". This masked the 200ft call, but we got the 100ft and subsequent calls. If we had been using more than 7.5% torque and the rate of descent was in excess of 1,500ft/min (7.6m/s), then the system would assume our rapid descent was unintentional and give us a "sink rate, sink rate, pull up" warning. A normal approach at normal rates of descent will trigger the undercarriage warning at 150ft. Ferguson overdid the flare at the bottom to show me the "tail too low" tail strike warning. A high-speed pass along the runway at 150ft caused the "too low, terrain" caution. As Ferguson reduced the speed, this changed to "too low, gear".
Glideslope caution
After a nearly 3h flight, we had insufficient fuel to carry out an ILS approach and it was getting close to sunset (the aircraft being cleared for day operations only). But I was shown a video where the pilot deliberately got low on the ILS glideslope when below 1,000ft radalt. A soft voice cautioned him, becoming more urgent as he fell further.
The EGPWS' excessive descent rate, rising terrain, descent after take-off, terrain clearance and descent below glideslope modes work all the time. The rest are customer options that are built into every box, and activated at customer request for no extra cost. Honeywell can, if necessary, remove unwanted alerts such that caused by a hill or even a large air craft under a holding pattern (envelope modulation). Smart stuff.
With its examination of the causes of CFIT accidents, Honeywell has done a magnificent job of incorporating the lessons into its monitoring and alerting systems. I can think of no possible eventuality that is not included.
Presentation is excellent, with enough detail to make the pilot instantly aware of what is around the aircraft that might cause conflict or is safe. I like the EGPWS' passive operation, whereby the system monitors quietly and does nothing until it sees possible trouble. I found the equipment easy to use, and the cautions and warnings appropriate.
I hope helicopter pilots will not use the EGPWS to push flights into deteriorating weather and terrain conditions for which they are not qualified or sufficiently experienced. This happened with the advent of GPS, and I suggest the system is labelled, like GPS, "not for sole navigation purposes." This concern aside, I consider EGPWS to be one of the major milestones in the advancement of aviation safety.
Source: Flight International
