Aeroplane performance calculations and safely applying them.

Our friend, Gavin Weir, CFI, Auckland Aero Club has written a really useful article for us today. He writes about something that is rather tricky for some of us and I hope you find the article not only interesting but containing a a few gems of advice for you.

All yours, Gavin!

Gavin Weir, CFI, Auckland Aero Club

Performance calculations – the bane of the student pilot preparing for flight test, or the license holder at BFR time. We are sometimes dismayed to have to pause to calculate the performance of our aeroplane on a given day, but why are we doing it in the first place? Take-off and landing distances are of course critical to flight safety, and Part 91 places the onus on the Pilot in Command to have determined that the runway in question is of sufficient length. There are several ways this can be achieved.

The group rating system established by CAA NZ is one method whereby if the group rating of the runway is equal to or larger than the aeroplane, then a take-off or landing may be successfully made IF the aeroplane is operated in accordance with the Pilot Operating Handbook. There is another gotcha in the group rating system however, the conditions used in establishing the performance number for the aeroplane is ISA (the International Standard Atmosphere). If the pressure is lower, or the temperature higher than ISA, then the performance of the aeroplane will be worse than may be expected based on the group rating alone – it would be a good practice to complete performance calculations if the group rating of the runway is the same as the aeroplane, and even if the runway figure is one larger than the aeroplane if the conditions are markedly worse than an ISA day (or there is any suspicion of a damp or wet runway for landing).

The primary method of determining take-off and landing performance is by calculation. The aeroplane Pilot Operating Handbook provides the means of doing this be it via performance graphs or tables. Pilots MUST be comfortable with the procedure laid out in the manual for that aeroplane type to ensure that when required the appropriate calculations can be made. This method provides what appears to be a very accurate calculation of the distances required for take-off and landing – consider then what must occur to achieve these numbers.

The aeroplane itself must be in good condition with the engine delivering normal take-off power, the tires at the proper inflation, the pilot performing the take-off or landing accurately in accordance with the published procedure and the conditions must match those used for the calculation. How long is the grass? Is the soil underneath soft or firm? Is the slope uniform? Is the wind direction and speed accurate and steady? Temperature, pressure, humidity….

One note to make here regarding the calculation of distance required – definitions.

·         Take-off distance required (TODR) – from brake release to 50’

·         Landing distance required (LDR) – from 50’ above the landing threshold to a stop

These are the figures that must be calculated and applied to determine if a take-off or landing can be successfully made. The ground roll figure is just that, the on the ground bit of the manoeuvre, and are of an informative nature rather than the overall distance required. Pilots are required to apply the TODR and LDR to the distance available.

Pilots are responsible for the safe operation of their aeroplane and the safety of those on board and external to the flight – to ensure a healthy safety margin is maintained it is universally recommended to apply a personal minimum to the calculated performance distance required. This may be a percentage such as adding 20% to the final figure, or a fixed extra distance for example adding 100m (think for landing, when the aeroplane comes to a stop having been flown accurately in accordance with the procedure, how much runway distance left before reaching the fence would the pilot be COMFORTABLE having? Presumably more than “just enough to turn off the runway”!

C-172 performance data from the flight manual

So what can pilots do to ensure compliance with the rule and absolute safety in their operations?

•  Firstly, be proficient in the published maximum performance techniques for the aeroplane being flown – these do differ from type to type and not applying the correct technique WILL have a significant effect on performance.

• Practice! Grab an instructor if desired to get feedback and tips. Calculate the distances required accurately, don’t take shortcuts to make it “quicker” or “easier”.

• Consider the conditions, if planning a morning arrival and afternoon departure, obtain the forecast afternoon winds and temperatures, not the cool of the morning for that take-off calculation! If this information is not available or uncertain, use a worse-performance value that will be definitely beyond the conditions to be expected.

• Don’t rely on a capricious breeze to “make it work”, always err on the side of worse performance and more margin.

• Try to avoid rounding during workings, and always round in the direction of worse performance – this will avoid cumulative rounding errors giving a false answer of adequate performance which could prove incorrect in the act of taking off, for example…

• Apply a personal minimum on top of the calculated figure AND STICK TO IT, be honest with yourself and allow for slight handling errors, and a comfortable amount of space remaining at 50’ on departure, or when the aeroplane stops on landing.

• Set acceleration abort points on the take-off where if the acceleration feels sluggish or unusual the take-off is abandoned. The ground roll figure from the calculations will give the pilot a view of when lift-off should occur, see if there is a landmark around this point on the runway to confirm flying speed is reached at the right distance – if not, stop!

• For landing a decision height on final to commence a go around if unstable on approach, and a decision point not more than 1/3 into the runway whereby if a touchdown has not occurred then an immediate go around is executed will allow performance monitoring through the manoeuvre.

• Always remember safety first and stick to your minimums – just because it’s only 10m inside the final calculated figure including minimums, doesn’t mean that one should go anyway because there’s a margin built in – that’s a slippery slope of reducing margins to the point where one day there may be no margin left.

• Have a rough idea of the aeroplane’s normal performance figures, say a 250m ground roll and 500m take-off distance, that way if the answers you get are way off, it’s an immediate red flag that something went wrong.

• There’s an app for that. Of course there are! And there are some quite good ones such as Soteria’s Wings. But again the conscientious pilot should be aware of WHAT the app is doing, what margin (if any) is being applied, and have that rough idea of what numbers you’re expecting to see, so if there’s a data error, you will notice that the answer seems off.

It may have been noted that no reference has been made to “rule of thumb” assessments of field length – these are a setup just waiting for an accident. Just because “it worked last time” or “Joe Bloggs told me this aeroplane is fine on this runway” or whatever permutation of the same gets bandied about, these are neither accurate nor acceptable at any time. Consider the Herald Test – would family and friends like to see a front-page article of an accident caused because the pilot relied on an arbitrary assertation that “she’ll be right”?

(Melissa here… At Air New Zealand, we were always taught that just because something has ‘always’ been done a certain way, it doesn’t mean it’s the right way. Always be responsible for your own safety!)

Remember – it is the pilot in command’s responsibility to ensure that performance is suitable for both take-off and landing.

Be safe, be conservative, and enjoy a long career of happy flying!