December 8, 2016

What the manual says and what it doesn’t say


What the manual says and what it doesn’t say

Donald Anders Talle


Over the course of the last 26 years I had the opportunity to fly a slew of different types of aircraft from a number of different manufacturers. One common denominator among most is that their pilot operating handbooks (POH) bear little similarity to each other.

Of course aircraft from the same manufacturer and roughly same time period have similar layout and content, but that’s about as much credit as I can give the light aircraft industry for standardizing this important document.

Don’t get me wrong, I’m not necessarily placing any blame here, just pointing out an important fact. Even with the advent of GAMA (General Aviation Manufacturer’s Association) guidelines, there are still vast differences in the way pilot operating handbooks are laid out, and what information they include. As a result, a couple of important issues arise: 1) What the manual says or implies can be done, does not necessarily mean it’s a good idea to try, and 2) What the manual does not say, does not necessarily mean that it cannot be done.

Starting with a seminal event of the 1970’s, GAMA’s publication of the “Specification for Pilot’s Operating Handbook,” manufacturers have had guidelines for the standardization of these manuals. This was a great leap forward since prior to that time most pilot operating manuals had very little information, especially in the performance section, and the organization and presentation of that information left much to be desired.

However, even with better guidelines some discretion still exists in many aspects of POH development and design. Although regulations sometimes deserve most of the blame, one of these areas of some discretion is on which performance information to include and how that information may be presented (e.g. graphs or tabular format).

While some charts or tables are always required by regulation, like takeoff ground roll and distance, some are not, like accelerate-go for light multiengine airplanes. And it seems that whether or not a manufacturer deems non-required information important enough to include is what determines whether it finds its way into a manual.

For example, consider two very similar aircraft: the Beechcraft Duchess, and the Piper Seminole. The Beechcraft Duchess (BE-76) has a performance graph for accelerate-stop as well as acceleratego, while the Piper Seminole (PA-44-180) has only the accelerate- stop for early model manuals, neither accelerate-stop nor accelerate-go for a stretch of models (I think that was a publishing error!), and then only a short-field accelerate-stop graph in the more current manual.

Anyone who has flown both of these aircraft knows that they are quite similar in both weight and performance. So why then would they choose to treat performance graph inclusion differently? Only the manufacturers know for sure, but when it comes to accelerate-go, I’ll guess that the lawyers had something to say. After-all, doesn’t the inclusion of an accelerate-go graph imply something, like just maybe that the aircraft is capable of continuing a takeoff climb after engine failure at liftoff?

Having flown the Duchess for many years, I can’t recall too many days where it was capable (even when lightly loaded) of a successful accelerate-go; of course I knew how to read the graph and wouldn’t have tried it anyway.

But wait, there’s more than one graph to be consulted in determining accelerate-go capability in the Duchess. Beechcraft also included a “Take-off weight to achieve positive single engine rate of climb at lift-off” graph. It turns out that even though you can produce a figure for accelerate-go for just about any condition or weight, you might not even get airborne in the first place (according to the other graph).

Sound like an accident waiting to happen? Maybe that’s why Piper chose not to advertise accelerate-go at all! After all, no light-twin (under 6000 lbs with Vso under 61 kts) is required to have single-engine climb performance in the first place! Sometimes, the absence of information signals that the underlying operation may be risky at best. Continued single engine flight in most light twins is just that; risky at best.

Have you ever taken off from a grass field in an aircraft that had no performance table (or even no instructions for that matter) for doing so? Did you wonder why some aircraft have provisions for soft-field takeoff and some don’t?

How about a paved runway takeoff, but on a runway that has a slope? Or one that is wet? I have a whole slew of aircraft manuals that give no information other than for a “hard surface, level and dry.” Does this mean that you can’t takeoff on a runway that has an upslope of one degree, or land on a runway that is wet? Of course not!

But a general understanding of the impact the condition of the surface has on takeoff and landing performance is critical in lieu of specific manufacturer guidance. A soft surface can increase takeoff distances by 20% or more. These surfaces include grass, snow, gravel, etc.

Wet, long grass may lead to the worst performance for takeoff and even worse for landing. While it’s counterintuitive that long grass should increase landing distance, it’s a matter of fact that braking cannot be performed on grass as would be performed on a normal paved, level and dry surface.

As a result, even with the increased friction due to contact with the grass, the ground roll will be longer than for a paved runway. I suppose the exception would be for an aircraft that did not have brakes in the first place!

Now let’s consider a wet grass runway (factor approximately 1. 3 for increased takeoff distance), an up-sloping runway of 2% (factor another 1.1), and the end result is an overall increase in takeoff distance of about 40%. As it turns out, all the factors that would serve to increase takeoff distance are cumulative. On the other hand, it might be the case that multiple conditions for takeoff cancel each other out.

For example, a down-sloping runway may cancel out the negative effect of a slight tailwind or short-grass runway. Bottom line: think about the condition of the runway and be ready to apply a reasonable amount of “fudge factor” to the calculated performance; some for the conditions, and a little more “fudge” for imprecise pilot technique.

One last note on this topic: some manuals appear to provide little or no guidance on varying runway conditions but in fact hide it in small print somewhere near the table or graph. Singleengine Cessna manuals come to mind! Be sure to scour every performance table or graph for associated conditions and “notes” on factors that impact the performance figures.

Let’s talk about operating with a tailwind. From very early on in my training, taking off and landing with a tailwind was talked about as if it was an absolute no-no, and surely would end in disaster if ever attempted.

In practice, I’m now convinced that those naysayers were either taught incorrectly during their early training, or simply misunderstood the intent of the cautionary words on the subject.

Upon review of older aircraft manuals, one finds very little guidance for the impact of wind on takeoff or landing performance. Many have provisions for a headwind component, but some have no information about affect of wind on performance.

Starting in the 1970s I see provisions for not only headwinds but also tailwinds up to 10 kts (at least for the three manufacturers I reviewed for post- ’70s information). And low-andbehold, the GAMA specs, which into effect about that same time, talk about including tailwind information for takeoff and landing! What happened?

Well, airplanes didn’t change, but the way airplane information is presented to pilots did. Now just because tailwind performance information is presented doesn’t mean it’s a good idea to operate with it, but recognize that small tailwinds impact nothing other than the amount of room it takes for takeoff or landing.

Can you takeoff and land with more than a 10-kt tailwind (assuming your aircraft manual has figures for that kind of wind)? Probably, but if you fly your aircraft too “far off the performance chart” you’re playing test pilot, and you’d better know what you’re doing or an accident may be lurking in your future.

Is it safer to always takeoff with a headwind component? Probably, but sometimes it’s not practical. Either way, if in doubt, follow any recommendations the manufacturer provides on the subject. By the way, for large aircraft, many airports will retain an optimum departure direction right up until there’s roughly a 10-kt tailwind, prior to switching to a less optimum departure flow.

I once conveyed some of my concerns about aircraft manuals to a fellow pilot, who replied coyly, “you know, the manual doesn’t say you can do it, but it also doesn’t say you can’t!” While I would never argue which such clear logic, ethics require me to point out that this is not quite the right attitude to take on the subject.

Regardless of what the manual says, or doesn’t say, we are ethically required to do what is regarded as safe. Enough said!