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 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 well even with a 20 percent power “loss.” Not to mention, the next step that should be accomplished after POWER has been added is to consider the PROPS and, if more power is needed, push them forward before retracting FLAPS and GEAR.
(Power, Props, Flaps, Gear … that has a familiar ring to it, doesn’t it?!)
Here’s my strong suggestion: Yes, for the normal landing, leave the props alone until the RPM decreases. For many King Air models, this won’t happen until in the flare or even after touchdown. The 300-series is the exception, with the props going into an underspeed condition – slowing down, coming off of the governor – usually on short final. However, the instant the landing becomes abnormal in any way, forget the noise concerns and push the prop levers fully forward early in the landing procedure, usually about the time the gear is extended. These abnormal situations include, to me, not just the Short Field and Single-Engine situations that Beech addresses, but also include (1) very low ceiling and visibility precision approaches in which the chance of a missed approach is greater, (2) landing with very gusty winds, in which not only will power be jockeyed a lot more than normal on the approach but, again, the chance of a balked landing is greater, and (3) unusual and challenging visual approaches to airstrips in mountainous terrain.
Suppose that your King Air model rarely, if ever, carries passengers for the landing. Freight haulers, sky-diving jump airplanes, and special mission military operations come to mind. Now interior noise is not a factor and, to be frank, exterior noise from a landing King Air is relatively minor even when the props are turning at maximum speed. So it probably makes sense to complete GUMP and move the prop levers forward early for these operators.
As most of you know, the maximum propeller speed of various King Air models has been decreasing with the passage of time. From a 2,200 RPM maximum in the early 90s and 100s, it went to 2,000 in the 200-series, then to 1,900 in the F90-series and the C90GT variants (and the Blackhawk -135A installations), and finally down to 1,700 in the 300-series. “Great!” you say, “The lower RPM reduces noise and must be good thing!”
Well, it seems to me that the old adage “For every good, there’s a bad,” applies here quite well. Here’s the bad: If other factors are equal, the slower a propeller turns the more difficult it is to utilize Beta and Reverse. Let me try to explain; a graph may be helpful here. Imagine starting at a speed of 140 KIAS or more in your 90-series or 100-series model, pulling power back to Low Idle, and then holding altitude as the airplane slows. There’ll come a time when the propeller speed began to decrease, since the low blade angle limit of the propeller governing range – the LPS, or Low Pitch Stop – had been reached. The RPM would drop below 2,200 well before it would decrease down to 1,900.
Take a look at the graph:
As you can see, for the same propeller and the same Low Idle N1 speed, about 110 KIAS is necessary before the propeller speed drops below 2,200 RPM, yet about 95 KIAS is required to get below 1,900 RPM. Realize that Beta and Reverse are achieved by repositioning the movable Low Pitch Stop (LPS). Thus, until the propeller blade angle is being controlled by the LPS – and that only takes place when in an underspeed condition – utilization of Beta and Reverse is impossible.
The F90 made its appearance in 1978 and was the first King Air to have 1,900 RPM as its maximum propeller speed. For the first time, Beech actually added a comment into the POH concerning this fact. It states, “WARNING: Propellers will NOT Reverse at airspeeds in excess of 95 knots IAS.”
Consider this: If an F90 at maximum weight makes a No Flap landing, it should be at 127 KIAS crossing the 50-foot threshold point on landing. At touchdown, it is probably still going between 110 and 120. That means that no Reverse is available until about another 20 knots is lost while rolling on the runway. Good thing we selected a long enough runway!
Many F90s, F90-1s, C90GTs or Blackhawk-converted 90s, and Raisbeck-converted 90s and 100s, often do their cruising and approach phases with the prop speed pulled back to 1,700 or 1,750 RPM. Do you see what’s coming? My goodness, it will now take probably less than 80 knots to be able to enter Beta and Reverse with the governor set for such a low speed! So I recommend that the prop levers be advanced fully forward – or at least to 1,900 RPM for the Raisbeck systems that still have a higher maximum RPM setting – when flying these models unless you plan to roll a long distance on the runway to make a distant turn-off taxiway, without lifting the power levers until you are quite slow.
“But wait,” you may be thinking, “I’ll just go ahead and run the props forward at touchdown and then I’ll be ready to use Beta and Reverse, right?” The problem with that idea is, since you’ll still be on the governors after touchdown, you will get the resultant prop speed increase, additional drag, and very likely even some asymmetrical drag unless both props are rigged identically. The only way to avoid these annoyances is to wait long enough to observe the RPM decrease before pushing the prop levers forward.
Speaking of asymmetric drag, if you are experiencing some of this while flaring – perhaps it seems the airplane wants to start a little sashaying dance left and right – I think I know the cause and the solution. It only happens with the later style, Type II, prop synchrophaser systems. In this system, there is no fixed master and slave unit, but instead the slower propeller always tries to flatten its pitch to speed up to and match the faster one … in a very limited RPM range, of course. I believe when this is taking place, yet with the range of blade angle travel being limited by the LPS, we sometimes encounter this left versus right “battle.” Solution? Easy – turn the Prop Sync switch off before landing … just like you need to do with the older style, Type I system.
As mentioned briefly before, the 300-series airplanes are the exception to the need for a relatively low airspeed before reaching an underspeed condition, before being able to use Beta and Reverse. Their Flight Low Pitch Stop is set at a surprisingly large blade angle, causing them to reach an underspeed condition at a much higher airspeed than other King Airs. Their propeller governing range extends from 1,700 RPM maximum to about 1,450 RPM minimum and 1,500 is the common cruise and approach setting. When the power levers are reduced to Idle in the flare – this series really wants to float on landing! It is rare that the RPM will not immediately start to underspeed even below 1,500. So you may choose to not wait until after touchdown to place the propeller levers forward in these models, but instead get it done on final when you observe the decreasing RPM.
Conclusion? For normal operation, it is standard King Air operating practice to not move the propeller levers forward to the High ROM position until on the runway. That’s what I teach and do. However, except for the additional noise, there is no reason not to do it on the downwind leg or when approaching a Final Approach Fix. If you prefer to keep the habit patterns you have already developed, feel free to place the prop levers forward just as you did before. And, for everyone, when faced with an abnormal landing situation, be sure to select maximum propeller speed nice and early, no later than 500 feet above touchdown, if not well before then.