Power settings for planned descent?

  • Does anyone have a good reference (youtube video or stuff to read) for recommended power settings (rpm and mp) when doing a normal descent of 500-1000 feet/minute? The accompanying manual+tutorial is a bit light on the topic.

    #notapilot 🙂

  • @theissondergaard There is no single "right way" to do it, but this is how I manage descents. It assumes cruising at 75% power. Warning: this works because the Arrow is not that slippery. If you do this in a Mooney or Bonanza without an initial power reduction it will probably result in excessive airspeed.

    • Start a 500 ft/min descent by leaving power alone and trimming nose down 5 miles away for each 1000 ft you need to loose. Be mindful to stay in the green arc of the airspeed indicator. (e.g. if cruising 7500 ft and the airport pattern alt is 1000, start 500 ft descent 33 miles out.)

    • Reduce power if necessary so you do not exceed 150 knots ground speed or if you are experiencing turbulence.

    • For every 1,000 ft you descend, reduce manifold pressure 1 inch and enrich the mixture slightly to maintain whatever power setting you've settled on given wind speed, turbulence, etc.

    • Landing light on below 3,000 ft (arbitrary, pick an altitude that works for you.)

    • This should get you to your pattern altitude 5 miles away from the airport but at well above pattern speeds. Level off and reduce power to 21 inches MP and 2400 RPM.

    • Slow to 110 knots IAS and drop the gear. If you are not slowing fast enough, either drop the gear or, if above gear extension speed, just climb a little to slow and then drop the gear. You will slow quickly with the gear down.

    • Add power and trim as required to maintain 90 knots in level flight and join the pattern. Perform a GUMPS check no later than midfield downwind: Fuel tank selector should be set to tank with adequate fuel, fuel pump on, 3 green on the gear lights, mixture full rich (or as required for high density altitude operations), Prop RPM lever full forward, seatbelts on. (At this point you are ready for a go-around if needed, optionally, if you are in a noise sensitive area, you can limit RPM to 2500 until you make the the MP change in the next step).

    • On downwind abeam the touchdown point reduce MP to 17 inches, and select the first notch of flaps. Increase rpm to max if not done already to be ready for go-around.

    • When runway is 45 degrees behind you, turn base, and add 2nd notch of flaps and slow to 80 knots

    • After turning final and making the airport is assured, put in the last notch of flaps and slow to whatever speed that works for you. in real life that's 78 knots for me (or 70 knots for short field) until I am over the threshold. Check gear down and safe once more on short final. Carry a little power into the flair, focus on the end of the runway, close the throttle a few feet above the runway and try to hold it off the runway by pulling the yoke slowly toward your belt and wait to impress your passengers 🙂

  • Every pilot has his preferred descent and approach procedure... I find that a good rule of thumb for descending is to always slow down first, then begin your descent. This will increase your angle of descent for any given descent rate, and reduce the horizontal distance required for the descent. Before descending to your planned approach altitude, first slow down to your planned approach speed.

    Reduce power to 10 - 15 MP and increase the prop to full Max RPM. Slowly raise the nose to slow down to about 100 knots and then apply Flaps - 10 to help control your speed during the descent, and to reduce the nose up pitch required for level flight during the final approach. Adjust the trim to establish and maintain a descent speed of about 85 knots, and adjust the power as needed to maintain a descent rate of -500 to -700 fpm, until you reach Pattern altitude (1,000 Ft. AGL), then slowly increase power to level off.

    Keep Flaps - 10 and maintain 85 knots during the approach. Don't lower the landing gear until just before you intercept the glideslope and are ready to begin your final descent. This will minimize the additional drag from the landing gear, and the amount of power needed to maintain your speed and altitude during the approach.

    After lowering the gear and beginning the final descent, reduce power a little and set Flaps - 25, then trim for 80 knots and readjust the power to maintain the glideslope descent. At about 500 ft. AGL set Flaps - 40 and trim for 75 knots. Increase power slightly to maintain the glideslope descent.

    Using a descent and approach speed of 85 knots with the gear UP also provides a good safety margin in case of engine failure... Best Glide Speed is 79 knots at MTOW with Flaps and Gear UP, so all you have to do is raise the flaps and re-trim for that speed.

  • @RetiredMan93231 Let me apologize in advance for being a bit off topic for a simulator forum. My comments here pertain more to real life operations and efficiency than anything to do with flying the JF Arrow III.

    I am not a fan of low power descents. For one reason, at high RPM and low MP, especially in a descent, the prop begins to drive the engine which generates "negative torque". That can damage the bottom end of the engine and should be avoided. I've seen a number of placards and warning arcs on tachometers that state to limit high RPM low MP operations.

    Another reason is low power descents will shock cool the cylinders. Lycoming suggests avoiding low power descents here.

    Lastly, a relatively higher speed full power decent as high cruise speeds is very economical fuel wise. You've spent a lot of fuel getting to altitude and have a lot of potential energy stored up. You can boost you MPG and overall operating efficiency by using either cruise speed or slightly higher speeds in the descent. The last thing you want to do for efficiency is drop flaps or gear early in the descent. You end up spending the stored potential energy heating up the air rather than getting to your destination.

  • Incidentally, i ran into this on YouTube:
    This guy gives a rule for descent: every 1in of MP = -100fpm in decent. So if you want to descent at 500fpm from straight and level flight, pull back the throttle 5in of MP. No idea if this only applies to the Comanche shown, and no idea if this is a good idea or not. Just thought i'd share.

  • @BernieV, Thanks for sharing your valuable incites as a real world Piper Warrior pilot... I totally agree that you should avoid dropping gear early in the descent, because it just adds a lot of extra drag, which must be offset by higher power and fuel consumption. I also agree that you should not let the prop generate negative torque, that's why I use flaps -10 to add some drag during the descent. But, perhaps also setting the prop to 1400 - 1500 RPM before beginning the descent would be a better procedure than setting it full forward. I believe that if the throttle must be set higher than idle to maintain your descent rate , the prop is providing positive, or neutral, torque... In any case, I'm sure there is more than one way to perform a proper descent and approach, and that I still have a lot to learn as a pilot... But, since I can no longer pass the medical, I'll have to settle for simulated flight only. Fortunately, with the great advances in realism of this simulator, and this outstanding JF Piper Warrior aircraft I can continue to do that...

  • Hi all, thanks for outstanding responses to my initial question. This was exactly the kind of input I was looking for. I understand that there are many correct (and even more incorrect 😃 ) ways of doing this, so I really appreciate all the perspectives.

    I'm trying to get to grips with how to manage a constant speed prop, so my initial understanding of "lower your power to drop altitude" needed some refinement when I now have TWO levers to be wrong about 🙂

  • @theissondergaard said in Power settings for planned descent?:

    I'm trying to get to grips with how to manage a constant speed prop, so my initial understanding of "lower your power to drop altitude" needed some refinement when I now have TWO levers to be wrong about

    It does take some getting used to... especially after flying a fixed prop plane for a long time. It still doesn't feel natural when I advance the throttle and I don't hear the engine rev up! I compare it to learning to drive a car with only one gear, then driving one with an infinite gear shift and much higher performance... Just treat the prop like the gear shift on a car and you'll be right most of the time! Unlike a car, you also have to manage Fuel Flow and EGT with the Mixture control, which gives you even more control over the final engine performance and power output, but it can also be more confusing!

  • @RetiredMan93231 We are all just one medical away from the end of our flying days. Sorry to hear it. I am not too far behind you. I've just retired, so the financial aspect of maintaining my own plane is looming larger as well. When the day comes that I sell my bird, I will be very happy that I have this model to fall back on.

  • @Cristi-Neagu I did a few tests starting at max cruise speeds at 9000 and 6500. By varying the throttle only, you can expect an initial descent of 125 ft/min for every 1 inch of manifold pressure reduced. A 4-4.5 inch reduction will get you 500 ft/min. If you hold the same altitude and allow the plane to settle after a power reduction, you can expect about a loss of 2 knots TAS for every reduction of manifold pressure of 1 inch.

    Other constraints you need to be aware of when descending;
    Max gear down speed is 129 knots IAS
    Max Fap Extension speed 103 knots IAS
    The Arrow (including the JF Arrow) has a placard to avoid continues operations below 15 inches and between 1950 and 2350 RPM.

    A sage AP I trust once told me to limit MP reduction to 1 inch per minute to prevent the cylinder heads from cooling too fast. This is not something you will read in the POH, but its good advise unless you like buying and breaking in cylinder heads 🙂 You can't do this in the pattern or when on instrument approaches, but you can when planning descents which is why I mention it here.

  • @BernieV said in Power settings for planned descent?:

    A sage AP I trust once told me to limit MP reduction to 1 inch per minute to prevent the cylinder heads from cooling too fast.

    I heard that too, and i think i heard it from Rod Machado in the FS9 flight school. The training material supplied with that game was really, really good.