Understanding some numbers
True airspeed (TAS)
We commonly use true airspeed (TAS) issued from the AFM for flight planing. TAS reliability is based upon a brand new aircraft flown by a test pilot corroborating the manufacturer engineering forecast. Some say this is not the real world. It is to a very close extent, at least it was, when the new model was out for certification.
Performance graphs for any type whether a Piper Clipper or a Boeing Dreamliner are issued for a new aircraft equipped with minimal hardware sticking out in the relative wind like antennas. The airframe being well polished, new propellers or fan blades derive power, brand new brakes and a straight airframe are the luxuries test pilots assume as normal. The test pilot benefits also from a clear mind and high situational awareness when flying since he/she is prepared to test all sorts of flight phases. Over to our side of the operating world, aircraft and pilot degradation (alright, I will use « human factors challenged pilot) contribute as much unfavourably to section 6 of the AFM.
This degradation abounds everywhere:
- Propeller wear: nicks, cuts, dents or simple chord reduction. The 1/2 inch (12 mm) average tolerance reduction in chord at the tip really hurts thrust.
- Engines not leaned to manufacturer specs. By the way TAS, for cruise is often provided for best power ie 100˚F rich of peak. Should peak EGT be used, be aware that the actual power output will be reduced by close to 4%.
- Engines not maintained properly: different spark plugs not approved for the type, annoying little check valves in the induction system, etc… you name it!
- Tachometer out of tolerance.
- Bent airframes.
- Worn, chipped or missing paint.
- Warped, cracked, missing plastic fairings.
- Improperly rigged flight controls: think of an aileron for example, sticking out in the airflow, then the trim required to fly straight… What a drag.
- Air gaps: door seals impersonating overcooked bacon.
- Pilot not flying straight.
The point here is not flipping over in screaming incontinent dementia. It is merely to acknowledge reality of cumulative performance erosion away from ideal book numbers.
It is reasonable to give oneself performance factor reduction of not more than 10%. Call this the « fudge factor ». In the airline world this factor is actually measured and incorporated automatically in the flight planing system for each individual aircraft. MEL’s have performance bias to be accounted for when an airframe part goes missing.
When flying short sectors with lots of fuel, one really does not care about all this. It is when a long flight with minimum fuel is planned that reality starts banging on the windshield. So much for AFM TAS numbers in cruise!
With this perspective, reducing drag can become very appealing. A well polished machine (commercial operators are exempt!), pampered propellers, well rigged controls and well fitted fairings do wonders. Even performance enhancing STC’s make a difference in rejuvenating if not beating the AFM numbers. BUT keep in mind that drag increases to about the square of speed. The individual benefits are not that cumulative.
Indicated airspeed (IAS)
Indicated airspeed (IAS) for general aviation aircraft often are provided for simplicity’s sake for a maximum gross weight scenario. Our concern in this department is stall speed (Vs). The lighter the aircraft, the smaller the stall speed will be. The difference is not huge, but quite observable. For example, you fly out to practice stalls during training with half fuel in your favorite 172. The ASI clearly heads counterclockwise towards the low peg and you are still hanging in there, level. For one, stall speed will be lower (not the angle of attack) and there is another item to consider: airflow entering the pitot, more on this shortly.
The same applies for an approach. If one decides quite legally to adjust approach speed to actual weight: Vapp = 1,3Vs at the threshold (calm winds), then the « regular float would be agreeably reduced during the flare portion of the landing. Indeed, in this case one is flying closer to the gross weight stall speed creating concern, no doubt, but nowhere closer to the actual stall speed.
Planning for wind shear and turbulence becomes a critical part of your decision making when calculating an approach speed. With reason, many would argue would a runway be that short that one needs to reduce speed? Within the physics, if you were trained to fly accurately why not fly this way? Maintenance cost are kept under control: brakes, FOD (foreign object damage) striking the airframe, props, etc…
Ground speed (GS)
On the subject of windshear, ever thought integrating the groundspeed indication off the GPS during approach? The headwind component at your height can be easily deduced. Compare this to the wind given (observed) at the airport. No one cares about this when the winds are light all level. It is quite a different story when flying in the vicinity of frontal passage for instance. Let’s say you experience a 20 kt headwind on final and a 5 kt headwind is reported on the ground, you know something is going to happen by the time you get there. Hey, the reported wind speed can be identical as the one at your height but the headwind component may be very much different with the added bonus of a good crosswind to booth!
Another problem with IAS: it lags behind aerodynamic reality, not by much, but let’s call it 5% difference when accelerating or decelerating vs actual angle of attack at flight load of 1G. The aircraft’s momentum being an issue along with the instrument inherent errors of position, tubing and the clockwork within. Then throw in IAS error due to sideslips or high AOA (air pressure entering the pitot tube at an angle). In a stiff sideslip, the error is significant enough that IAS drops right back to it’s resting peg. Are you stalling? Of course not, but you could be subject to an impending stall if you do not pitch down or add power to counter the extra drag (reducing AOA by the way).
Until one enjoys a certified AOA indicator, a prime source of information in stall avoidance is the ASI when used with good judgement. Factors that may interfere with ASI reliability are:
- • Ice
- Bugs nesting (buy a lottery ticket if your pitot takes one in flight!)
- Heavy mildew accumulation in the tubing low points (aircraft operating often on grass fields take in all sorts of vegetation during take-off and landing).
- Heavy rain.
- Unremoved prior flight pitot covers (unfortunately happens all the time).
- Damage from bird strike on radomes causing airflow disturbance around the pitot tubes.
When doubt becomes an issue, always remember the very basic: Constant attitude = constant airspeed. Situational awareness gets shot out view when IAS becomes unreliable. When one is visual the issue is problematic, when in IMC it’s quite another game level! Situational awareness is regained only by improving scanning and returning to a stable flight condition. « If something feels bizarre that’s because it is! » Whether you experience sluggish ailerons, an aircraft becoming abnormally quiet, pitch attitude being way too high irrespective of the power set, more than 1G being felt, you will stall with the exception of sitting in a modern jet fighter.
Who ever said flying was like riding a bicycle?