‘Taking Flight’ (Part Three)
CLICK HERE for Part Two.
‘Taking Flight’ (Part Two) looked at the preparations and execution of getting airborne. However, there are variables outside of the pilot’s hands that nevertheless must be considered and countered to ensure a safe take-off. From environmental effects to emergencies, we will now consider subtle safety strategies to counter these different conditions and see that there is more than one way to take flight.
The Powers That Be.
Whether man takes to the land, sea or sky he will always be at the mercy of the elements to some degree. They can be seen as a threat or a challenge, but either way nature is an inescapable aspect of aviation. From crosswinds and climate to wet runways and windshear, the range of conditions are as diverse as the planet itself.
There will be days when those conditions exceed the performance of the aeroplane or our own level of skill and the prudent choice in these situations is to stay on the ground and fly another day. However, more frequently the elements will be within the grasp of both crew and machine, so due consideration should be given before advancing the levers and accelerating down the runway.
The wind can be our friend when it is blowing straight onto our nose for take-off. Reducing the ground roll and increasing the angle of climb and obstacle clearance, a headwind is the ideal ally provided by nature. Unfortunately, pressure systems, noise abatement procedures and local topography do not always ensure that the wind direction is our friend. At times it will blow from abeam and present a challenging crosswind, while a wind from behind will penalise performance in the manner that a headwind can aid it. Tailwinds will both extend the ground roll and erode the angle of climb. To this end it is critical that take-off performance charts are consulted regarding the take-off distance required with a tailwind, while the aircraft limitations will generally state a maximum tailwind component regardless of the amount of available runway.
The major consideration in the event of a crosswind is the issue of directional control. The ‘downwind’ wing is shielded by the fuselage to a degree, while conversely the ‘upwind’ wing is in receipt of greater air flow, encouraging it to fly. Consequently, the upwind wing needs to be held on the runway during the take-off roll with the appropriate use of aileron. Compounding this directional issue is the effect of the crosswind on the aircraft’s fuselage and the tendency for the aeroplane to ‘weather-cock’ into wind. Through a combination of control inputs, which may actually see rudder and aileron in opposition, the goal is to maintain that centre-line during the take-off roll. Once airborne, the aim is to smoothly return the controls to the balanced inputs required for the climb out as the aircraft is now free to weather-cock into wind. The challenge then is to fly the extended centreline without being blown downwind.
Temperature is another environmental consideration for the take-off. Warmer temperatures can be detrimental to the take-off on a number of counts. Firstly, the heated air is less dense and stifles the performance of the aerofoil to lift the aircraft from the runway. So much so, that for every degree Celsius above ISA, it is equivalent to raising the airfield a further 120 feet above sea level. For instance, a 30 degree day would equate to an airfield at 1800’ higher than its true elevation. Further to this, the propeller is less efficient in the thinner air and the engine suffers from reduced volumetric efficiency with a corresponding drop in power output. All of this equates to a longer ground run and reduced climb performance which once again needs to be verified preflight in the aircraft’s performance manuals. With humidity also affecting air density and take-off performance, the old catch-cry to be wary when conditions are “Hot, High or Humid” makes very good sense.
Conversely, cold climates may equate to better take-off performance due to their positive effect on air density. In these regions, the greater threat lies in the presence of frost, ice or snow. These forms of moisture can severely destroy the lift characteristics of an aerofoil, so as always a thorough preflight of the aircraft is essential, with increased vigilance towards contaminated surfaces. Furthermore, when present upon the runway, visible moisture may not only retard the take-off acceleration, but risk being ingested into engines as it is thrown up by the undercarriage.
The surface does not need to be contaminated by ice or snow to hamper the take-off roll. Sometimes it is the natural surface of the runway that proves to be an effective retardant in the form of long grass or desert sands. Obviously, the presence of water will compound the issue, but unkempt grass can provide quite a surprise to the student raised solely on black asphalt and painted centre-lines. Sometimes the Flight Manual will recommend a ‘soft field’ technique which advocates the use of a higher flap setting to allow the aircraft to become airborne at a lesser speed. However, the catch comes after take-off when the climb performance is degraded by the extra flap extension, so ensure that obstacle clearance is not a particular issue.
The good news is that environmental factors have been around since the Wright Brothers and much has been learnt in the years in between. Consequently, aircraft performance manuals cater for most situations and if you abide by the book figures, you should be covered. If the prevailing conditions are so unique that they are not catered for, or should they exceed the published limits, then the only safe option is to stay on the ground and wait for conditions to improve.
The Big Bang. (...and sometimes not)
An engine failure on take-off is regarded as one of the most critical situations a pilot can encounter. In a single-engined aircraft, gravity and inertia become the powerplant and only limited options and time lay ahead. In a multi-engine aircraft, the outlook may be more promising if the failed engine can be secured and the aircraft is able to fly away to a safe altitude. I emphasise ‘may’ as only the foolhardy believe that a light twin’s second engine is always a guarantee of climb performance.
Whether in a single or a twin, an ‘escape plan’ should be formulated in the event of an engine failure. What is the terrain situation on upwind? Are there parallel runway operations? Are there forced landing fields or a nearby airport? If I am struggling to maintain height in a twin, at what height will I start to turn back towards the field and will I turn left or right? Clarifying these issues prior to take-off is invaluable and may in fact dictate your departure plan with everything working. An early or delayed crosswind turn in a single may avoid some nasty terrain should the engine go silent. Better safe than sorry.
Furthermore, it would be remiss to consider engine failures without remembering that they are not always a cut and dried ‘bang’ followed by silence. It may only be a partial failure, or even sound a whole lot worse than it is. The repeat offenders of a blown tyre on take-off or a seat belt outside the door banging on the fuselage have both led pilots to believe that the aircraft engine was about to self destruct. No matter how much noise or vibration may be present, verify the aircraft performance. If the aircraft is still performing satisfactorily, don’t rush into anything, but keep climbing for precious altitude where you have options and time to carefully analyse the problem.
Check back soon for the conclusion of 'Taking Flight'.