Single vs twin

Single vs Twin Which is Safer?

(From FAA Aviation News/March - April 1991)

Spring has sprung (in most of the country, that is), and it is the time when a pilot's fancy turns to doing more flying in the balmy air. Spring is the time when new plants sprout, new animals are born; so perhaps you have considered having your certificate 11 grow" a new rating. Perhaps that new rating you are considering is a multiengine class rating. You have heard other pilots remarking that it only takes a few hours to earn, and there is no requirement for a written test. Why not? A few hours in the air in a 11 real" airplane, and you walk away with an authorization to fly airplanes with more than one engine. Besides, everyone knows two engines are better than one-right? � Editor

According to an old saying, "There is safety in numbers." If the saying is true ' if one engine is safe, are two engines twice as safe? The following short, simple, single-versus-twin-engine pilot test may help you decide. The test is designed to test your knowledge of safe operating procedures for one and two engine light general aviation aircraft. Depending on your number of correct answers, you may be: a passenger, a student pilot, a pilot, or an aircraft survivor. Match the following questions with their correct answers. The correct order is given at the end of the article.

Questions:

  1. When are two engines better than one?
  2. Why have two engines?
  3. When is one engine better than two?

Answers:

  1. When you crash.
  2. So you can pick the spot for an emergency landing.
  3. When one quits.
  4. I don't know/I don't care.

Score based on the number of correct matches:

0 = passenger; 1 = student pilot; 2 = pilot; 3 = survivor.

The questions are variations of the single engine versus the light-twin-engine aircraft controversy pilots have argued about since Orville Wright made his first flight in a single-engine, twin-prop aircraft. Each question contains an important safety message for pilots of both single and light-twin-engine airplanes.

Although two engines provide a twin-engine aircraft a degree of safety through redundancy, that safety factor can be offset by the pilot's lack of knowledge about light twin operating characteristics. Misconceptions about "two are better than one" have caused many pilots grief. The reason is simple. Most light twins (for the purpose of this article, those under 6,000 pounds gross weight and/or with a stall speed of 61 knots or less) lose about 80 percent or more of their power when an engine fails, rather than the 50 percent one would expect. That 80 percent or more power loss is why, under certain conditions, a light twin may only have enough power after an engine failure for the pilot to pick a spot for an emergency landing. The aircraft may not have enough power to hold altitude or fly safely on only one engine.

The problem is some twin pilots may decide to risk continued flight when the safest decision may be a controlled emergency landing in a place of their choosing rather than risk an out of control crash. Single-engine pilots do not have the same type of problem. Their decision process is simple. They lose an engine; they land!

Since an engine failure can ruin any pilot's day, the following is a review of some practical takeoff safety tips for those pilots who may not have flown much during the winter. Maybe some of the ideas will help some pilot prevent an engine-out emergency or at least minimize the risks of one. The survival key for both single and light-twin-engine pilots is their flight planning before the engine starts, not after it stops.

As in any article discussing flying ideas and safety tips, the pilot operating manual is the authoritative guide for the safe operation of a specific model of aircraft.

According to the latest available National Transportation Safety Board (NTSB) general aviation safety report, Annual Review of Aircraft Accident Data for General Aviation Calendar Year 1987, the two most dangerous first phases of flight during the period 1982 to 1987 were takeoff and landing. Landing was the most dangerous accounting for about 25 percent of the accidents, and takeoff was second at about 20 percent. This article will focus on takeoff techniques with special emphasis on twin-engine operations, since FAA Aviation News recently published a story on landings titled "The Stabilized Landing Approach." (May-June 1990).

The first flight planning step for any flight is a review of the aircraft's performance data. The data provide takeoff, en route performance, and landing information needed by the pilot in deciding if the flight can be flown safely. When reviewing and using the performance information, each pilot should use very conservative estimates for the takeoff distance and climb performances listed in the operating manual. The average pilot and your basic used aircraft may not be able to match the handbook's performance information.

After reviewing the performance data and computing weight and balance information, each pilot must decide if it is safe to takeoff, fly en route, and land at the destination airport.after considering such environmental factors as density altitude, wind direction, obstacles, and runway conditions such as length, slope, and type of surface at both the departure and destination airports. The information will also help satisfy the requirements of FAR  91.103, Preflight action, which states in part, "Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight." The FAR then lists specific aircraft performance, airport, and runway requirements.

In addition to the normal flight planning requirements, a twin pilot must also determine if a safe flight can still be made after losing an engine. In many cases after losing an engine, a light-twin-engine aircraft cannot, or should not, continue the flight because of the loss of power and such adverse environmental conditions as high density altitude, high terrain, or IFR minimum enroute altitude requirements that can often exceed the aircraft's single engine capabilities.

As part of each preflight planning session every pilot should also determine the minimum runway length needed for takeoff. In some cases, such as high density altitude, the minimum length needed may almost equal or exceed field length. A safe pilot may decide this is an unacceptable risk. If the runway length is adequate, the horizontal distance needed to climb to a safe maneuvering altitude, not just the distance to clear the standard FAA 50-foot tree at the end of the runway, should also be calculated. Under some conditions, such as high density altitude or airport elevation, some aircraft cannot climb fast enough or high enough to avoid some of the obstacles near some airports around the country.

Because density altitude is one of the most important factors in determining an aircraft's performance and ability to fly, it should be calculated before each flight to ensure adequate aircraft performance. High density altitude can reduce an aircraft's capability below acceptable safe limits. It can also destroy what little single-engine capability a twin aircraft may have. But how many pilots routinely compute density altitude (DA) or even remember what information is needed to compute it? (DA is pressure altitude corrected for non-standard temperature using either a flight computer or chart. Pressure altitude is the altitude read on an altimeter when 29.92" is set in the altimeter setting [Kollsman) window.)

After computing density altitude and runway takeoff distance, another important flight planning item each pilot should (as some operations require) compute is the aircraft's accelerate-stop (A/S) distance. A simple definition of the term is that distance needed to accelerate an aircraft to rotation speed, for the aircraft to lose power at that moment, and then for the pilot to be able to stop the aircraft on the remaining length of runway based upon airport conditions and aircraft load. The A/S decision point is what separates single-engine pilots from multiengine pilots. UP to A/S, each type of aircraft can stop on the runway. After A/S and up to a certain altitude a single-engine aircraft is off the runway and into something, be it grass, fences, trees, or houses. However, a twin pilot may be able to continue the takeoff and return for a safe landing. The key word is may. For, like their single-engine counterparts, sometimes the safest decision twin-engine pilots can make is to execute a controlled, survivable landing off the runway in a place of their choosing rather than risk trying to go around in an aircraft that can not safely fly. The only way to know if your aircraft can safely takeoff with only one engine operating is by understanding your flight manual and your own operating techniques and aircraft. Some flight manuals provide detailed performance information that can be used to compute continued takeoff capabilities with only one engine operating. Just be sure you have read and understand all of the small print that explains the specific conditions that apply. If your manual does not list the information, maybe your aircraft is not capable of continuing a takeoff with only one engine operating.

The A/S point is arguably the most critical decision point in the twin pilot's GO/NO-GO decision process. The number of decisions a twin pilot has to make at the A/S and later during the takeoff procedure, such as making sure Vmca, minimum controllable airspeed, plus a suggested safety factor of at least five knots or the manufacturer's recommended speed is reached before rotation, knowing Vx and Vy for both single engine and normal two-engine operations, and being able to control and fly the aircraft with only one engine operating are what make flying twin-engine aircraft so complex.

Understanding and calculating A/S distances are not enough. Pilots should prepare themselves for possible takeoff emergencies by reviewing their aircraft's emergency procedures. One good review technique is the "what if" scenario. Pilots can prepare for any possibility by asking such "what if" questions as, "What if I lose an engine" and then reviewing the correct procedure and options. Valuable time can be saved during an emergency if the pilot has memorized the important emergency checklist steps as part of the initial aircraft checkout. Once the emergency situation is under control, the pilot can then review the complete checklist to make sure every item is done.

After reviewing all the flight planning steps and going out to the aircraft, the best way for any pilot to avoid or minimize an engine problem on takeoff is a careful preflight and ground check using a checklist.

And the ground check does not stop with only an engine runup. Every pilot should check to see if their aircraft is developing full power early in the takeoff roll. By detecting a power problem early in the takeoff roll, the pilot can abort the takeoff before passing the critical A/S point. The power check is done in two easy steps. First, the pilot checks the aircraft's instruments for proper indications. Obviously this means the pilot must know the aircraft's normal indications. Then the pilot cross-checks the instrument readings against an "outside" reference. The check is made by comparing how much time or distance the aircraft normally needs to accelerate to rotation speed to the current takeoff roll. If after the normal amount of elapsed time or distance is used (distance can be measured by runway distance markers, a tree, brush, or some other point along the runway) and the required airspeed is not obtained, the pilot should abort the takeoff and find out why. The time to abort a takeoff is before the aircraft runs out of runway, not after it runs off the runway or into the trees. The importance of aborting a takeoff when something is not right cannot be over emphasized. Lives have been lost when takeoffs were not aborted.

A well publicized case of an aircraft not producing enough power to takeoff and fly was the Air Florida B-737 crash at Washington National Airport on January 13, 1982. After liftoff, the aircraft hit a bridge near the airport and crashed into the Potomac River. Seventy passengers and four people on the bridge were killed in the accident. The National Transportation Safety Board's report listed as one of the probable causes of the accident the "... captain's failure to reject the takeoff during the early stage when his attention was called to anomalous engine instrument readings." (Editor's note: Snow and ice were major factors in the accident.)

As the example shows, not only is it important to be able to determine if an aircraft is developing enough power to take off and fly, it is equally important to know how much distance is needed to stop it safely. This is why an aircraft's accelerate-stop distance is so important. This distance is what separates the single-engine pilot's limited choices from the twin-engine pilot's options.

A single-engine pilot has few choices after passing the A/S point and having an engine failure. In most cases the only choice is selecting what not to hit. This is where prior planning is important. A safe pilot, either from being familiar with the local area, or having reviewed the airport environment before landing, should know the best emergency landing spots straight ahead (making only slight turns to avoid obstacles) off the end of the active runway, considering such things as, powerlines, obstacles, trees, open areas, houses, fences, roads, and the location of high population sites, such as schools, near the airport. The safe pilot will then have a plan ready in the event of any type of emergency.

Like their single-engine counterparts, twinengine pilots can also make a controlled emergency landing straight ahead. Unlike their single-engine counterparts, twin-engine pilots may be able to continue their takeoff on one engine and return for a safe landing. The key word is may.

The problem is most light general aviation twins are not required to hold or gain altitude with only one engine operating. And that one operating engine may not be producing even the limited performance listed in the aircraft's operating manual since engine performance decreases over time. (This is the basis for the saying about being able to pick the spot where you want to crash,) Because of a twin's limited performance with only one engine operating, its pilot must also use the correct single-engine techniques to fly the aircraft to have any chance at all for a safe flight. Anything less than perfect technique can result in loss of altitude, control, or both.

No matter how well both twin- and single-engine pilots do their flight planning and preflight their well maintained aircraft, accidents will continue to occur during takeoff. Engines will fail. Pilots will lose control. Accidents will happen. To reduce those risks both during takeoff and later phases of flight, pilots must understand their operating environment; their aircraft's performance and operating systems; and their own flight skills and abilities when deciding on a safe or survivable course of action when confronted with an emergency. In some cases, a wise pilot may decide to either delay the flight, reduce the aircraft's load, or modify the route of flight to avoid or minimize the risks of a takeoff or en route engine out emergency, even if the aircraft does have two engines. Because they know there may be times when even two engines are not enough insurance for a safe flight.

But, whether you fly a single, or twin-engine aircraft, hopefully, this article has provided you with some interesting safety ideas to think about. Regardless of the number of engines on your aircraft, the best insurance policy you can buy for a safe 1991 flying season is a thorough flight review with' your local certificated flight instructor (CFI). A flight review is especially important if you have not flown recently, or if you are not current in type. (The CFI must also be current in the aircraft used for the checkout.) In addition to ensuring a safer pilot, the checkout could also meet the pilot's requirement for a biennial flight review and/or the flight portion of the pilot's next set of "Wings." ("Wings" is the FAA's Accident Prevention Program's Pilot Proficiency Award Program.) One of the things you may want to discuss with the instructor is how to calculate your own accelerate/stop distance. If you have access to a long runway, with your aircraft at gross weight and using your normal takeoff technique, accelerate to rotation speed and then abort the takeoff and stop. The total distance from your start point to where you stopped is your rough A/S distance. To add an element of surprise to the test, have the instructor pull the power at rotation speed and note the distance. The distance should increase by several hundred feet because of your normal reaction delay. Once you have an estimate of your personal A/S distance, always add some distance for the unknown. Remember that rolling takeoffs add distance to A/S distances. Always leave yourself an out.

If you are flying a twin, after you have determined your own A/S distance, take advantage of the instructor's presence by testing you and your aircraft's ability to execute a single-engine go-around. Simulate a single-engine go-around at a safe altitude and see if your aircraft and skill will allow a successful go-around. Some aircraft are not capable of making a safe single-engine go-around. One FAA safety pamphlet says a single-engine go-around may be impossible unless you have several hundred feet of extra altitude above the terrain and an airspeed above Vyse. The situation is particularly critical it you lose the engine during a normal go-around.

The moral of the story? Listening to a lot of hangar flying about light twins is not what sharpens your edge in flying them. The only way to minimize the risks in any kind of flying is to learn, learn some more, and learn again. Flying multis is fun and challenging, but never, ever take that extra engine for granted.

And it does feel so good to see these words on your certificate: Airplane-Multiengine Land.


The following sources provided ideas and information for this article: Mr. George Lutz of Springfield, VA; the FAA's Accident Prevention Program's pamphlets titled, "Planning Your Takeoff" and "Flying Light Twins Safely;" the FAA Flight Training Handbook; and the NTSB Aircraft Accident Report NTSB-AAR-82-8.

Correct quiz answers: 1c, 2b, 3a.