Aircraft W&B check 3

[a5130]

Hi All,

I have been tasked to check the weight & balance of a Q400 aircraft if a stretcher kit is installed at a specified reference point. Could someone outline the step-by-step procedures on how to do this? I have the stretcher weights and arms, weights of the seats to be removed and the location.

Note: By doing this check, I would be able to determine the worse case scenario, that is, how many stretcher can be installed with the how many seats to be removed.

 

[dgapilot]

Does the operator of the aircraft have a weight and balance program now? Do you have the empty or operating weight and CG of the specific aircraft involved? How about fuel weights and station numbers? Does the gear swing change CG location? Lots of data you didn't identify are needed to figure worst case scenario. On airplanes like the DHC-8-400 (I'm assuming that is what you mean by Q400) the load and fuel are always trade offs. You will need to know what route you want to fly, and what alternate is needed to figure the required fuel load. You also need to know what the landing or min fuel load anticipated will be so you can calculate both take off and landing. If the gear swing changes the CG, you also have to figure both conditions with gear up and gear down.

A good place to go for basic weight and balance calculations is

http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/FAA-H-8083-1A.pdf

Hope this helps.

 

[rb1957]

just to clarify, you have weights of seats, stretchers, and "occupants" ... including extras like overhead bin luggage or required medical equipment ?

you know where you want to install the stretchers (defined on a LOPA), and how many seats are removed to allow this. personally i'd be surprised if there was much difference (between the stretcher and the removed seats, particularly between a loaded stretcher and the equivalent number of pax) ? i mean a stretcher has to remove at least 3 seats (ie 6 pax) ?

there are two ways to do this ...
1) as an absolute ... start with a specific airplane LOPA, remove seats, add stretchers, or
2) as an increment ... just calculate the delta due to removing seats and adding stretchers.

btw, "worst case" ? why ? surely stretchers would be loaded closest to the front of the cabin (it'd be Really hard to maneouvre a stretcher down an 18" aisle). I'd put the stretchers at the front of the cabin, moving aft as I have to install more.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

If the stretcher has oxygen bottles in it, expect the local airworthiness authority (or your boss) to tell you there are areas of the cabin where they shall not be installed.
Look up "Design Considerations for Minimizing Hazards caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure" from the FAA website (AC 20-128A).


STF

 

[VEBill]

It's not precisely my area, but from what I've seen, W&B calculations are done all the time and the exact procedure to be followed *should* be in the aircraft manual somewhere. And there *should* be plenty of examples of all the previous calculations on file somewhere.

You not only need the procedure, but the same manuals must have the C of G / W&B *limits* listed.

Some aircraft can be on the edge of their limits due to modifications over the years, so small changes can be important.

 

[rb1957]

the procedure ain't rocket science ... weight (added or removed), station, moment ...

"Some aircraft can be on the edge of their limits due to modifications over the years, so small changes can be important." ... i think that's why you should do an incremental W&B (rather than starting with some specific a/c) and it is up to the installer to make sure he's operating within the Flt Manual limits. In fact, if you're doing the STC to install the stretchers, your LOPA should define where stretchers can by installed and final cabin arrangements (if you have stretchers in the rear of the cabin, do you install seats ahead of them, presumably after the stretchers, or do you fly with an empty cabin (other than the stretchers) ? The problem I see is that there are many possible configurations, so doing a W&B to cover them all seems extensive, and I think it'd more straight-forward to say "recalc the W&B for the strethers installed, each stretcher weighs ZZZlbs, stations as per LOPA ...".

Unless you're doing this for a specific a/c ...

another day in paradise, or is paradise one day closer ?

 

[a5130]

Thank you all for your comments and suggestions, much appreciated.

These are the steps that I will take to determine if installing the stretcher will change the C of G.

i) Calculate the effect of seat removal i.e. total weight and moments. 1 stretcher = 3 seats removed, 2nd stretcher, another 3 seats removed and so on.
ii) Calculate the stretcher installation effect i.e. stretcher weight added at known locations/stations.
iii) Calculate the overall effect i.e. effect of seat removal & effect of stretcher installation (total weights removed/added & total moment)
iv) Comparing the final figures with the aircraft W&B records to see if it will have an effect on the C of G.

End///

 

[rb1957]

removing 3 seats is worth 100 lbs (near enough) ... stretcher alone should be less than this.

then one patient compared to up to 6 pax (and their carry-on) ...

sounds like it's a wash to me

another day in paradise, or is paradise one day closer ?

 

[a5130]

For your information, after calculations, installation of the 6th stretcher assembly will have an effect on the CG, which will require a new load data sheet.

 

[rb1957]

that seems a little "odd" ... why the sixth ? is it that the moment increases with the arm ? are you the operator, or the modifier ? sounds like the operator ?? i was thinking more in terms of the modifier.

how did you account for "up to" 6 pax ? ('cause i've got to believe that 1 loaded stretcher weighs less than 6 pax.)

another day in paradise, or is paradise one day closer ?

 

[VEBill]

Again, I'm not an SME in this area. But...

Too little weight in a given location can be just as dangerous. Never assume that you can remove weight and be safe; the weight might be necessary to keep that end of the aircraft down.

Also, never assume that the change you're doing is less than the ever shifting passengers and thus not important. That is all taken into account in the limits, and you cannot assume that the passengers will or will not actually be there; pushing up against the mins or maxes in the this direction or that.

W&B is something that has to be done 'by the book', double checked, and by qualified persons.

Good luck.

 

[berkshire]

When all else fails, and you have completed the modifications, put the aircraft on a set of scales.
You can be horrified at times when, what the aircraft actually weighs, and where the CG really is, compared to your calculations, are far apart ,and at other times pleased when they are where you figured they should be.
I have had aircraft where they have had a factory weight and balance be as much as 30 pounds different and the CG be over 3" away from the factory's numbers.
B.E.

You are judged not by what you know, but by what you can do.

 

[verymadmac]

Berkshire was that 30 lb on small aircraft, I have seen a boeing 737, that was 500 lb under (last weighted by the conversion facility).

There seems to be something about weight and balance that confuses a lot of people.

A5130, first is it just the net weight change you are trying to define or are you getting in to the operation aspects of the weight and balance i.e. the "as prepared for service weight". The operator should have a definition for what the "as prepared for service weight" is (often it will be the same as whats in the OEM's weight and balance manual but not always). It may include the mass of the flight crew, the flight attendants & associated bags, as well as food carts, etc ,etc.

 

[berkshire]

Mac,
Yes that is on small single engine aircraft. It was common practice for some manufacturers to not weigh the aircraft as they came off the line, and use a calculated weight, based on an average of the aircraft weighed further ahead on the line. And the 30 pounds I was talking about was over not under.
Without naming names the companies were based in Wichita Kansas.
The thing that bothered me about the original post was that the questioner appeared to be removing seats and adding stretchers from the front of the aircraft, which depending on how many he did, would tend to make the aircraft, tail heavy, which prompted my comment about weighing it when he was done.

You are judged not by what you know, but by what you can do.

 

[verymadmac]

Sorry that should say "Boeing 737, that was 500 lb under recorded."
There was an airliner manufacture that until recently weighted the aircraft twice and used the lower of the two weights.

I would add that if the aircraft was weighed after installation, an exacting record of the aircraft configuration needs to be made, as there nothing worse that the conversation along the lines of, we weighed the aircraft 6 months ago but the records don't define which LOPA (or LOPA revision), or part there of was fitted.