How to Obtain Aircraft’s Published Performance After Years of Operation

Airplane maintenance and good performance are closely related.

The numbers published by the manufacturer are considered as optimistic as an aircraft is brought into service.

With the passage of time, this aircraft goes through various wear and tear – bugs on the leading edge, prop nicks and not to forget the bad landings.

The published numbers can be realized even after years of operation.

In case the airplane’s actual performance is different from the published figures, it is a sign that the condition of airplane has diminished as time passed and hours of use.

The difference in performance may be due to the level of maintenance an airplane received over the years.

You should expect the actual performance of aircraft will match the published numbers only when it has received proper maintenance and any shortcomings are fixed in a timely manner.

I am not talking here about a missing screw or two which escaped inspection. I am talking here about a combination of discrepancies that prevent from obtaining a book performance.

A Level Playing Field

Performance issues may be due to airframe and the numerous simple items that should be addressed first before moving on to labor-intensive and more difficult items.

Accurate instrumentation is one such simple item. For example, mechanical tachometers are notorious for being inaccurate. They should be checked regularly with a calibrated instrument.

A tachometer that is off by just 100 RPM can severely effect your airplane’s performance.

Static systems are quite sensitive and should be maintained in good condition. It will effect the terrain and transponder performance along with airspeed accuracy.

Static ports should be kept unobstructed, clean and free from any distortion. Static lines connecting the static port to instruments should be checked for cracks, discontinuity, leaking alternate air ports and broken moisture traps.

The pitot static system is checked only when it is re-certified, however, a problem may occur when an adjacent system is being fixed or checked.

Here are simple checks:

  • A climb or descent is noticed on the VSI when the airplane’s window or door is closed (with engine off on the ground).
  • A momentary descent or climb is noticed on the instruments when the cabin heat valve is opened in flight.

There can be something wrong with the static system in case you notice such things and should be checked.

Leaking static systems can show erroneous airspeed indication which can effect aircraft’s performance. Your climb performance will differ even if there is a variation of five knots.

When was the last time the accuracy of airspeed indicator was checked? Has any other instrument every been taken out for calibration or overhaul?

Instruments do get old, diaphragms leak, markings fade and needles stick, which makes them inaccurate and less responsive.

Instruments become inaccurate as they age and give distorted information to related gauges, whether they are analog or digital.

During preflight, attention should be given to areas near pitot tubes, static ports and related equipment. Check for any scratches or ripples, dens and wax or dirt buildup.

There should be no bird droppings, bug nests, ice and / or snow in and around these areas. Make sure they are clean during preflight and you don’t takeoff with them.

Aircraft Rigging

since a long time, the problem of rigging has been troubling mechanics and they try to fix it using the least expensive method. It is possible to fix these problems using these methods, however, they often create other problems in future.

This is the reason why an aircraft that is similar to yours cruise a handful of knots faster than yours, or may be slower.

An example of fixing a rigging problem is to drop one flap slightly, which creates more lift on one wing. This method is effective, but it also result in drag. Over time, this method can be used more than once, and the opposite flap is being dropped at times. This makes the problem even bigger.

There will come a moment when you will say enough is enough and want to start things from the beginning.

To rig an airplane properly it takes time and money and it needs to be flown many times before everything is right. Every airplane cannot be rigged using the drop-a-flap method, but each one of them need to be rigged for hands-off cruise flight.

In case you feel the need to hold the aileron or rudder to maintain co-ordinated, level flight, than it call for the aircraft to be checked by a maintenance technician.

In case the airplane surprise you during flight than it requires more than just simple adjustments. Was the airplane flown by someone else and the damage was not reported? A recent storm caused damage to it? Was it flown in extreme turbulence or has it exceeded some limitation such as flap operating speed?

Much of the performance data published is based on actual flight testing by the manufacturer. A brand new aircraft is used for testing including instruments, airframe, propellers, power plant and other equipment.

These airplanes are flown in favorable conditions by a professional test pilot. They are clean of bugs and dirt, optimum center of gravity, usually at gross weight and perfectly rigged.

When you go through the performance data of any aircraft, make sure you pay attention to the conditions under which these number were obtained. These numbers need to be adjusted as per the condition that will be there at the time of the flight.

Flight Controls, Fairings

When was the last time you had a good, careful look at the flight controls of your aircraft? Here are the things that need to be checked:

  • Make sure the tension right in the cables
  • Make sure they are securely attached with no free play
  • Make sure the rod ends are properly lubricated and they move freely within limits
  • Check for any grind or squeak sound and if they are hard to move to full travel
  • Check if the flaps are loose or sloppy. Check if there is any side movement and if they touch the fuselage (not good)

There is no exact measurement but there should be no free play in flaps as it can effect overall performance.

Flaps should be lowered with the recommended speed limit, or else it can damage the flaps and upper wing areas ahead of flaps. This is one more reason to make sure the airspeed indication of the aircraft is accurate.

Affect on Aircraft Performance due to Propeller and Powerplant Items include:

  • Alternate air or carburetor heat not closed fully. Warm air leaks from associated ducting into engine intake system.
  • Rodent or birds’ nests, snow / ice pellets obstructs airflow in the air intake system or dirty air is filtered.

Mixture and Throttle Controls

  • Wear of contact points or point block can retard magneto timing
  • 150 to 200 RPM drop during pre-takeoff magneto check will give a decreased performance
  • Low cylinder compression due to verified mechanical issues such as scored cylinder bores, burned valves, cracked cylinder heads etc.

Loose Exhaust Baffle

  • Valves don’t open or close fully as one or more camshaft lobes may be pitted or worn. High metal content in oil may result in internal engine problems. All this decrease performance ultimately.
  • Check for excessive electrode gaps and / or cracked, worn ignition harness, worn spark plugs
  • Mixture and throttle control don’t adjust properly and don’t travel completely for mechanical stops
  • cooling air ducting broken or missing or one or more engine baffles
  • Exhaust or muffler system partially blocked with debris from broken internal baffles
  • aerodynamic inefficiency caused due to nicks in propeller blade and pits not dressed properly

Weight and Balance

The overall performance of an aircraft is affected considerably by the way it is loaded. Performance is also affected by supplemental or optional equipment and how well they are secured.

For example, an aft CG condition may result in higher cruise speeds in some aircraft. An airplane’s CG is also affected with fuel burn and landing gear retraction.

One can experiment by loading the airplane differently and noting the differences. Make sure you are within limits and you have done your weight and balance calculations properly.

Importance of items related to airframe include:

  • Cowl flaps may be poorly adjusted or loose and do not close completely
  • Airplane may not track straight on the ground due to steering or landing gear not aligned, low tire pressure or a dragging brake. It can result in increased takeoff distance.
  • Incomplete or improper repairs after a major damage to airframe
  • Leaking static system or effected by disrupted airflow near static ports can give fault airspeed indication

Conform to the Type Design

Recommendations from the manufacturer should be the standard in maintaining any aircraft. Aircraft and its equipment that are FAA type-certified should adhere to its type design and supplements in order to airworthy.

There have been many instances where engines and components such as magnetos, carburetors and spark plugs, spinners and propellers that are used are not type certified.

Manufacturer’s recommendations should be followed for maintenance and inspection to ensure optimal performance from 0 hours to TBO. For example, if there is a reduction in climb performance it calls for an investigation of engine performance.

The point here is to make you think about your attitude towards maintenance and the relationship it has in achieving published performance. There are some maintenance where you will have to hire a certified technician. There are numerous preventive maintenance tasks that can be performed by the owner.

Talk to your maintenance technician and work along with him to maintain your aircraft in optimal condition. You should never rush through the pre-flight procedure.

Keep a note of the recommended schedules for maintenance and sort out the discrepancies in a timely manner to make sure your aircraft gives best performance.

Leave a Reply

Your email address will not be published. Required fields are marked *