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Having Twins is Risky

Having Twins is Risky

For this article I thought we might look at something a bit different for a change – the climb performance (or lack of it) of a typical light twin engine aircraft after one engine has been shut down.

Most people, pilots included, assume that a twin engine aircraft is safer than a single engine aircraft as it can continue to fly following the failure of an engine. Whilst this is a reasonable assumption, it is not always true. Reference to an Australian Transport Safety Bureau (ATSB) 2005 report shows that approximately one third of all power loss accidents in twin engine light aircraft occurred during non-asymmetric power loss. The majority of these were related to fuel management issues resulting in double engine failure and a forced landing – in other words the availability of a second engine was of no benefit whatsoever!

According to the ATSB report the remaining two thirds of power loss accidents in twin engine light aircraft were related to loss of power on one engine. The reasons for these power losses were more varied with fuel management, fuel system problems, simulated and real engine failures being amongst the quoted reasons. It must also be remembered that apart from the issues common to both single and twin engine aircraft, statistically there is twice the chance of a real engine failure occurring in a twin, after all, all else being equal, there are twice as many engines! The trick to a successful outcome, of course, is to always take maximum advantage of the availability of the second engine and not let the increased engine failure probability get you. Operating a twin should be safer than operating a single, you as the pilot must make sure that that is always the case.

So how do we do it? Well first of all we must understand what happens to our twin engine aircraft when one engine fails. This article is not intended to be an exhaustive brief on how to fly a twin engine aircraft – your flying instructor or training pilot is responsible for that. What I want to do is cover some of the more salient points.

Firstly, if one engine fails during cruise or descent we should have no problems shutting the engine down and continuing to a safe landing. Engine failure is most critical during take-off and, to a lesser extent, during climb, particularly if obstacle clearance is an issue. When one engine fails we lose 50% of the available engine power but something like 90% of available climb performance! In addition to this we may have a very serious controllability problem, particularly if the airspeed is allowed to reduce toward the single engine minimum control speed (VMCA).

During multi engine training pilots are taught the correct actions required to correctly identify a failed engine and carry out the engine failure drills, particularly during the take-off phase.  After all, that is by far the most critical flight phase in a light twin engine aircraft. During training the instructor will ‘fail’ an engine over and over and over again until the trainees’ response is both accurate and instinctive. This is as it should be.

I have recently come across a case where a pilot suffered an engine failure at the worst possible moment on a flight – just at the point where the undercarriage was selected UP. With the option of aborting the take-off now not a real alternative for him, he elected to continue the take-off, just as I would have. He completed all of the required engine failure drills, feathered the inoperative engine and had the situation under control with sufficient airspeed to enable a safe, albeit minimal climb. Unfortunately, for reasons beyond the scope of this article, while maintaining a height of approximately 100 feet AGL, he turned back toward the airfield he had just departed from. The aircraft subsequently crashed and burst into flames following a progressive loss of airspeed.

So what are the salient points to be learned from this?

•         Even when the aircraft is correctly configured for a single engine climb the achievable climb performance is minimal, maybe 200 ft/min or less.

•         Any manoeuvering at all will reduce climb performance due to the increased drag. Even a gentle turn for instance incurs some increased drag due primarily to the increased load factor. The longer the turn is maintained the worse the situation becomes. Do not be ‘suckered’ into trying to return to the apparent safety of the airfield until a safe height (500-1000 feet AGL) has been achieved.

•         A small angle of bank should be maintained (approximately 5?) toward the operating engine. This has the effect of not only lowering VMCA, but also reducing drag slightly. Following engine failure straight flight, in equilibrium, with the wings kept level, requires a sideslip angle with deflection of both rudder and aileron to balance forces and moments. The disadvantage of this lateral and directional balance of forces and moments is that, although straight flight with wings level can be achieved, the resulting sideslip increases drag slightly thereby decreasing climb performance.

The last point I wish to make is that the ramifications of an engine failure are not over when the engine failure drills have been successfully completed, the aircraft correctly configured and climbing. The subsequent actions carried out by the pilot can be just as critical. During training it is usual practice for an instructor to ‘give back’ the ‘failed’ engine following successful completion of the engine failure drills by the trainee. Whilst it is difficult (and possibly dangerous) for an instructor to persevere beyond that point in flight, I consider it vital that the trainee be thoroughly briefed on the subsequent actions and the dire consequences of not following them.

Twin engine aircraft should be safer than single: it is up to the pilot to make sure they are!

One Comment

  1. This is a well written article about the silent hazards which always are somekind of a ghostly and unwanted passenger, traveling with us, when we are flying a twin. FI’s sometimes forget to talk about some important details when a pilot checks out on a twin. Especialy not mentioning the fact that even a shallow turn is extreamly unhealthy when there is not enough ground clearance present.

    Keep up the good work.

    Regards and wishes from Switzerland

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