Personal/Corporate, Training

From the instructor’s seat

By Johan Nurmi | June 1, 2004
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Fighting the Domino Effect

When one simple thing goes wrong in flight, others that I like to call "the variables" will kick in. This will usually lead to a domino effect in which everything goes wrong and an accident is inevitable.

Pilots must be trained to do a couple of things to prevent the domino effect from taking hold. First, we must always have a reserve plan in mind to quickly solve that first problem that crops up. Second, if something does go wrong, we must try to correct that problem immediately. Examination of an accident will illustrate some of these "variables" and the benefits of a reserve plan and quick action.


In this accident, the pilot of a Robinson R22 BII flew a friend up in the mountains. This pilot had very limited mountain experience and was a low-time pilot.

The peak of the mountain was about 5,000 ft. The pilot was going to fly up a ravine that leads up to the mountaintop. The ravine is extremely beautiful, and there is a creek beneath it with many interesting waterfalls. Its walls steeply rise 900 ft. The ravine is so narrow there is not room for two helicopters to pass. From one side to the other is less than 150 ft.

As this flight progressed, the weather was starting to get bad. Visibility was about 5 miles and the ceiling was about 3,000 ft. Conditions were VFR when the pilot departed the airport where he rented the helicopter. The sky was overcast, with dark, stratus clouds, and the ceiling and visibility started to drop slowly. Winds were out of the south at 10 kt., but were increasing as a cold front moved south. It was a cold day, and the pilot was told to only fly locally, with carburetor heat on during his entire flight. He had been informed in a written contract about the school safety rules, which he had signed. One of the rules was not to land on pinnacles or in confined areas.

The pilot had called the Flight Service Station and received a standard weather briefing some 50 mi. from the departure airport, since he lived far from it. The weather briefer informed him about the weather and reported the actual weather for the area from which he had called. The briefer informed him that present conditions were VFR. The pilot never asked about the weather outlook. When he received the briefing, he thought the weather was okay. So he and his friend jumped into his car and drove the 50 mi. to the airport.

The school's manager asked the pilot to be safe, and stay in the vicinity of the airport. The pilot started up the engine. After warm-up, he and his friend took off and flew to the south.

When the pilot and his friend flew into the ravine, they had a great time. The incredible view from the helicopter impressed the friend and they were both excited about the beauty of the mountains in which they were flying. There was a little bit of turbulence and the Robinson was pitching, rolling, and yawing due to the increased wind in the narrow ravine.

But the pilot thought he could handle it and continued flying through the ravine to the great waterfall that was at 3,000 ft msl. up in the ravine. There, just in front of the big waterfall was a small landing area. The pilot decided to do a steep approach to the landing zone, then shut down the engine. He and his friend wanted to hike up to the waterfall and take some pictures of a place where very few people had set foot.

As he started his steep approach, he pulled full carburetor heat, and slowed down his airspeed below 30 kt. He now was inside the height-velocity diagram, which for the R22 starts from about 12 ft and goes up to 625 ft. (non sea-level airport).

This means that if a pilot has an engine failure inside this altitude, he has only a small chance to walk away from it. In most cases when the engine quits inside the height-velocity diagram, the impact will be harsh, and there will probably be fatalities.

The back side of the power curve means that if a pilot slows down the airspeed below 53 kt. in the R22, the helicopter requires more power to go slower and less to go faster. On the front side (53 kt. and faster), the aircraft requires more power to go faster and less to go slower.

During his steep approach, the airspeed gradually decreased below effective translational lift. This is defined as "extra lift obtained by forward airspeed." As the pilot slowly increases his forward airspeed above 30 kt., he will obtain this lift, and the helicopter will climb without the helicopter pilot lifting the collective. On the other hand, when the pilot slows down his airspeed below 30 kt. on his final approach, he will lose effective translational lift and therefore also lose lift and main rotor thrust. As a result, the aircraft will slow down and descend. The pilot must therefore lift the collective to maintain altitude and increase power.

As the pilot lost his effective translational lift and found himself on the backside of the power curve (with full carburetor heat applied), he was caught in a strong downdraft, on the leeward side of the 5,000-ft. mountain. The Venturi effect was also present, which basically means that the winds moving in the ravine will double as the pressure drops, and the RPM started to decrease.

Even highly experienced mountain pilots can have problems with strong downdrafts in the mountains.

The pilot knew he could only pull about 23 in. of manifold pressure on the engine without pulling down the RPM. As he saw the decreasing RPM, the helicopter started to go down vertically and forward airspeed slowed down remarkably. The manifold pressure exceeded 23 in. Then the RPM warning horn and RPM cautionary light came on at 97 percent.

In a situation like this, a helicopter pilot must react rapidly and know how to regain his lost RPM immediately. Otherwise, it will continue to drop, and a rotor stall will occur. In rotor stall, the blades come to a full stop and experience excessive blade flapping, and may cut off the tail boom section. In the R22 and R44, the rotor blades will stall below 80 percent plus one percent per thousand feet of altitude.

Before a helicopter pilot plans to land on a mountain top—called a pinnacle landing—he must check his out-of-ground effect (OGE) hover diagram, which simply means the maximum altitude in mean sea level at which the helicopter may safely land. This situation is the "red line" for mountain landings. So, if the pilot finds he is inside the height-velocity diagram, on the backside of the power curve and beyond the maximum safe altitude for landing, he should not attempt a landing.

The R22's Max OGE in the summer is based on the weight of the helicopter, its pilot, passenger and fuel and the existing temperature. The pilot uses the OGE diagram to check his available landing altitude. He then should subtract 10 percent for safety if the engine has run more than half time before overhaul, which is 1,100 hr. in the R22. This pilot never checked the max OGE hover ceiling.

The pilot also did not follow the "hand method," which determines how much total weight he will have before departure. In the R22, this is relatively simple. In the center-of-gravity section in the Pilot Operating Handbook, we can find our basic empty weight (875 lb.). Then we add the weight of the pilot, passenger and fuel. We then subtract that sum from the max gross weight of 1370 lb.

In some cases, a pilot never knows he is exceeding the max gross weight. If he does, max out-of-ground effect hover cannot be determined.

This pilot never calculated center of gravity before take off. He and his friend exceeded the max gross weight by 50 lb.

As the RPM decayed to 97 percent and the horn and light came on, the pilot should have lowered the collective and increased the RPM back to 104 percent. He could have also stepped on the right pedal to send some of the power driving the tail rotor back to the engine. Some 20 percent of engine power drives the tail rotor. When the pilot steps on the left pedal of American-built helicopters, he is using that 20 percent to control tail rotor thrust, which is needed to fly the helicopter in trim above 50 ft, or to keep a straight ground track on final approaches. As the pilot steps on the right pedal, he no longer is using that power, which is therefore available to power the main rotor. The pilot who experiences low RPM can also use effective translational lift to get out of a low RPM situation. As the helicopter gains forward airspeed, the main rotor produces more lift and main rotor thrust.

As the RPM decayed below 97 percent, the pilot tried to increase RPM, but failed to get it back due to inexperience and all the other variables that caught him in that ravine. As he saw the RPM go far below 90 percent, he decided to enter an autorotation down into the ravine in order for him to save his and the passenger's life. In the autorotation, his airspeed and RPM were too slow. When he flared, there was not enough lift in the rotor. As a result, he crashed the R22 on the side of the ravine wall. The tail rotor hit first, then the helicopter rolled on its side to the left. Miraculously, both the pilot and the passenger survived without life-threatening injuries.

Variables that lead to the domino effect and cause a pilot to crash can be avoided, as we can see in analyzing the actions of the accident pilot.

These variables include:

Knowledge of the weather—The pilot should have request a weather and outlook briefing at the departing airport. He should have analyzed weather conditions. Weather obviously played an important role in this accident. Statistics show 17 percent of all accidents are weather-related.

Experience level—This pilot was not familiar with mountain flying or with recognizing turbulence and downdraft. He was ill-prepared to cope with strong winds or to avoid the leeward side of big mountains. These are critical in avoiding accidents. Even very experienced pilots have respect for high mountains.

Decision-making—The pilot did not react immediately when the RPM fell. With proper training and better awareness he could have recovered his lost RPM and safely turned away from this ravine. The R22 and R44 both have a governor. But the governor will not help the pilot control the RPM when the pilot is exceeding his maximum manifold pressure.

Know Your Aircraft

Weight and balance—The pilot never calculated c.g. If he had, he would have offloaded 50 lb. of fuel to stay below 1370 lb.

Knowledge of your aircraft—The pilot never checked his maximum out-of-ground effect hover ceiling. If you try to land higher than your OGE, you are going to be involved in an accident. You will lose your RPM and risk a rotor stall. Also, as he made the steep approach, the pilot applied full carb heat to avoid carb ice. It's actually an automatic process in the R22. As the pilot lowers collective, the carb heat automatically comes on. But pilots should always monitor the carb heat indicator so the needle won't come on in the yellow arc (if it does, the pilot should pull full carb heat). In this accident, the pilot had full carb heat, and it decreased the engine power (manifold pressure) by one inch. The pilot should have lowered the carb heat instead of pulling it up, because of the RPM loss.

Judgment—The pilot should have grounded himself when he saw the relatively low stratus clouds, bad weather, and windy conditions. He showed bad judgment by going into the ravine given his inexperience.

Attitude—This is a severe problem for a many in this industry. We sometimes think we are never going to be caught in an accident. "It can happen to everybody else, but not to me." Let's lay down our own ego. We are not the best pilots in the world! We can always be better trained, with better knowledge, faster reactions, and let's face it, we will never know it all.

Awareness training at the factory every year is a good solution for keeping yourself updated with new information, and it will lead you to become a better pilot. Keep studying articles about accident prevention, fly regularly, and keep up autorotation skills with your CFI. It is never too late to improve your weak areas.

Johan Nurmi is an FAA Gold Seal instructor pilot and owner of USA Academy of Aviation, Inc. in Murrieta, California.

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