Monday, June 18, 2012

Balancing Act, Made Easier

The plan was simple. I was to receive an aircraft check-out and the instructor simply wanted a completed familiarization sheet and three trips around the pattern. I'm no slacker when it comes to check-outs, but this plan seemed reasonable because I had given nearly 500 hours instruction in this aircraft type. The fam sheet was the usual series of questions requiring an Easter egg hunt through the aircraft flight manual, but the flight turned out to be anything but perfunctory. My first landing was humiliatingly flat: I struggled to de-rotate smoothly, but even with yoke all the way back the nose wheel touched a mere 100 milliseconds after the main wheels. Flummoxed, I reached into my bag of tricks, but the second landing wasn't any better. After the third flat landing with full back elevator, the instructor announced my check-out was complete. The first words out of my mouth were "Something is wrong with this airplane." The instructor assured me all was normal, but I was skeptical. Arriving home, I created a weight and balance spreadsheet for the plane, plugged in the data, and discovered we had done something I'd sworn I would never do. We'd flown an airplane with the center-of-gravity (CG) outside the forward limit.

Assumptions & Error-prone Arithmetic

Years earlier, the aircraft in question had its propeller changed and a different, heavier model installed with the appropriate log book entries and paperwork. The new prop made an already less-that-ideal forward CG situation worse. The incorrect assumption made by me, my instructor and pretty much everyone else who'd flown the plane was that, like most light GA aircraft, the weight and balance was fine with two people in the front seat and fuel to the tabs. We all know what happens when you assume ... Having uncovered the CG issue, temporary ballast was placed in the baggage compartment and eventually field approval was received to permanently mount several pounds of lead ballast in the tail.

If a task is tedious and time-consuming to do by hand, pilots will be less likely to do that task and just make assumptions. Instead of assuming, pilots should be encouraged to use technology to help make weight and balance calculations easier to accomplish. Weight (or mass, if you prefer) and balance calculations themselves are conceptually simple, but if you're in a hurry to depart, or under external pressures (not wanting the bore or alienate your passengers), or if you make assumptions (as illustrated above), you could make a mistake. That mistake could embarrass your or it might kill you.

All pilots know the basic weight and balance drill:
  1. Add up the weight of the aircraft, the weight of the fuel on board, and the weight of all the passengers and bags.
  2. Calculate the moment for each item (weight * station) and add up all the moments.
  3. Divide the total moment by the total weight to get the loaded CG.

W&B Apps

If you want speed and accuracy in weight and balance calculations, use a computer. There are many specialized apps out there for smartphones and tablets, but you can roll-your-own weight and balance spreadsheet, too.

CFI Tools was one of the first suite of iPhone/iPad apps to provide sample weight and balance envelopes and takeoff/landing calculations for popular GA aircraft. If you don't find your aircraft you can create your own by entering the CG/weight limits. There are separate apps for calculating performance, crosswind, weight and balance, and so on. There are iOS and Android versions as well as a smaller subset for Windows Phone 7. CFI Tools Preflight Wx+ app packages all the individual apps into one, but currently is only available for iOS. With this app, there's no excuse for failing to calculate expected performance, density altitude, and weight and balance. These various apps aren't free, and they're a bit clunky, but they still provide pretty good bang for the buck.

Real-time takeoff & landing, CFI Tools PreflightWX+

Roy Kronenfeld's free Aviation W&B Calculator for the iPad can be customized to provide weight and balance for most any aircraft. You can also customize for your own aircraft or visit the web site, which has a selection of templates for a lot popular types supplied by other pilots. If you use someone else's data, remember to trust, but verify! All for $10.

Weight & Balance calculator's convenient data entry.

Weight & Balance calculator's summary page.

Anyone who flies a Cirrus SR20 or SR22 should already know about Robert Urschel's amazing and free app Cirrus Perform for the iPhone and iPad. Separate versions of this apps are provided, one tailored for the iPhone and one the iPad.

Cirrus Perform W&B on the iPad.
Cirrus Perform Departure data on the iPad.

Not to be outdone, Robert also has created a free weight and balance iPhone app for the Cessna 172S model. It runs just fine in 2x mode on the iPad, by the way. Robert's apps don't just calculate weight and balance. With a network connection they'll download the current METAR for a specified departure or destination, show the runways available, and give you interpolated takeoff and landing performance based on the runway you choose. Pretty cool considering these apps are free.

Cessna Perform weight and balance on the iPhone.
Cessna Perform surface Wx retrieval.
Cessna Perform takeoff data.
If you are in the enviable position of owning or flying a Pilatus PC-12, there's the free PC-12 Digital Airplane Flight Manual HD for the iPad. It appears to be very nicely designed and seems to have all the bells and whistle a Pilatus owner or pilot could want.

If you're a spreadsheet nerd, it's relatively easy to create a weight and balance spreadsheet with a graph. Don't use any spreadsheet without first verifying that it is based on the correct data and providing the right results.

Garbage in, Garbage out

The only, let me repeat, the ONLY place to start when calculating weight and balance is the aircraft's Approved Flight Manual or the current weight and balance record found in the aircraft. If you get anything from this post, it should be this:

The generic empty weight and CG shown in a PIM or POH may be significantly different from the actual weight and CG of the aircraft you plan to fly.

Over the years, I've encountered many weight and balance records that had serious errors. Some errors crept in when equipment was added or removed from the aircraft, the mechanic or technician calculated the weight, arm and moment of the affected item, and then added or subtracted the weight and moment to arithmetically compute a new empty weight and CG for the aircraft.

I've even found incorrect weight and balance records that were prepared by the factory that produced the aircraft: One from Socata and one from Diamond. In one case, an arm of zero was used for one of the main wheels during weighing. In the other case, the moment/1000 was swapped for the arm for an item that was added at the factory. In both cases, it appeared I was the first person to raise the flag that something was amiss. Kinda makes you wonder how often pilots really are calculating weight and balance, doesn't it?

Important Stuff

Flying an aircraft within the correct weight and balance envelope is important for a variety of reasons, but the three important ones are load limits, aircraft handling, and performance calculations

An aircraft's maximum positive or negative load limits (found in the Limitations section of the AFM) can easily be exceeded when the aircraft is overweight. Part 23 specifies the aircraft must withstand a 50% additional load, but remember that the loads that have been imposed on an airframe are cumulative and largely invisible to the naked eye when inspecting the aircraft. If you're flying an aircraft that's 10 years old, it's safe to assume the 50% additional load safety factor has been used up.

When the aircraft is loaded outside the allowable CG range, all sorts of undesirable things happen. A forward CG cause the landing characteristics of the aircraft to become ... um ... challenging, as illustrated in my example above. An aft CG can make a plane easier to stall and stall recovery more difficult. And if you get into a spin, good luck.

In the greater scheme of takeoff safety, exceeding the maximum gross takeoff weight is one of the more dangerous things you can do since there is no performance data for an overloaded condition. You may be able to takeoff and climb just fine on a cool day at sea level. On a hot day at a high field elevation, you could easily endanger your life and the lives of your passengers, not to mention persons and property on the ground (cf. 14 CFR 91.13 - Careless or reckless operation). Consider this video of a Bonanza attempting to takeoff from Cameron Park, California. It ain't pretty, but it graphically illustrates the consequences of not carefully considering takeoff hazards.

Know Before You Go

It hasn't happened often, but on occasion I've seen pilots about to fly with too many passengers, pulled them aside, introduced myself and pointed out the danger in their plan. Some pilots may feel that it's none of there business what other pilots plan to do and that pilot-in-command authority is sacrosanct. I don't happen to agree with that philosophy. We're all human and we all make mistakes. Just remember there are mistakes that will embarrass you, and mistakes that can kill you. Avoid the latter.


David Cheung said...

Thanks for the insightful post, John. I've used CFI Tools since I got my iPad a while back and have always been grateful at the ease of use that it brings, considering I'm such a lazy pilot.

BTW, Roy Kronenfeld's free Aviation W&B Calculator for the iPad is no longer free, but listed as $9.99 in the App Store.

John Ewing said...


Thanks for pointing out my error. All fixed ...

Terence Wilson said...

Thanks for the thoughtful article John. I've been using WnB Pro ($5 for iPhone), it's has all the features you would expect and comes well reviewed.