High Performance Twins & Blueline Considerations
Turbocharged High Performance Twins and Blueline Airspeed Considerations
One point in this thread is that a checkout in twin engine airplanes should include an engine failure after takeoff from a simulated airport hard-deck of 3000 feet AGL. This can be done safely, easily and without harming the engines in most any non-turbocharged light twin. However, if the airplane you’ll be flying is turbocharged or for any reason you are reluctant to abruptly fail and engine in this simulation, a performance test can still be accomplished by very gradually reducing power and then feathering the engine. After the engine is feathered the single engine climb performance can be validated. If it is sufficiently positive then it will only be that much better 3000 feet lower after a takeoff from an airport at that lower elevation.
Now let’s talk BLUELINE.
After the engine is feathered, the pilot should experiment with various airspeeds above and below “Blueline” (Vyse) as marked on the airspeed indicator.
When the engine is feathered on the twin as in the above recommendation, turbo-charged or not, the pilot should experiment with the performance capabilities of the airplane. As I have mentioned earlier in this thread, don’t use simulated “0-thrust” as that will give false performance results either positive or negative. With the engine feathered (real 0-thrust) it is now most important to determine the climb performance capability of the airplane at various airspeeds above and below blueline, such as blueline plus and minus 5 to 10 knots. Surprisingly, you will see that in effect, best rate of climb in feet per minute is practically the same for plus or minus 5 knots of blue line. Therefore, in a practical sense, the blueline should really be marked as 10 knots thick instead of being shown as only one exact airspeed. You will also see that when the airplane is at a light weight, such as with one person and half tanks of fuel, Vyse is, in effect, reduced by as much as 5 knots.
So as an example, if blueline (Vyse) is 100 knots at maximum gross weight in a typical high performance twin, best rate of climb in feet per minute will be immeasurably demonstrated to be the same at 95 to 105 knots. So in this case, if the airplane is very light, the best single engine rate of climb airspeed will actually be about 95 knots, or 90 to 100 knots considering the reduction in Vyse for the reduced gross weight.
What does this all mean? Simply this, if you were to lose an engine just after takeoff with no runway remaining and no option for a safe landing, the gear was retracted and you checked your airspeed as 5 knots below blueline, use what has been learned from the 3000 foot AGL test, hold that airspeed (Vyse-5k) because there is no performance value in pitching forward to obtain blueline, plus such a pitch down is certain to cost altitude when none can be given up. Furthermore, if the airplane is lightly loaded, and your tests at 3000 AGL show a best performance rate of climb airspeed to be blueline minus 5 knots, then as much as 10 knots below the marked blueline may be used to prevent a loss of altitude. Pitching forward to obtain the marked blueline which would be an increase of 10 knots in this case, will probably cost as much as 50 - 100 feet of precious altitude. So it would be best here to hold the current airspeed of blueline minus 10 knots (Vyse-5k) and climb to a safe altitude, then gradually increase airspeed for further climb. Of course, in any twin engine airplane Vmc is a critical airspeed that must be constantly considered for safety too, and in the above example of blueline minus 10 knots this may be dangerously close to Vmc in some airplanes. Also, when flying with and engine shut down/feathered it is always a good practice to be within gliding distance of an emergency use airport should the remaining engine fail for whatever reason.
And in conclusion on the topic of blueline, let’s talk about blue line on final approach.
In a practical sense, blueline airspeed is used on final approach in most every twin engine airplane for both single engine approaches and normal two engine approaches. Why is this? The answer here is that there is no good reason except that it pre-bugged, blue is a pretty color, and that speed is not too fast or too slow, it’s just about right.
Blue line is usually held on final approach until 300 to 500 feet AGL whereupon full flaps for landing are normally extended and airspeed reduced to the normal over the threshold Vref speed of 1.3 x Vso.
So should blueline be used on final approach in a typical twin? The answer is yes, but not for any real aerodynamic reason. It’s just a pretty color.
Your questions or comments on this topic are welcome……
Are there anti feather pins on this plane? How about other light twins you are flying?
