General Flying: Letting down from altitude without overspeeding the airframe

[Chris In Milwaukee]

Hi everyone,

While the plane is being built, I've been flying a C-140 to acquire the tailwheel skills I need. But when I need to actually get somewhere today, I also have a Cessna 172RG Cutlass.

Last weekend, I flew up to Grand Marais, MN. Beautiful spot to visit, and a potentially challenging airport on a hilltop with gusty winds at almost 90 degrees to the runway (at least on the days I was there).

Normally I like to fly low, but due to wildfires in both Canada and Minnesota, on the way back I needed to fly high to stay above the smoke and haze. It was a great flying day, very smooth at 8500 feet where I was cruising along at 136 kts true and 165 kts ground speed. But eventually all good things must come to an end, and I had to descend. As I dropped the nose and trimmed for 500 ft/min, I was creeping up to around 140-142 kts. The top of the green arc in this plane is 145 kts, and the air was smooth, so I wasn't too worried, but I like to stay away from the yellow if I can.

Because the Cutlass has a constant speed prop, it's not like the fixed 172 where you can just pull the throttle back to slow down. You pull back too far, and of course you get the "gear is up you dope" honk when the MP goes below 12 inches (knowing that jaredyates has an issue with flying the warning tones ). I'd heard the stories of folks flying behind a CS prop that says at high RPM, props make great brakes. So I spun the prop up to 2700 rpm. It didn't seem to make a difference, or very little that I could tell. My in-flight checklist for descent just says "Power as needed". So I just put the prop back to 2300 rpm where it was before and watched my speed on descent, reducing angle as necessary.

Once I was on the ground, I was thinking about how to deal with descents of this nature, so I thought I'd reach out to you guys. The one thing I didn't do was run the prop in the other direction (<2300 rpm). Would that have helped? I recall reading that the courser the pitch, the less drag it has (like feathering).

Thoughts are welcomed!

Best regards,
Chris

[Baloo]

Great question Chris,
Up in the higher elevations it can definitely be a bit of a challenge. The hard part is reducing power enough to be able to descend without shock cooling the engine while still allowing you to get down without going too fast.

A few things that I have done in the past include:
- Plan your decent early if possible and start reducing your power slowly in level flight over the course of a few minutes. Let the speed decay as the power is reduced and one your speed is back then start your gentle decent. Doing it this way makes it much easier to manage your speed/decent rate because if you point the nose down and then reduce power it is too late and the thin air makes it very difficult to go down and slow down. Every 2000 feet or so level off and add a bit of power to keep the engine warm and happy, rinse and repeat.

- You could also try lowering some flaps at altitude after you have slowed down. Yes this will reduce your max speed but it may enable you to increase your rate of descent in a pinch or at the very least it will bring your deck angle right down.

- In terms of RPM setting, yes in theory moving the prop to full fine should help you slow down if you have a fat prop but is the extra noise really worth it. If found in most light singles pushing the prop in doesn't do as much to help your descent at altitude as it does close in to the runway.

- Don't forget to richen the mixture as you descent. The last thing you want to do is level off and apply modest power only for the engine to cough.

These are just a few things that do but each to their own.

[jaredyates]

The shock cooling issue is about as contentious as the primer and TIG/OA arguments. I fall on the &quot;not going to be concerned about it&quot; side, but this is a decision for each engine owner. There is good reading on the topic in Kas Thomas's book Fly the Engine which is here: http://amzn.to/24Lq81X

[Baloo]

Thanks Jared, Ill check it out. When I was flight training 15 years ago we would be in the practice area doing forced approaches etc. We had one instructor that would pull the engine to idle to start the exercise. The school policy was to &quot;warm&quot; the engine every 1000ft with a shot of power. If we forgot to warm the engine the instructor would have us open the window and stick our arm out into the cold Canadian January air until we returned to base to teach us what our engine felt like. I guess my fingers are still thawing out and wrong or not its a lesson that has always stuck.

[JimParker256]

John Deakin has an excellent article on this topic here: Pelican's Perch #66 The whole series is excellent, and you may want to read through the entire "Where Should I Run My Engine" series (Index). But in this article he specifically addresses the "Crowbar Descent" (how to lose a LOT of altitude quickly) near the bottom of the article, stating that you first reduce the RPM to the lower end of the green arc. Next, you lower the nose gently to maintain airspeed, then do what he terms the "big throttle pull": Reduce the throttle smoothly (over 5-8 seconds) all the way down to 15 or so inches of MP. Keeping the airspeed up should avoid the gear up "screamer" - at least that's how it worked on my Commander.

Ironically, I sold my Rockwell Commander 114 to finance the Patrol AND to purchase a 1965 Citabria 7ECA (O-200 and oleo gear) to acquire the tailwheel skills and to fly while building the Patrol. At this point, 100 knots is pretty much "max" cruise speed... Funny how quickly your perspectives change! With the Commander, I prepared for my trip to Sun-N-Fun by practicing maintaining airspeed down to 100 knots from well outside the airport area, to the pattern entry and all the way to short final, as stated in the NOTAM for the SNF arrival... Now with this plane, I'll have to do it again, but this time trying my best to keep the speed up to 100 knots (115 mph is max cruise)! Bleeding off airspeed on short final (without flaps or constant-speed prop) is a skill I need to acquire once the tailwheel is somewhat under control. (I started to say "tamed", but realized that implies that a tailwheel can ever be tamed - and one thing I've learned is that if you ever think "I've got this!" that tailwheel will decide it's a great time to teach you a lesson about over-confidence!)

[JimParker256]

Duplicate post - sorry!

[jaredyates]

Because the Cutlass has a constant speed prop, it's not like the fixed 172 where you can just pull the throttle back to slow down.

If this is the case, you might have discovered the best airplane ever! This is likely not the case though, being that physics will intervene. Here's how I would do it. Next time you are in a cruise configuration, pull the black knob back until the gear horn starts going off, then push it in until the horn just goes out. Depending on how it's all rigged up and what I can remember from my Cutlass experience, that should be somewhere around 12" MAP. If yours is going off at 18", is that something that can be adjusted? 18 sounds much higher than I remember, but it has been a long time since I flew one. Ask someone like Mr. Barger, he knows I'm sure. Is the gear horn based on MAP or throttle lever position? If it is based on lever position, then that will be your setting all the way down. If it is based on MAP, then you can further decrease the throttle as MAP climbs during the descent.

Once you bring the throttle back, the airplane will likely pitch down to maintain the trim airspeed, but good technique would have you do the same with the yoke. Leave the other knobs alone, except gradually richen the mixture very slightly every thousand feet or so as you descend. See what your vertical speed is in that configuration. If it is 500 feet per minute, for example, then you'll be able to figure out your descent angle. This will be your descent configuration for the engine levers, until you do your GUMPS check and push the red and blue knobs forward for the possible go around.

Once you have your descent angle, start with the place in space you are trying to get to, such as the pattern altitude on the downwind, the crossing restriction issued by ATC, etc. Then build your desired vertical path up from that point, and find where it meets your cruise altitude. That is the point where you need to start down. So here's an example. You are at 8500 feet, and you want to arrive at 1500 feet. That's 7000 feet to lose. Your descent rate is 500 feet per minute, so that means 14 minutes of descent. You can cheat and look at your GPS. When your ETE hits 14 minutes, start down at 500 feet per minute, and you should arrive there just about in time, depending on how the winds will change. If you'd like to get all complicated you can think about how TAS is going to decrease as your altitude decreases, but metaphorically this is ax work and not scalpel work, so there's no need to get out the micrometer.

If you don't have a good GPS ETE or if you want to have more pilot dignity, you can do your own time calculation. Decide on an average ground speed during the descent. This is going to be a guess, since you don't know how the winds are going to change on the way down. You can approximately rule out the TAS change by averaging your TAS and IAS. Then guess on a speed correction due to winds, based on what you think your average headwind or tailwind will be. So if you figure your ground speed is going to be 150 knots on average during the 14 minute descent, then you know that you'll cover 35nm during the descent. This is what you use that e6b for. Put the pointer on 150 and look out from 14 on the B scale. That means you'll need to start down around 35 miles from your destination. The actual numbers will vary based on your real life numbers, and while it sounds complicated, it's kind of like weight and balance or restaurant tipping. Calculate it a few times and you'll get a feel for the right range.

One complicating issue is that your cockpit indications of airspeed and vertical speed are based on your passage through the air, but your destination and desired descent angle is based on geometry over the ground. So if you have a headwind, you can start down later. If you have a tailwind, you can start down earlier. In the case you cited, you had a pretty generous tailwind, and if that is combined with a gear switch that is set a little high, then you might just fly along forever, at least seemingly. This is the sort of thing we deal with all the time at work, being that we have long descents through lots of variable winds. Starting down early or late burns more fuel, and in the Cutlass it is likely very small, but on bigger planes the numbers really start adding up.

As for the prop question, this is something you can learn by experiment. Take the plane up to an altitude that you would use for stall practice, just to ensure that the ground is not a concern. Bring the throttle to idle (this is really important) and push the prop all the way in. Nose over to establish a fast descent, like 100 knots or more if you can. As long as your blades are on their low pitch stop, the prop will wind up like it does on a fixed pitch. The prop speed will increase to the point of the RPM that you have set on the governor (full RPM in this case), at which case the blades will leave the low pitch stop to maintain the selected setting. Now, with the engine still at idle, pull the prop control all the way back. I'll resist the temptation to spoil it unless you really want me to. While you are in the descent, you can work the blue knob back and forth to see what changes. Keep the throttle at idle you won't have to worry about hurting anything by moving the prop control.

[Chris In Milwaukee]

You're correct. It's approximately 12 inches of MAP based on throttle position. Fixed in the original post. It's actually a dual-actuated system, and will sound the horn if you're less than 12 inches of MAP or more than 20 degrees of flaps with the gear up.

[Chris In Milwaukee]

I have the -cheater- of which you speak, as both the Garmin 400 and the iFly 720 both have vertical-speed-to-target warnings when I hit 500 FPM descent needed to get there. :-)

[patrickh99]

Deja vu, I flew from NW MN to NE IL last weekend, and also had to go high to get above the smoke. We ended up at 10,500' to get above it , where it was smooth as glass with a 30kt tailwind. I had a similar issue on descent, which I solved by doing exactly what Jared described: 9000'/500fpm = 18 min. of descent. So I started my descent about 25 min out, to arrive at pattern altitude well outside the pattern. I left the prop and throttle controls in, trimmed the nose down, and let the speed go up into the yellow. Enjoy the extra speed as long as it is smooth.

I had a fixed gear C182, so I didn't have one option you didn't mention: drop your gear. As you know, it changes your descent rate dramatically.

And to take it to the extreme, for emergency descents: drop your gear, pull the power, and roll into a 45-60 deg bank.

[Chris In Milwaukee]

Thanks for your thoughts. It's nice having gear that can be extended at any speed. I was bringing down in plenty of time, but there was just that one thing of how I was going to keep it from going too fast.

I'll definitely be reviewing some of those works referenced above for techniques. When I get to instrument training, I'm going to have to figure this out! :-)

[Battson]

This is an easy one!

Pull the RPM right back, and that keeps the MAP high while generating very little thrust. Just following engine manufacture's recommendations on maximum over-square, it's a factor of 7 for Lycoming e.g. 21" MAP and 1400 RPM is the limit.

I use this if I really need to get down in my BH, because of terrain. Usually I plan my descent properly and descent earlier to avoid running the engine too cold.

[JimParker256]

This is a serious question - I'm looking for legitimate feedback and understanding.

To begin with, I don't understand the factor of 7. I can't even figure out how it applies in the case you mentioned (21&quot; MAP and 1400 RPM). When I multiply 21 inches of MAP times 7, I get 147, not 1400. If I do it the opposite way, multiplying 1400 RPM times 7, I get 9800... I just can't see how the factor of 7 correlates between MAP and RPM...

Even moving past that point, I'm curious if you've actually found a reference from Lycoming for the recommendation to limit oversquare operation to a factor of 7? I've looked in the Lycoming 360 and 540 operator's manuals as well as the Continental O-200 manuals, and cannot find any reference to any limitations on oversquare operation, nor any limitation on operating with either high or low RPM versus any particular manifold pressure.

The rule-of-thumb I've always heard people mention who are concerned about oversquare operation is to avoid running the engine at any MAP higher than the RPM divided by 100 (thus 2100 RPM and 21 MAP, or 2500 RPM and 25 MAP). But even that doesn't make sense to me, since both the Lycoming and Continental manuals publish approved data that clearly violates that rule of thumb ... Ditto the various aircraft operating manuals (AFMs and POHs) at my disposal -- ALL of them show cruise power settings that clearly violate that rule of thumb, so the manufacturers seem to disagree with the oft-quoted rule-of-thumb...

A couple of well-known engine experts in other forums (Mike Busch, George Braly, Walter Atkinson, and John Deakin, just to name a few) have stated that the &quot;oversquare&quot; limitations date back to supercharged radial engines, where it was possible to run extremely high MAP (like 65&quot; or more!) and such operation was to be avoided at low RPM because of the internal stresses on the engine. They have further stated that neither Lycoming nor Continental place any limitations on any of their normally aspirated engines operating oversquare. Since I haven't been able to find a reference in the Lycoming or Continental engine manuals, and I am wondering if this might be one of those &quot;aviation myths&quot; that are handed down from pilot-to-pilot, without ever being really questioned and clarified. It's been a long time (a really long time) since I took my fluid dynamics courses, but I cannot understand how there could be any correlation between a low pressure (MAP below 29-30 inches means the pressure in the intake manifold is lower than standard atmospheric pressure) and engine RPM.

I'm hoping someone here can either clear this up for me (us?) or just finally have it confirmed that this &quot;oversquare&quot; thing is nothing to worry about...

Thanks!

[Battson]

You've taken me literally in a mathematical sense with the word 'factor', I just chose loose language in my hurry. What I meant was, the difference is 7 (21 - 7 = 14) less the place-holding zero.

There isn't text reference to the 7 in the Lycoming Key Operations manual, there is a graph of the Performance Curve which contains the information among a bunch of other info. You can interpolate the exact result depending on power setting from the graph, but I simplified it to a rule of thumb which I can actually remember and apply generally at the power settings I am interested in.

Whether your engine crankshaft is 'counterweighted' with the 72801 weights also comes into play, in terms of the limits (so-called, more about added weights for tuning). You want to avoid high speed and low power with the counterweights installed.

The old over-square avoidance rule of 2500 and 25 inches is hokey. Lycoming's own publications dispelled this as a quote 'myth' a couple of decades ago. See the section of Key Operations covering 'An Explanation of Power Settings'. That section of Key Operations goes on to explain how to extract the information I have referenced above, from the Performance Curve graph (my rule of 7). Also read Mike Busch / Savvy Aviator for a lot more details.

[JimParker256]

Thanks, Battson. The 7 makes more sense now. (And yes, I tend to take things literally. I'm definitely a right brained engineer - or enginerd, as my son says!) I'll do some more studying.

[Chris In Milwaukee]

I appreciate all of the advice. Thanks to you all. Lots of different things to try! I don't know what made me keep from pulling the RPM back and trying that. After thinking about it, there was no way I was going to "outrun" the prop. One more tool in the bag o' tricks.