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Mass & Balance Calculation for PPL Skill Test

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Mass & Balance Calculation for PPL Skill Test

Old 19th Nov 2019, 21:58
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Mass & Balance Calculation for PPL Skill Test

Evening all:

I've got my Final Skills Test coming up in a Cessna 152.

Iíve been told that I need to do a Mass & Balance calculation for myself (75kg) and my instructor (86kg).

I have literally no clue where to start. Any help finding the formula or limits would be a huge help.

Thanks all!
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Old 20th Nov 2019, 00:06
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Welcome Ted,

I hope that between your instructor, and ground school, someone has provided you with instruction as to a weight and balance calculation. This is basic stuff. Though it's generally fairly hard to mis load a 152 (unless you're piling cargo into the back), other types are easy to mis load, and unsafe to fly out of balance limits. For my extensive experience in aircraft maintenance and modification, W&B is the most regularly screwed up thing done in documenting an aircraft. I have several times taken off in an airplane later found to have a basic error in the W&B, leading to my calculations being wrong, and the plane was less safe to fly for it.

The POH for the 152 has good information in Section 6, including examples. Admittedly, the information is in pounds, so converting your weights to pounds would make the calculations easier. That said, if, while following the example in the Cessna POH, you're feeling challenged, you are entitled to some remedial training, 'cause someone has not trained you as you have paid. Yes, we here can walk you through this, and will help, but this is part, an important part, of training for which you have paid.

Can you be more specific as to what additional wisdom you seek?
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Old 20th Nov 2019, 00:12
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Originally Posted by ted202 View Post
I've got my Final Skills Test coming up in a Cessna 152.
Are you sure about that? If you haven't been taught W&B what else haven't you been taught?
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Old 20th Nov 2019, 01:27
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Go to page 83 of the attached PDF for the weight & balance info for the 152, a sample loading problem is on page 91. All in Imperial measurements though.

http://www.t-craft.org/Fleet/C152-1978_POH.pdf
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Old 20th Nov 2019, 03:47
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@ted202: Are you saying that during your PPL(A) theory you did not have flight planning and mass&balance lectures? Then during your training you did not do an Operational Flight Plan (+ ATC Flight plan mandatory if for example flying to controlled airspace) with mass&balance at least before each cross country flight (take off weight, fuel burn, landing weight) and a fuel calculation including reserve for your trip? Normally it have to be done even for aircraft with glass cockpit. You should also have an SOP of the school with some words about mass an balance in addition to what you find in the AFM.

If your school does not provide Operational Flight Plan that you can (or are obliged to in order to be allowed to fly) fill in with your calculations before each flight then what you have to do is use the AFM and read chapter 6 in the AFM for Cessna 152 (I did fly that one too and I believe the AFM should be similar) and if you got a complete AFM you will have an envelope there (figure 6-11) that you can make copies of and use to plot your Take off and landing weight, you will also find performance information in chapter 5 that you should understand and know how to calculate if you are going to fly to some foreign airfield to be able to prove to the examiner that you understand it and not just "its a long runway". And do not forget to check the documentation of the actual aircraft you are flying for the exam since every aircraft got different equipment and can have different basic empty weight its not like all C152 are the same, all C172 are the same, all C182 are the same etc.

@Pilot DAR: It may be fairly hard to load a Cessna wrong to get out of the envelope, but fairly easy done when taking passengers (during a time building or leisure flight for example) especially on Cessna 172 to overload it. Actually the only fatal accident in Poland of Cessna 172 (the pilot was experienced airline pilot still skipped m&b) because the aircraft was highly overloaded and unable to climb in a proper fashion from a difficult airfield when trying to avoid obstacles it hit the treetops and fell into a city park killing 2 of 4 occupants including the pilot. So mass and balance is important in every aircraft and in Cessna especially to not overload it. So no matter the aircraft doing the right thing is important....
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Old 20th Nov 2019, 05:52
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Originally Posted by ted202 View Post
Evening all:

I've got my Final Skills Test coming up in a Cessna 152.

Iíve been told that I need to do a Mass & Balance calculation for myself (75kg) and my instructor (86kg).

I have literally no clue where to start. Any help finding the formula or limits would be a huge help.

Thanks all!
Itís usually referred to as W&B.
Iíd be concerned and talking to my instructor if I was you.

Are you saying youíve no concept of W&B?

Have you done the calculations for performance? Take off to 50í from a wet grass runway with no wind at MAUW for instance?
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Old 20th Nov 2019, 06:28
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All in Imperial measurements though.
Actually, not since 1824. In the United States, there is a system called 'Customary Units'. Metrication was resisted in the US in the late 19th Century partly on religious grounds.

Have a care when referring to units of measurement in a POH or Flight Manual originating in the US. You're fairly safe with the smaller units of length, though things get strange with longer distances. Units of mass for our purposes are mainly equivalent to the old Imperial units but units of volume are way off. The most glaring of these is a US gallon. Generally in the UK, we dispense fuel and oil in litres, so you need to know how to convert from litres to Imperial gallons and US gallons. As a very rough guide and probably good enough for a PPL Skills Test, one litre of AVGAS on an average day weighs 1.6lb. Hopefully in ground school you've touched on how this can change, principally with temperature. I like to say that 1 kilo is about 2.2lbs.

Another thing we tend to mix and match is Mass and Weight. At rest on the Earth's surface, they have a rough equivalence . The Mass of an object remains largely the same most places in the Universe, whereas weight is very changeable. Concorde in the cruise weighed measurably less than it would have done on he surface, due to its distance from the centre of the Earth, but its mass remained the same (for the instant the measurement might have been taken - in practice both were going down at a prodigious rate as fuel was burned!)

TOO
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Old 20th Nov 2019, 11:24
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A religious objection to the metric system? How stupid can the world be? What exactly was the perceived problem?
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Old 20th Nov 2019, 12:53
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@Pilot DAR: It may be fairly hard to load a Cessna wrong to get out of the envelope, but fairly easy done when taking passengers
Read carefully! What I said was:

Though it's generally fairly hard to mis load a 152 (unless you're piling cargo into the back), other types are easy to mis load, and unsafe to fly out of balance limits.
It's surprisingly easy to load most other Cessnas out of limits, and care must be taken! As a very new pilot, I actually misloaded a `172 with half tanks, one adult and two kids aboard! The fact that "she seems to fly okay" is by no means that the W&B is within limits, or acceptable. A wrong loading may show itself viciously (even in a 152) if you approach the stall. I have had to test modified Cessnas for ballast installations, and there's a lot more to getting a W&B right than "it seemed to fly okay..."
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Old 20th Nov 2019, 15:19
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Using your figures in this simple calculator, seems to put you almost at MAUW, but with a forward C/G.
24.5 Gallons, 75kg = 165lbs, 86kg = 190 lbs..
So you should be Ok, as long as he doesn't sit on the tailplane…

http://www.internationalflyingclub.org/src/wb152.html
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Old 20th Nov 2019, 15:41
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@Pilot DAR: I understood as if you were writing about other types like if you meant other types of SEP(L) that are not made by Cessna. In case someone read it the same way, I just believe its important to underline that Cessna require the same level of airmanship as any other aircraft even if they are built to help pilots survive mistakes.

Since actually there exist people who believe Cessna almost fly by itself and you can not do stuff wrong.... that is like the main cause of fatal accidents. Of course it can save you or save itself from damage when doing some mistakes like a bad flare during landing where for example an AT-3 would crash its landing gear a Cessna survives it without damage to the aircraft.... also as 30 years long list of accidents show it also often (just one accident involving loss of life in 172 in Poland. And the only fatal one involving a 182 was caused by showing off aerobatic maneuvers on low altitude by.... chairman of an aeroclub and sadly with 3x children on board) save the pilot and passengers life in aircraft written off type of accidents. But if someone neglects the safety rules it ends the worst possible way.

What is worth noticing the Polish fatal accidents (I do not know where to find complete safety database for the rest of the world) be in 172 or 182 happened with experienced pilots so I believe emphasizing that Cessna does not fly by itself and also require proper mass&balance + following the rules regarding altitude, maneuvers and IMC conditions is just as important as with any other aircraft.
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Old 20th Nov 2019, 23:11
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The OP - one of the most worrying seen for a while. What on earth is being taught ffs!!!!
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Old 21st Nov 2019, 01:46
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Concorde in the cruise weighed measurably less than it would have done on he surface, due to its distance from the centre of the Earth
and the latitude and direction in which it was travelling. (effect of earths rotation, known as the EŲtvŲs effect)
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Old 21st Nov 2019, 02:58
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Concorde in the cruise weighed measurably less than it would have done on he surface, due to its distance from the centre of the Earth
So... the crew had to calculate a new W&B for the high altitude portion of the flight?
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Old 21st Nov 2019, 03:05
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Originally Posted by Pilot DAR View Post
So... the crew had to calculate a new W&B for the high altitude portion of the flight?
But the M&B would be OK!
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Old 21st Nov 2019, 11:39
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Well no-one has yet explained what W+B calculations are all about. So let's try and put the OP in the picture..

When doing Moments at college, it was explained to me that, If you take any point on a body at rest, then the sum of the clockwise moments will equal the sum of the anti-clockwise moments. The Moments are defined as being the weights times their distance from that point.
To make thing simpler, you could take moments around the C/G, then the moment of the C/G would be zero, and would simplify the maths.

It doesn't matter where you put your datum point, anticlockwise will always equal clockwise. For the Cessna the datum point is somewhere near the back of the bonnet, but the calculations would work just the same even if it was at the tip of the prop-spinner. ( looking at the worked example, it is 39 inches in front of the seats, and the centre of mass of the oil is 14.7 inches in front of it.)

Best of luck, and study the POH for your particular aircraft.
.
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Old 21st Nov 2019, 12:42
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For the Cessna the datum point is somewhere near the back of the bonnet
... which may vary if the plane is further from the car park.

Seriously.... the datum for any plane is specifically stated in the POH and the weight and balance forms, and very easy for the pilot to find. For single Cessnas, it is usually one face of a stated portion of the firewall, which can be accessed through the cowling. Other types use other datums, some forward of the plane. I have never encounter a datum behind the C of G of an airplane. The moments can be calculated relative to the datum as stated. Understand that weight and balance moments, which the pilot must calculate, will differ from the aerodynamic moments. The pilot should be aware of the aerodynamic moments, which affect how the plane reacts in a stall, while very certainly respecting the weight and balance moments, as those assure compliance with the aerodynamic moments.
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Old 21st Nov 2019, 17:13
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No need to make it more complex than the OP needs to pass the test. Just do the calcs as set out in the POH and his PPL text book. If he then has any questions about the whys and wherefores he can ask his instructor. Oh wait.....
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Old 21st Nov 2019, 21:22
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From my experience of training on 150/152 it is hard to load them outside their C of G limits, but easy to overload them.
You may find you are very close to max weight and have to reduce fuel load.
Note it is weight not mass as mentioned mass would be the wrong unit of measurement.
Good numbers to remember on fuel are 189lt=50 us gallons = 40 imperial gallons = 300lbs

It is important to learn how to do the calculation as you move on to more complex aircraft with multi tanks as the C of G can move as fuel is burnt on some aircraft, to the point that it is impossible to raise the nose on landing or it goes backwards with loss of pitch stability.

So I think the advice given is critical, not difficult maths bit of addition & multiplication. Best luck with test.
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Old 21st Nov 2019, 22:36
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As a current flying instructor I am very surprised at the original post. Your instructor/ flying school have done you a disservice putting you forward for a flight test without the required skills and knowledge.
By the time you are ready for a skills test you should have a very good idea about Weight & Balance. (Mass & Balance for whenever the authority decide to change it back again). It’s a fundamental of flight planning and you should have done a theory exam on it too. It’s really not a good situation ‘having no clue’ at this late stage. Take the advice the others have given you and give your instructor a hard time about it too.
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