Typical G-Forces For A Commercial Flight?

N243NW · Mon Dec 18, 2006 11:50 am

Hi all,

I've been doing some flying around as a virtual airline pilot, mainly using the 717 and 737 and perfecting various stages of flight for each. However, I have no idea how comfortable my "passengers" actually are, since I don't know if I'm climbing or descending gradually enough.

I was wondering if anyone here could give me a typical figure for a comfortable intiation of descent in terms of G forces. Are passengers okay with .7 Gs? .8 Gs? .9 Gs? At what point does a passenger start to feel discomfort? It would help a lot to know about what I should aim for, since in place of actually being able to feel what the aircraft is doing, I have a nifty little G meter in the X-Plane software I can refer to.

Thanks!
-N243NW Big grin

oly720man · Tue Dec 19, 2006 6:40 am

I'd have thought that the g force would be next to nothing. The g force at the start of acceleration on the runway, on rotation at take off, along with braking at the other end are the highest "planned" g forces in a flight. At take off the g force is around 0.4g.

In rough terms do F = ma where F is the thrust, m is the aircraft mass and a is the acceleration.

For a B777-300 with 2 GE90-115, total thrust (flat out) is 230000lbs or 1023000N. MTOW basic for the B777 is 580000lbs, or 263000kg. Acceleration is 1023000/263000 = 3.89m/s2 = 3.89/9.81g = 0.3965g (horizontally).

At the start of descent you're probably looking at less than 0.1g as a deceleration. It's nowhere near as noticeable as the take off acceleration. In this case the 0.1g is horizontal. It's still 1g vertical.

Buyantukhaa · Tue Dec 19, 2006 7:45 am

Quoting Oly720man (Reply 1):
At the start of descent you're probably looking at less than 0.1g as a deceleration. It's nowhere near as noticeable as the take off acceleration. In this case the 0.1g is horizontal. It's still 1g vertical.

Eh... while the earth will do its best to cause a 1g downward acceleration, the wings prevent that. So your vertical accelaration is 0g (assuming flying in a straight line).

SlamClick · Tue Dec 19, 2006 7:57 am

Using an autopilot's vertical modes will typically give you a quarter-G push or pull to start a climb or descent. So from .75 to 1.25 is the maneuvering range with time spent at 1 outweighing by a wide margin. If some planes (CFM-737s for example) if given a close-in leveloff after takeoff will combine that quarter-G pushover with pulling the thrust way back. The combination of dethrusting and pushing may feel like negative G but it is not.

Turbulence of greater than one G variance is probably pretty remarkable. The rate may approach 10G per second but for only about a tenth of a second. Sustained G, positive or negative, lasting a second or more is just not that common.

N243NW · Tue Dec 19, 2006 8:30 am

SlamClick,

So by these standards, you'd say that I wouldn't cause any passengers' stomachs to turn uncomfortably if I initiate a descent that causes the airplane to experience .8 or .9 vertical Gs?

Interesting to hear about the forces on a passenger during takeoff too.

-N243NW Big grin

roseflyer · Tue Dec 19, 2006 9:25 am

A 30 degree bank exerts 1.15 Gs and that is the highest bank that will be experienced under normal conditions. If you get to 45 degrees, the force goes up to 1.41Gs and a 60 degree bank is 2 Gs, but that is beyond anything that would happen unless there was an emergency.

oly720man · Tue Dec 19, 2006 4:42 pm

Quoting BuyantUkhaa (Reply 2):
So your vertical accelaration is 0g (assuming flying in a straight line).

yes, relative to the normal 1g acceleration - weight.

speedracer1407 · Tue Dec 19, 2006 8:12 pm

Quoting N243NW (Reply 4):
So by these standards, you'd say that I wouldn't cause any passengers' stomachs to turn uncomfortably if I initiate a descent that causes the airplane to experience .8 or .9 vertical Gs?

Since your'e only talking about .8 and .9 G, and not 1.1 and 1.2 G, I suspect (and please correct me if I'm wrong) that you may be counting up from zero...in other words, "zero G" is straight and level flight. As Oly720man pointed out, when talking about vertical G-force, you have to start at earth's naturally occuring 1 G. So experiencing .9 G would make your body feel slightly lighter. 1.1 G would push your body into the seat slightly.

I hope i'm not off on a tangent that you didn't intend, but if you mean .8 or .9 G GREATER than normal 1.0 G level, turbulence-free flight, then you're talking about huge G forces that would certainly be uncomfortable for passengers. If we accept this scale, then sustained .1 or .2 G would cause drinks and passengers to begin floating around the cabin, and 1.8 or 1.9 G would make it difficult, maybe impossible for passengers to get out of their seats.

For a comparison, find a friend with a serious sports car (Porsche 911 or something), and have him sustain a relatively tight turn around a circle at the absolute limit of the tires' adhesion. That's about .9 Lateral G (starting from zero).

SlamClick · Wed Dec 20, 2006 12:26 am

Quoting Speedracer1407 (Reply 7):
As Oly720man pointed out, when talking about vertical G-force, you have to start at earth's naturally occuring 1 G.

That is exactly correct. If I say .9G I am talking about one-tenth of a G less than you feel sitting in your chair right now. If I say 1.1G that is a very slight acceleration of only one-tenth more than you feel normally. Most people could not even feel that.

Quite often the .25G push over that the autopilot might use to start a descent will not even wake up sleeping passengers.

Quoting RoseFlyer (Reply 5):
a 60 degree bank is 2 Gs, but that is beyond anything that would happen unless there was an emergency.

That is correct. In a type rating checkride in training done under Part 121 Subpart N, (Pre-AQP airline training) we had to demonstrate "steep turns" in each direction. We would enter at 250 knots and use a 45° bank. We'd do a 180 in one direction, roll out for a moment, then a 180 in the opposite direction to roll out on the original heading, on altitude, on-speed. We would never turn this steep in line operations, with 25°-30° bank being about normal. At high altitudes with a heavy load more like 15°

N243NW · Wed Dec 20, 2006 4:22 am

Quoting Speedracer1407 (Reply 7):
Since your'e only talking about .8 and .9 G, and not 1.1 and 1.2 G, I suspect (and please correct me if I'm wrong) that you may be counting up from zero...in other words, "zero G" is straight and level flight.

Actually, Speedracer, it's the contrary in this situation. I should have clarified that this is .8 or .9 times the force normally felt in cruise (i.e. 1G is straight and level flight). I'm not counting up from zero. So from what has been said, if I initiate a descent or level off at cruise altitude using a control input that causes the plane to experience .8 or .9 Gs (.1 or .2 less than normal gravity), most passengers won't notice or will barely notice.

Thanks for all your help!

-N243NW Big grin

SlamClick · Wed Dec 20, 2006 5:08 am

G-Loads by quarter G increments.

+2.00 Twice the pull of gravity (60° bank) Your cheeks are sagging.
+1.75
+1.50
+1.25 Normal when autopilot is beginning a climb
+1.00 Normal, straight and level flight
+0.75 Normal when autopilot is beginning a descent
+0.50
+0.25 Very light in your seat.
00.00 Zero-G, weightless but so is your seat
-0.25 Truly weightless - plane is falling faster than freefall. You will actively float "UP"
-0.50 All the loose debris is flying around.
-0.75
-1.00 You are firmly against the overhead.
-1.25
-1.50
-1.75
-2.00 Hard against the straps or squashed on the ceiling. Your eyeballs are bulging.

787atPAE · Wed Dec 20, 2006 10:10 am

Quoting SlamClick (Reply 10):
G-Loads by quarter G increments.

+2.00 Twice the pull of gravity (60° bank) Your cheeks are sagging.
.
.
.
-2.00 Hard against the straps or squashed on the ceiling. Your eyeballs are bulging.

I heard from someone/somewhere that bad things happen when the negative g's are too much, like you start popping blood vessels in the brain. I heard this happens around -1g for "disgusting fatbodies" like myself. I know loss-of-conciousness can happen between +5 to +11 g's depending on how fit you are and what equipment you are wearing.

Can anybody confirm what the limit is in the negative g direction before we start popping vessels?

Also, has anybody ridden Mission:Space at Epcot in Orlando, FL? That ride gets you up to 2 g's for several seconds. That was well worth the hour long line!!!! Smile

SlamClick · Wed Dec 20, 2006 10:37 am

Quoting 787atPAE (Reply 11):
Can anybody confirm what the limit is in the negative g direction before we start popping vessels?

Well, my son (good health, athletic type) popped a blood vessel in his face leaning over the back of the couch sorting out an electrical connection.

That was 1G negative, sustained.

Starlionblue · Wed Dec 20, 2006 11:49 am

Quoting 787atPAE (Reply 11):
I heard from someone/somewhere that bad things happen when the negative g's are too much, like you start popping blood vessels in the brain. I heard this happens around -1g for "disgusting fatbodies" like myself. I know loss-of-conciousness can happen between +5 to +11 g's depending on how fit you are and what equipment you are wearing.

Well, the opposite of a blackout from positive Gs is a redout, where your vision gets red from all the blood collecting in your head. While I don't know an exact number, negative G tolerance is much lower than positive G tolerance. I would guess -1.5 to -2.5 somewhere for sustained. Aerobatic pilots frequently get into the -4 to -5 range, but they don't sustain it for nearly as long as, say, a fighter pilot sustains 4-9G.

Quoting SlamClick (Reply 12):

Well, my son (good health, athletic type) popped a blood vessel in his face leaning over the back of the couch sorting out an electrical connection.

That was 1G negative, sustained.

Ouch. Brings up the important difference between sustained and non. It's quite possible to survive dozens of Gs for a very short period, but beyond about 5-7 most people will faint. You can take more with training and a G-suit, but the limit is still around 9-10.

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