The difference in adding a person to the backseat

Voodoobones

Voodoobones

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Voodoobones
I have had my PPL for a few years now but only recently flown with two passengers. I'm what you would call a chicken pilot so I don't try many things that I am uncomfortable with. Well finally I decided it was time to fly with a third person in the 172. Up until this point it was always me and one other person.

I asked a lot of question and talked to a lot of other pilots about what I should expect with the extra weight in the backseat. Everyone said I would be fine and that I shouldn't have a problem. "You won't notice much of a difference."

I did my weight and balance and everything checked out okay. But with that being said I definitely noticed the extra weight. I took off from a 9000 foot runway so I would have more than enough room for this new experience. As soon as we lifted off I could feel that the plane felt a lot heavier than I was use to. I made a shallow climb out (Did I mention I'm a chicken?) and continued on my local scenic. I notice that the plane had a tendency to bank to the right due to the extra weight on that side. Other than that it wasn't much of a problem. Then came the landing and what I learned from it.

My approach was stable and I kept my speed up a little bit higher than normal. As I got over the number I pulled power and just as I was flaring the bottom dropped out and I plopped down hard with a nice bounce for embarrassment. You would think I would of learned from the first landing but no.

On my way back to my home base I thought about why I botched the landing. It just didn't click though. Once again I had a stable approach and as I got over the numbers I pulled the power and plopped on the flair.

Have you guessed my error yet?

It wasn't until I was in the car alone analyzing my landing when I realized that I should have carried more power into the flair due to the extra weight.

Anyone else have a similar experience?
 
Ummm.... not necessarily "more power, " but certainly a tad more energy (airspeed) to compenate for the added weight to maintain the same overall approach and landing picture.

In other words, you could maintain the same airspeed and energy level with a steep descent and power to idle. Or you could fly a very flat approach at an airspeed with lots of power. We typically fly somewhere between the two, as it gives us runway visibility, a bit more control and options, it doesn't scare the pax, and too many of us are not as adept at the significant change required to go from fairly steep angle downward to level (or "flare/flair").

What happens to most pilots unfamiliar with a rearward CG shift due to more weight in back is not adjusting for that weight -- you set everything up the way you do when solo, yet the airplane's flying alot slower than it usually does at that trim+power+attitude setting. So when you levelled off above the runway ("flare"), you did not increase the angle of attack enough to maintain flight (more weight, more lift required).
 
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Should I come in at a higher airspeed and carry that through to the flair? Maybe not pull the power so early?
I want to try it again but maybe I will take a CFI with me to get a better understanding.
 
Maybe don't worry about it and learn your airplane. I doubt yours was the first hard landing the thing ever had, nor will it be the last. They are built to handle it. For all you know you hit a sudden down draft. While it is good to be cautious, one does have to push the envelope to achieve anything.

Oh, and welcome to POA!
 
steingar said:
Maybe don't worry about it and learn your airplane. I doubt yours was the first hard landing the thing ever had, nor will it be the last. They are built to handle it. For all you know you hit a sudden down draft. While it is good to be cautious, one does have to push the envelope to achieve anything.

Oh, and welcome to POA!
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Very true about learning. It's just that I don't want to learn at the expense of my PAX. Maybe I should go up with some people that aren't afraid of a good bounce or two. :goofy:

Thanks for the welcome.

Jim
 
You could always just add some ballast. Fifty pounds in the back seat and fifty in the luggage should move the CG back quite a ways with just the pilot.

This way you also get to avoid the "embarassment" of imperfect landings. :yesnod: BTW, any landing you walk away from is a good one.
 
Voodoobones said:
Should I come in at a higher airspeed and carry that through to the flair? Maybe not pull the power so early?
I want to try it again but maybe I will take a CFI with me to get a better understanding.
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Before you fly, you should spend some time on the ground with a CFI that understands and can relay how weight affects angle of attack.

What you discovered was that something has to change to accomodate the increased weight. You probably lifted the airplane's nose the way you usually do when solo for the "flair" (I don't use that term, but that's not important now :rolleyes:).

You may have had "added airspeed," but keep in mind a 172 is fairly draggy, and once you pulled power and lifted the nose the speed decayed significantly. So all the "extra speed" did was add a few dozen feet to the runway length consumed.

So the only option you had before the bottom dropped out was to increase lift enough to maintain flight. We really don't want to "stall" our airplanes in (people call them "full stall landings" when all they mean is the stall horn is honking).

Rather, you want to maintain controlled flight until the wheels gently kiss the ground and begin rolling,. Why we do this at minimal controllable airspeed is to reduce the ground roll.

Think of it this way -- you could land at 100 knots in a 172. But the landing would be very flat and the ground roll very long.
 
How fast were you on short final in the 172 (no need to be any faster then 65 knots. I generally come in at about 55-60)? You really don't need to carry power into the flare on a 172. They land find at gross with the engine at idle.

It's all about technique.
 
jesse said:
How fast were you on short final in the 172 (no need to be any faster then 65 knots. I generally come in at about 55-60)? You really don't need to carry power into the flare on a 172. They land find at gross with the engine at idle.

It's all about technique.
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Landing fine at gross would imply taking off at more than gross. Are you telling on yourself Jesse. :wink2:
 
Well - at gross or 8 lbs under gross - whatever.
 
I guess my point is as a pilot you are going to encounter new situations all the time if you really fly. You probably won't if all you do is fly circles around your home airport, but some of us think that's a bit boring. If you need a lesson to do anything new you will never do anything new. My thought was to go out, use your own self-confidence, and be a pilot.

Odds are you need a CFI to check you out in a new-to-you aircraft. You shouldn't need one to put a person in the back seat or a piece of luggage in the back. And if the landing was a bit hard, who cares? The pax? They don't know the difference anyhow.
 
If I had to guess, I'd say it's either coincidence, or else the result of you trying too hard to "compensate" for something that doesn't really require compensation.

In my experience, you don't really need to do anything different. The V speeds you've probably had burned into your head are most likely based on MGW. So you don't need to add a few knots to your approach speed or your Vy, these speeds are actually more "correct" when you've got an extra butt in the seats.

Consider that a 172 with 53 gallon tanks holds 300lbs of fuel. Fly with full tanks on one day, then half tanks on the next, and that's a 150lb difference. Does the plane fly completely differently? Not really. But since the weight isn't staring at the back of your head, and asking you lots of questions, you don't think about it, you don't try to compensate for it, you just fly the plane, which is what you should do.

In general, coming down final with a lot of power in, and then suddenly "chopping it" at the last second, is a good way to end up with a "thump" landing, as the bottom drops out. It's better to ease the power off gradually.
-harry
 
jesse said:
How fast were you on short final in the 172 (no need to be any faster then 65 knots. I generally come in at about 55-60)? You really don't need to carry power into the flare on a 172. They land find at gross with the engine at idle.

It's all about technique.
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I will keep it between 65 and 70 kts on short final and then bleed it off. I was just caught off guard at the end when it suddenly dropped. When it is just me and another person I have no problem so I think the issue here is that rearward CG that I am not use to.

steingar~
I understand what you are saying. I don't grab a CFI every time I want to expand my horizons. I fly up in the Pacific Northwest and I am always exploring new places. But at the same time there is no reason to re-invent the wheel. If you can find someone that can give you good advice then learn from them.

I have a couple of friends that I will ride with to learn new procedures and techniques. These beautiful remarkable machines that we control can bite you very hard and I know when to ask for help. It also increases the chances that I won't forfeit my kids college tuition by paying for a bent airplane or worse yet not being there to see them graduate. :wink2:
 
I agree with Harry in that I am uncomfortable with the term "pull the power." I have no problem with a smooth reduction of power.

Part of the effectiveness of the horizontal stabilizer in maintaining pitch attitude is the discharge air from the propeller. When you are using enough power to actually pull the airplane through the air, that is a positive force; as you reduce throttle setting with drag constant you are reducing thrust, and that imbalance results in gradual reduction of lift (which you counter by increasing angle of attack slightly). "Chop the power" and you lose that elevator effectiveness more or less instantaneously without time to react. The airplane has inertia as it moves through the air and it cannot react instantaneously. Use the pitch/power relationship like porcupines make love....very carefully.

Bob Gardner
 
bobmrg said:
I agree with Harry in that I am uncomfortable with the term "pull the power." I have no problem with a smooth reduction of power.
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I'm confused. Why are you uncomfortable with "Pull the power"? When I run the throttle I try to make it as smooth as possible. Doesn't pulling the power accomplish that? :wink2:
 
Getting back to the weight/airspeed thing for a sec: think of it this way... you've heard about how increasing the wing loading in a turn causes the stall speed to go up, right? Well, banking isn't the only way to increase the load on the wings- actually adding weight to the aircraft does the same thing.

So, more weight means the airplane wants to fly faster to do each specific thing you want it to do: not stall, descend a certain amount over a certain distance, etc.

What happened to you is that the wings had less lift available than they normally would at that same airspeed (even if you didn't hear the stall warning horn), and the vertical speed increased at the last moment.
I doubt very much this was an aft-CG problem; just adding one normal-sized person to the back seat of a 172 does not put the CG very far aft.There might have been a very small difference in the way the plane was responding to your elevator input, but the crux of the matter was probably your gross weight.


Gliders are a great example, because power does not figure into it, and gliders move forward because of their weight (gravity substitutes for thrust).
The 2-seat glider I fly these days has precisely the same glide ratio with one person aboard as it does with two, but the speed (Vbg) required to achieve this ratio is slightly higher with 2 people. It will fly the same horizontal distance for every foot of altitude lost, but it will go from the top of the glide to the bottom faster. It also stalls at a higher IAS, Va is higher, and minimum-sink speed is higher. It pretty much just needs to fly faster when it's heavier, in order to get the best performance possible at that weight.



But more important, during an approach, than thinking about "adding airspeed" is to simply remember that you are heavier, and therefore the plane wants to do everything a little faster.
You can't be looking at the ASI as you touch down, after all... to avoid "plopping" onto the runway when you are heavy, you just need to keep in mind that you should be moving forward a bit faster than when the plane is lighter.
 
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Jim - welcome to POA. Glad to see that you asked your question and are getting plenty of answers here.
 
This type of situation is why, when checking out someone for their first time in a plane with back seats, I include some flying (and landing) with folks in the back. Better to learn about the differences at higher weights and more aft cg's with me in the right seat than to be surprised by that on their own.

BTW, for the light singles most folks here are flying, the difference in final approach speed for more/less weight should be about 1 knot per 100 lb, i.e., if you've been flying your 172 at 60 knots on final with just two and partial fuel (about 2000 lb), you'll want to use about 64 knots on final fully loaded to have the same stall margin and time in the flare.
 
Seems like maybe you are using the same set of parameters that were specific to the W&B of the plane when you were training. Those parameters need to change as the W&B changes.
Example:
1700RPM, 55kts, 750fpm descent = good landing solo
1700RPM, 55kts, 750fpm descent != good landing with 3 people

I was taught to use pitch and power to maintain airspeed and rate of descent without the rote power settings.
That said, I fly a Cherokee 6 with a huge CG envelope and my landings get better as I load it. I think this is due to the elevator being having more authority as the CG moves aft.
Also, "pulling the power" requires a major change in AoA to maintain your sink rate. I find I have to go from about nose level on short final to +7 degrees to compensate for cutting the power or I get a big thump.
 
I just had a thought. I wonder if you used to pull the power right when the ground effect began to arrest youf descent. The ground effect was what was compensating for your power change. With the higher weight the ground effect is not sufficient to overcome the power change and you need to adjust your AoA.
 
In a 172, I've usually got the power all the way off well before the flare, even with a full load -- the airplane is almost kite-like, and unless you're flying an instrument-style 3-degree approach path, you just don't need much (if any) power the last quarter mile or so. In any event, while having some sort of starting point helps, I'm not much on holding "canned" power settings on final. I prefer to set trim to maintain desired speed, and adjust power as needed to set glide path. That way, compensation for differences in weight, wind, etc., is taken care of without great mental gymnastics to determine necessary power.
 
Voodoobones said:
I'm confused. Why are you uncomfortable with "Pull the power"? When I run the throttle I try to make it as smooth as possible. Doesn't pulling the power accomplish that? :wink2:
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I guess you had to be there, and I wasn't. To my mind, "pulling the power" means a fast movement of the throttle toward the idle position and is not "as smoothly as possible." Maybe I read more into your post than you intended.

Bob
 
Ron Levy said:
In a 172, I've usually got the power all the way off well before the flare, even with a full load -- the airplane is almost kite-like, and unless you're flying an instrument-style 3-degree approach path, you just don't need much (if any) power the last quarter mile or so. In any event, while having some sort of starting point helps, I'm not much on holding "canned" power settings on final. I prefer to set trim to maintain desired speed, and adjust power as needed to set glide path. That way, compensation for differences in weight, wind, etc., is taken care of without great mental gymnastics to determine necessary power.
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This is the technique and reasoning that was given to me when I was training. Also, one doesn't get fixated on a specific runway picture and airplane setup. You are controlling what needs to be controlled: airspeed and sink.
 
Hey thanks everyone for some terrific info. I'll be doing some flying this weekend so I will see if I can figure out what I was doing different with the back seat pax than when just having the front seat pax.

Hi P.J.! You lure me here and then bail out of the thread? Chicken. :D

No worries Bob. I'm with you on the smooth movement of the throttle. I think I read in "The Killing Zone" that fast movements of the throttle can disrupt the airflow in the venturi and cause and engine to stutter or worse. So I try to always make smooth changes to the throttle.

Jim
 
Voodoobones said:
No worries Bob. I'm with you on the smooth movement of the throttle. I think I read in "The Killing Zone" that fast movements of the throttle can disrupt the airflow in the venturi and cause and engine to stutter or worse. So I try to always make smooth changes to the throttle.

Jim
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Turbines are more sensitive to abrupt anything. Our small piston singles, not so much. The main concern is wear on the engine from rapid changes.

I've flown with folks who were taught to "pull power to idle" some place near the runway. Either they are very good and also instantly adjust for the loss of thrust, or we end up plopping on the runway. :eek:

If I'm aboard to instruct, I suggest, then demonstrate, a more controlled approach. :yesnod:
 
dmccormack said:
Turbines are more sensitive to abrupt anything.
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My experience differs. You can hurt a piston engine by large/rapid throttle movements, but not a turbine engine (at least, not a turbojet/turbofan -- my experience with turboprops is limited). Yeah, do the wrong thing, and you can compressor stall it, but normally rapid throttle movements won't hurt a turbine engine. With piston engines, possible problems range from banging up the counterweights to various rapid cooling issues (especially with turbochargers) or just choking the engine (try a real throttle slam on the "go" of a T&G or go-around some time). Slam the throttle on a turbine engine designed since the 50's, and all you're likely to see is a slow response, not damage or failure.
 
Ron Levy said:
My experience differs. You can hurt a piston engine by large/rapid throttle movements, but not a turbine engine (at least, not a turbojet/turbofan -- my experience with turboprops is limited). Yeah, do the wrong thing, and you can compressor stall it, but normally rapid throttle movements won't hurt a turbine engine.
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This is my experience with turbine engines too. When I first flew the King Air, which was my first experience with turbines, I was surprised that you could go from cruise to idle in one motion. No worries about shock cooling. When advancing the throttles you had to make sure you didn't exceed the maximum torque or ITT so you had to be a little bit more careful.
 
Ron Levy said:
Not issues with the jets I flew (J52, J79, and TF30).
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Most of the jets I have flown could go throttle to the forward stop without any problem although you had to be careful with the ones which had older design engine computers. The King Air could exceed it's maximum ITT on takeoff if you went full forward, especially in Denver in the summer.
 
Ron Levy said:
My experience differs. You can hurt a piston engine by large/rapid throttle movements, but not a turbine engine (at least, not a turbojet/turbofan -- my experience with turboprops is limited). Yeah, do the wrong thing, and you can compressor stall it, but normally rapid throttle movements won't hurt a turbine engine. With piston engines, possible problems range from banging up the counterweights to various rapid cooling issues (especially with turbochargers) or just choking the engine (try a real throttle slam on the "go" of a T&G or go-around some time). Slam the throttle on a turbine engine designed since the 50's, and all you're likely to see is a slow response, not damage or failure.
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Oops! True. I meant to say turbo -- not turbine.

I gotta pay attention ... :redface:
 
dmccormack said:
I reviewed my son's history research paper last night: "Son, it wasn't the Decoration of Independence...."

Spell check only goes so far. :yesnod:
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LOL!
My wife is a teacher. So I have the fun of a wife and an educator pointing out my deficiencies. I can't get away with anything. It's so not fair. :mad2:
 
I was taught to use pitch and power to maintain airspeed and rate of descent without the rote power settings.
That said, I fly a Cherokee 6 with a huge CG envelope and my landings get better as I load it. I think this is due to the elevator being having more authority as the CG moves aft.
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If I'm not mistaken, though elevator authority increases (up to a point) the further forward the c/g is (all within the manufacturer's envelope, please). I think of it as being a see-saw, with the fulcrum (c/g) closer to one person, so that the other person, to move the close person needs not be all that heavy.

When one moves the c/g aft, all that's happening is it's easier to get the nose up on the flare... Then again, I'm likely wrong... :rolleyes::eek:
 
jshawley said:
If I'm not mistaken, though elevator authority increases (up to a point) the further forward the c/g is (all within the manufacturer's envelope, please). I think of it as being a see-saw, with the fulcrum (c/g) closer to one person, so that the other person, to move the close person needs not be all that heavy.

When one moves the c/g aft, all that's happening is it's easier to get the nose up on the flare... Then again, I'm likely wrong... :rolleyes::eek:
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Close. Thinking of it as a seesaw is good.
The fulcrum is the Center of Lift, not the CG. The CoL of most airfoils is around the quater chord point (1/4 of the way back on the wing). The CoL can move forward or aft a little bit based on angle attack.
The CG is the person on one side of the seesaw.
The downward lift from the horizontal stabilizer is the other person.
If you move the CG aft (toward the CoL fulcrum) then less down force is required from the horizontal stabilizer to counter the weight, thus the elevator seems to have more authority.
 
D'oh! I knew that. Failure of a synapse to fire, I guess... :redface:
 
jshawley said:
D'oh! I knew that. Failure of a synapse to fire, I guess... :redface:
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The center of lift of the airplane has to be co-located with the CG but the center of lift of the main wing is relatively fixed as you stated. And unless you're unable to attain the pitch attitude you want with full up elevator input the issue isn't lack of control authority but rather higher stick forces with a forward CG. In any case, I can agree from personal experience that it's a lot easier to "flare" a Six when the CG loading is aft vs forward.
 
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