Page 18 - Volume 10 Number 7

Ask the Expert
Props Forward on Landing?
by Tom Clements
Ever since the first reversing propellers appeared on King Airs – with the introduction of the A90 in 1966 – there has not been universal agreement concerning where the propeller levers should be positioned before landing. The Beech Pilots’ Operating Manuals (POMs, earlier models) or Pilots’ Operating Handbooks (POHs, later models) were consistent in stating that the prop levers did not get positioned full forward until after touchdown for a normal landing. However, if and when the landing was not normal – the two examples covered by the POM/POH were Maximum Reverse Thrust (Short Field) landings and No Flap landings – then the published procedure was/ is to advance the prop lever(s) fully forward before touchdown. However, even though this has been official “factory standard” procedure for five decades, a number of King Air operators treat the King Air props very much like the ones on their previous Queen Air, Baron or Apache ... place them full forward for all landings. After all, what would GUMP be without the P?! (I know, wise guy ... GUM.)
Since most pilots transitioning into a King Air have previous experience in piston twins and almost assuredly received their multi-engine training in one, they have the “P” step – props forward – well- ingrained. So why would Beech change the checklist to leave props alone until on the runway?!
Noise. That’s the reason. Because of the free turbine nature of the PT6, it is easy for the props to turn at maximum speed even when relatively low airspeeds and low power settings exist simultaneously. Thus, when the prop levers are pushed forward, almost always the prop speed indeed goes to maximum ... with the resultant extra noise both inside and
16 • KING AIR MAGAZINE
outside of the cabin. On the other hand, most light piston twins will be in an underspeeding – “Off of the governor” – condition somewhere on the base or final leg and at that time the prop levers may be placed fully forward with no noticeable change in RPM or noise taking place. The benefit of this procedure is being more ready to reach full power in the event of a balked landing.
So does that mean that we are less ready for a balked landing in a King Air when we leave the props back at the cruise setting? Unlike the situation with some piston engines, there is no “over-boosting” concern when maximum torque is applied on a turboprop engine while the propeller is turning, even at the lowest speed setting of the governor. For example, a member of the King Air 200-series has a redline torque limit of 2,230 ft-lbs and a propeller governing range from 1,600 to 2,000 RPM. The maximum rated airframe shaft horsepower (SHP) is 850 and since SHP = Torque X Np X K, we can achieve 850 SHP only when both torque and Np (propeller speed) are at their respective limits: 850 SHP = 2,230 ft-lbs X 2,000 RPM X 0.00019.
It does not hurt a thing to set 2,230 ft-lbs while the props are back at 1,600 RPM, but now the formula shows we have only 680 SHP – a 20 percent reduction.
The conclusion here is that if we commence a Missed Approach or a Balked Landing by pushing the power levers forward to the torque limit – assuming ITT is not a limiting factor – while the prop levers have not been pushed full forward, no harm is done but we have not produced full power. Let’s face it, however: Unless we are talking about one-engine-inoperative operation, a twin engine airplane performs rather
JULY 2016
PAUL BOWEN PHOTOGRAPHY