<Warning, long rant follows>
This coming Bank Holiday Monday, I had promised a ride to some of my family. I gave them a few options re. where they wanted to go, and they chose Compton Abbass - they liked the idea of flying across the Salisbury Plains. I've been there once before, and it's a nice airfield in some beautiful scenery - should be a good day out!
Now I know that the PA28-161s that I fly aren't too good with four on board, even though one of them is my 13 year old cousin (who is more like 8 years old in terms of her physical size). So last night I did some very careful weight, balance and performance calculations.
Weight needs some planning, but is not a problem. With the 4 of us on board, we can get enough fuel on to get us there and back with around 1.5hours of reserves - more than enough. And the CofG is well within limits.
Next, I moved on to the performance calculations. Landing is no problem - although if my landing is anything like the one I did last week, where I used most of the 2000m runway at North Weald, I'll find myself in a field 1/2 mile off the end of Compton's runway!
Next I moved on to take-off. Calculated the take-off performance, factored in the grass runway, then added on 33% as per CAA recommendations. I was surprised to find I needed around 860m of TOR, compared to the 800-ish m of TORA at Compton Abbass. (I think it's 806m, I forget exactly.)
Now, I've done a fair bit of flying in PA28s over the past few months. Much of this flying has been me and an instructor, with full fuel - which brings the weight pretty close to gross. So I thought I had a reasonable idea of what kind of performance I could expect, and even though Compton Abbass is at about 800' (700' higher than I'm used to), the weather forecast for Monday is pretty hot, and I'd allowed for zero wind, I didn't expect to be a full 60m short of runway!
Suspecting that something was wrong, I checked the factor that I was using for the grass field, and the safety factor. Both correct. I double-checked all my figures, they were all correct.
It was still fairly early in the evening, and I was bored. On a whim, I decided to head down to the airfield and take a look at the POH.
Now, I don't know about the aircraft you guys fly, but this is how it works with the PA28s at White Waltham. The main body of the POH contains about a million performance charts covering just about every conceivable scenario. But we're not allowed to use them. Instead, we have to use the CAA-approved performance charts, which live in the CAA Supplement at the back of the POH. There are a few differences between the CAA charts and the Piper charts:
- The distances required for take-off, landing, etc, are slightly longer. Presumably Pipers were designed to be used in USA air, and don't work so well with UK air? Well, I don't have any better suggestions, do you?
- The distances are shown in metres as well as feet. This is possibly the only useful difference
- The CAA charts include the ability to factor in runway slopes. I guess a simple chart of %age factors for different gradients is insufficient for the few runways where there's any appreciable gradient, so they decided to make their charts more complicated instead?
- Whereas the Piper charts are very nicely presented and easy to use, the CAA charts are photocopies of hand-drawn diagrams on something that looks very similar to WHSmith's "Back To School" graph paper.
The other big difference is that instead of about a million charts, there are something like 5. This means that instead of having around 1/4million different choices of charts to use for take off performance, I only have one.
And this, ladies and gentlemen, is where the problem lies. You see, somewhere in the small-print of the CAA suppliment, just before the charts themselves, it has the magic piece of information that I'd missed. "Take-off figures assume <insert several assumptions here>, 0 degrees flaps, <insert more assumptions here>"....
Wait a minute! 0 degress flaps? Now, I may be wrong here, but I thought take-off performance figures should show the
minimum take-off run required. And the minimum, unless I've been taught incorrectly, is definitely
not acheived with 0 degrees flaps!
I flicked to the Take-Off section of the normal operating procedures chapter, and there, sure enough, it states that "a significant reduction in the ground run can be acheived by using 25 degrees of flap."
So, the question is, which
idiot in the CAA decided not to measure the short-field take-off performance???
I showed this to my instructor, who had finished work but was sitting around having a beer before she went home. The "official" Piper charts show two sets of performance figures - one with 0 degrees flap, and one with 25 degrees flap. We decided to find the difference between the two for the specific conditions under which I would be operating (turns out to be just under 8%), and apply this to the CAA charts. You'll be pleased to know that, once I did this, there is more than enough runway at Compton Abbass - so I will be heading down there on Monday, weather permitting.
There are two morals to learn from this:
- Read the small-print, and find out exactly what performance your performance charts are giving you
- The CAA are idiots!
Ok, rant mode off, and if you've got this far, I thank you for letting me unload my anger on you!
Blue skies and safe flying,
FFF
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