Understanding Twin Engine Aerodynamics

Twin engine aircraft are still the backbone of the corporate transportation world. The Beechcraft King Air is emblematic of what “makes business work” in the US. To “fly corporate” you still need to obtain a multi-engine rating and build that “twin time.” After the piston comes the jet and most of these seem to have at least two burners. Unfortunately it is increasingly difficult to find a light twin in the GA training fleet or on a local airport. Slippery carbon fiber, single engine, fixed gear planes are largely the small business airplane and as they move up the single engine turbo-props like the TBM, PC-12. Meridian have taken over in the flight levels.

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Along with the disappearance of the Apache, Aztec, TravelAir fleet, the knowledge and craft of multi-engine flying is also getting harder to obtain and master. This is a brief overview of the subject with a special emphasis on the problems of flying on one engine in the event of a problem. Further articles on this subject are available on our multi-engine training page…this is just a warm-up to the subject.

The wake-up call for small twins came in 1979 with a special NTSB report that studied 2,229 accidents over a five-year period. Of these, 610 accidents involved fatalities and 123 resulted from engine failures. In nearly 3/4 of these cases pilots lost control of their plane due to the asymmetric thrust from the lost engine. The percentage of fatal accidents involving engine failure was more that four times greater in light twins than in single-engine planes. Lack of pilot proficiency was cited as the cause of most of these accidents and four specific recommendations were put forth by the NTSB. Lets first examine the problem and then we will look at the NTSB ideas.

The basic problem is obvious if you study a wing mounted twin-engine aircraft. If one of those fans goes bad that plane sure is not going to fly straight or pretty. But the major problem is more what my former chief pilot used to say when he saw a light twin take off. “You know why that plane has two engines? Because it needs them both” Once you understand how pathetic the performance is on one engine, you are a long way toward safety. When an engine fails in a light twin you absolutely must sacrifice performance and achieve control instead. A good rule is only try to achieve performance once you have controlled the adverse forces. Though you lost half the power when an engine goes bad, 80% or more of your climb potential just died too.

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It did not help of course that manufacturers like Piper used to put pilots in the field who would demonstrate single-engine take-offs in the Aztec. This was fueling a myth that only could be achieved with superb piloting skill (and a very light aircraft) Load it up with fuel and family and most light twins can barely maintain altitude on an ISA day with an engine out. Pilot handbooks of the day were often written more to sell airplanes rather than insure safety.

Here is a fun experiment for a single-engine pilot to understand the dilemma of a twin engine failure. At a safe altitude, after clearing carefully, fly your plane at the best glide speed (Max L/D…minimum drag) at a level altitude. Make sure you get very stable and control the left-turning tendencies with rudder and carefully note the power setting. This number in most small singles will be 60-70% of your rated power (this assumes max gross weight) In a C-172 this is around 1900rpm.This is the minimum power it takes to maintain level flight. The difference up to maximum rpm is your climb potential. Usually, with coordinated controls, you can assume about 100fpm for each 100 rpm of surplus power available…500-700fpm. (Here is where power to weight ratios really count and streamlined airframes rule!) The point however is to gain respect for how little climb potential ANY plane has. A twin with an engine out lost 50% of it’s power but most of that is what allows it to climb! The remaining 50% just might keep you in the air (if you do everything correctly)

“Doing everything correctly” in the heat of the moment is a tall order for a twin driver. Imagine you are climbing out at max power and  max gross, (gear up) when an engine suddenly fails. That operating engine will twist the plane dramatically and the loss of lift on the dead engine side will dramatically amplify this force. One major flight control opposing this yaw and roll is the rudder. Since this air control surface loses effectiveness rapidly as you fly slower, it’s critical to lower the nose and maintain airspeed (while holding the nose straight) as a first remedy. Sacrificing climb to get control is essential. Once basic control is regained, there are many tweaks to optimize the performance that are beyond the scope of this article. Suffice to say we must avoid flying slowly of we lose rudder authority and the yaw force will win. The critical loss of control occurs at what is called Vmc or “velocity  for minimum control.” There are detailed specifications for Vmc in CFR 23.149 for manufacturers to determine and mark this airpeed (the “red line”) The important take-away must not get near this minimum speed or we will lose control. It would be better to reduce both throttles to idle and crash level lie a single than to roll over from the asymmetric forces. The high AOA coupled with the huge yaw lead to loss of control pretty quickly.

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The joke in multi-engine training is that since you mostly fly on one engine anyway (since the goal is mastering this exotic skill) shouldn’t the cost be half? You get exceptionally fast and skilled after five to ten hours of training (and hopefully even pass an FAA check ride and add this too your certificate). Unfortunately like all carefully honed skills this quickly becomes rusty without practice. Hours of successful multi-engine flying lead to complacency. This is why the NTSB statistics are so grim on twins.

And the recommendations? More recent currency for pilots, better POH information and increased outreach to rated pilots on light-twin problems. An interesting idea still not implemented by the FAA is for multi-engine pilots to accomplish their flight review in a twin to retain their privileges. Hopefully the overview is clear; the price of flying a multi-engine aircraft is maintaining superb currency. If you don’t do this “luck and hope” are two of your primary flight planning tools…never a great idea. Here is a great article to dig deeper…good luck!          ©2015, David St. George

 

 

 

Fly Accurately, Practice Often, Avoid LOC!

  • Fly Accurately: Know your pitch/power for all maneuvers…think “total energy”
  • Practice Often: Too much “enroute autopilot”? Go “yank and bank” (steep turns, “dutch rolls,” slips) Challenge yourself!
  • Stay Sharp and Avoid LOC: Practice out of your “comfort zone” with a CFI…become “bulletproof!”

The most pressing area of concern in general aviation safety is  I-LOC; “In-flight Loss of Control.” This sounds like a new and surprising problem, as if the plane will jump out of your hands but it really is a problem that has dogged aviation safety for years:  “maneuvering flight”. Whatever you call it, over 40% of pilot accidents happen in an area of flight where we only spend 5% of our time; maneuvering flight and usually below 1000ft agl. This would of course include that inherently tricky area of take-off and landing.

As a long time CFI and DPE let me assure you that flying is safe and what we are addressing here are the frustrating and almost predictable accidents that seem to happen regardless of training and interventions. In aviation safety, we have harvested all the low-hanging fruit; our planes are now super safe and we have incorporated all kinds of technology to help. We have displays with current weather and other planes in the cockpit and GPS to drive us  precisely to our destination. What does not seem to be “fixable” however is the pilot handling the controls when the demands of the task are suddenly overwhelming. Stall/spin accidents account for 21% of fatalities in aircraft…we need to learn to control our aircraft correctly for safety not just in the center of the control envelope but also toward the edges. This involves both basic aerodynamic knowledge and recent and correct practice at the controls of the aircraft. This Vg diagram depicts the maneuver envelope for a GA aircraft. The yellow oval is where we spend 95% of our time…safe and comfortable with little real demands of pilot skill. Pilots who spend all their time here are are not prepared if they are suddenly displaced from their “comfort zone!”VgCenter

The problem starts with the fact that the fundamental physics of safe aircraft control is not all intuitive. Flying safely requires deeply embedded trained responses based on extensive knowledge and good habits. Piloting skills need to be correctly taught initially and fully assimilated but also practiced regularly to be available, in fluid form, when a sudden upset demands action. Flying only in the center of the flight envelope will not keep you sharp enough to be safe…you need to be challenged with some dual flight on a regular basis. If a pilot wants to climb in a plane, just pulling on the yoke will not work (for long). If you want to turn, mashing a rudder in that direction will not work (for long). To be safe in a plane and avoid I-LOC, you must bank the plane in a coordinated fashion and understand and manage angle of attack. Mishandling both of these control inputs together is the heart of I-LOC…an aerodynamic stall (excessive AOA) with a lot of yaw (turning with the rudder)!

Angle of attack cannot be easily explained in an blog article, it really has to be demonstrated to be effective. Suffice to say here that AOA is usually almost invisible to the pilot. A plane in a snapshot in a level flight attitude could be climbing or descending or even stalled and falling, but its impossible to tell without movement. The trend would reveal the AOA as would the position of the yoke. The amount of back pressure and the extent of geometric pitch is a good rough estimate of AOA. Pilots absolutely must practice and understand this critical part of aircraft operation.This classic diagram from Aerodynamics for Naval Aviators clearly diagrams this problem.

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The recent “seawall approach” into SF international in a Boeing 777 was a classic student pilot level error in aircraft control. It is what we call “naive rendition” in the flight instructor world: what you *think* should make the plane do what you want…just pull up the nose and it will climb…NOT! Actually, nose pitch controls airspeed primarily and adding power is necessary to arrest a descent and “stretch a glide” to the airport when low on final…not intuitive! In this photo, both the nose high (scary) Aircraft A and the nose low (friendly) Aircraft B have the same angle of attack. The plane can quite easily stall with the nose below the horizon with G-forces induced by a turn.

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Turning coordinated using balanced rudder and ailerons seems to be increasingly rare in the pilot community and it is the heart of avoiding I-LOC. This essential skill is often missed in initial pilot training due to all the other “important” items that have to be covered in flight training…is my sarcasm showing? Nothing is more important than turning coordinated with balanced rudder and aileron! Pilots in training often miss this due to focus on glass panel avionics and other “shiny toy” distractions. Find a good instructor and practice this vital skill in your plane. Watch directly out over the nose when rolling the plane and the coordination will be immediately apparent. Watching the ball is absolutely the wrong place to focus…you are already behind the machine. “Dutch rolls” on a point will give you a good feel for the rudder and develop confidence in the outside visual cues.

Understanding slips and skids are also essential to safety in an airplane…but I doubt more then 10% of pilots could describe the process. (Keeping the plane from turning with the rudder is a slip and aerodynamically  stable…read this). Often used to land in a crosswind or add drag to increase the descent rate, a slip is a required maneuver for all private pilots candidates….learn and practice this maneuver with a good CFI.

Ultimately what I am am recommending here is a solid aerodynamic understanding of flight (AOPA course is excellent for this) and good “stick and rudder skills” to be safe and avoid I-LOC. Maintain a regular recurrency flight routine to stay sharp. Participation in the FAA Wings program is an excellent way to do this. Spend some time in our tail wheel aircraft to acquire an understanding of correct rudder usage. Practice some dutch rolls on your annual check-out. Above all stay sharp with your maneuvering skills…just droning on autopilot in the center of the flight envelope will not maintain the requisite pilot skills for safety. As the professionals joke “safety is not an accident” it take regular practice to stay safe.    ©2015, David St. George