SAMRA/SARON Airframes, Power Plants, Propellers and Aircraft Systems Questions

Full Authority Digital Engine Control (FADEC) manages engine operation using

• control of fuel scheduling only, leaving every other engine parameter, such as bleed air, ignition timing, and anti-ice scheduling, to be set, monitored, and adjusted manually by the flight crew using conventional analogue gauges.
• a digital system that always reverts automatically to a hydromechanical backup control unit the instant the primary digital control channel experiences any kind of internal fault or malfunction.
• a digital electronic control system with no separate hydromechanical backup, so a failure of that control typically means the loss of control of that engine's fuel scheduling rather than reversion to a manual mode.
• a purely mechanical and hydraulic governor system, with any digital instrumentation fitted only for cockpit display purposes and playing no role in scheduling fuel.

Explanation: 'Full authority' is the operative word: FADEC has complete authority over the engine, with no separate non-digital backup mode -- unlike earlier 'supervisory' digital systems that retained a hydromechanical backup channel. Most FADEC units are internally dual-channel for redundancy, but that is a different thing from an independent manual backup.

TP 690 Section 2 - Full Authority Digital Engine Control (FADEC)

The distinction between an 'anti-ice' system and a 'de-ice' system is that

• anti-ice systems are used only while the aircraft is stationary on the ground before departure, while de-ice systems are used only once the aircraft is airborne and in flight.
• anti-ice systems prevent ice from forming in the first place, using continuously heated surfaces, while de-ice systems allow a limited amount of ice to accumulate before removing it, using cyclically inflated pneumatic boots.
• de-ice systems are specifically designed to prevent ice from forming on a protected surface in the very first place, while anti-ice systems are instead used only to remove ice after it has already formed and visibly accumulated there.
• there is no meaningful operational difference at all between the two; the terms are simply interchangeable depending on which manufacturer or textbook is being used.

Explanation: Anti-ice systems (continuously heated leading edges, for example) are run before or as ice would start forming, to prevent accumulation. De-ice systems, such as pneumatic boots, are deliberately allowed to let a small amount of ice build up before cycling to crack and shed it -- the two approaches work on opposite timing relative to ice formation.

TP 690 Section 2 - Aircraft Systems, Ice and Rain Protection

Flap (or slat) asymmetry protection systems exist primarily to

• automatically extend the flaps fully on both wings the instant any asymmetry sensor triggers, regardless of flight phase. This is the reasoning typically applied when troubleshooting this type of indication.
• prevent flaps from ever being selected below a certain airspeed, unrelated to any asymmetry condition.
• increase the maximum flap extension speed whenever an asymmetric condition is detected, allowing faster flap retraction.
• detect an uncommanded difference in flap or slat position between the left and right wing, and stop further movement (or otherwise limit the resulting rolling tendency) before it becomes hazardous.

Explanation: If a flap or slat drive mechanism fails or jams on only one wing, continued movement on the other side would create a serious, asymmetric lift (and rolling moment) difference between the wings. Asymmetry protection systems monitor for exactly that kind of left/right mismatch and stop (or otherwise safely manage) further flap or slat movement before the asymmetry grows large enough to become a serious control problem.

TP 690 Section 2 - Airframes, Power Plants, Propellers and Aircraft Systems, Flight Control Systems

Galvanic corrosion, one of several corrosion mechanisms of concern in aircraft structures, occurs when

• a composite structure is exposed to ultraviolet light, with the resulting damage classified as galvanic corrosion regardless of any metal content.
• a single metal is exposed to dry air with no moisture present at all, with corrosion proceeding purely from oxygen exposure and no electrolyte involved.
• two identical, chemically matched metals are joined together, with the corrosion arising purely from mechanical fatigue rather than any electrochemical process.
• two dissimilar metals are in direct electrical contact in the presence of an electrolyte (such as moisture), causing one metal to corrode preferentially as current flows between them.

Explanation: Galvanic corrosion is specifically an electrochemical process requiring both dissimilar metals (with different electrochemical potentials) and an electrolyte to complete the circuit between them -- moisture (including humid air, condensation, or contamination) commonly serves as that electrolyte -- and it's why aircraft manufacturers take care with dissimilar-metal contact, isolating fasteners and fittings, and protective coatings at such interfaces, unlike UV degradation of composites or fatigue cracking, which are separate damage mechanisms.

TP 690 Section 2 - Airframes, Power Plants, Propellers and Aircraft Systems

An electric motor-driven hydraulic pump, fitted as a backup to the primary engine-driven hydraulic pumps on many aircraft, allows

• no meaningful backup capability at all, since an electric motor-driven pump is understood to be incapable of producing usable hydraulic pressure under any circumstance.
• the complete elimination of engine-driven hydraulic pumps from the aircraft's design, since electric pumps are understood to replace them entirely on modern aircraft.
• hydraulic fluid to be converted directly into electrical power, reversing the usual relationship between the electrical and hydraulic systems on essentially every transport-category aircraft design.
• hydraulic pressure to be generated using electrical power alone, providing continued hydraulic system function in situations where the associated engine-driven pump is unavailable.

Explanation: An electric motor-driven hydraulic pump generates hydraulic pressure using electrical power, providing continued hydraulic system function as a backup when the corresponding engine-driven pump is unavailable (for example following an engine shutdown) -- it is a genuine backup source of pressure, it does not eliminate the need for engine-driven pumps as the primary source, and it does not convert hydraulic fluid into electrical power.

TC AIM AIR - Hydraulic System Backup Power

Fire-resistant synthetic hydraulic fluid (such as a phosphate-ester-based fluid), as used in most transport aircraft hydraulic systems, is chosen primarily because it

• is fully compatible with all aircraft paints, seals, and finishes with no special handling precautions required.
• is chosen purely for its lower cost compared to petroleum-based hydraulic fluid, with fire resistance an incidental and unimportant property.
• has a significantly higher autoignition/flash point and greater resistance to sustained combustion than petroleum-based fluids, reducing fire risk if a hydraulic line ruptures near a hot engine or brake component.
• eliminates the need for hydraulic system filtration, since it is inherently free of any particulate contamination.

Explanation: Fire-resistant synthetic hydraulic fluids (commonly phosphate-ester-based) are used in transport aircraft primarily because of their significantly higher resistance to ignition and sustained combustion compared to petroleum-based fluids, which matters because hydraulic lines and components often run near engines, brakes, and other hot areas where a leak could otherwise pose a serious fire risk.

TP 690 -- Airframes, Engines and Systems: Hydraulics

Pneumatic de-ice boots are generally cycled (inflated and deflated) according to a specific timing sequence, rather than being left continuously inflated, primarily to avoid

• structural damage to the wing spar, a concern unrelated to how de-ice boots are cycled, since boot inflation pressure is defined as being transmitted directly into the underlying spar structure regardless of the cycling sequence used.
• excessive fuel consumption, since boot cycling is understood to have a direct and significant effect on engine fuel burn.
• 'ice bridging,' a condition in which ice accumulates over and conforms to the shape of a continuously inflated boot, becoming difficult to shed even when the boot is subsequently cycled.
• electrical system overload, since pneumatic boots are powered exclusively by the aircraft's electrical system rather than by bleed air or an engine-driven pneumatic pump.

Explanation: De-ice boots are cycled on a defined timing sequence (rather than left continuously inflated) partly to avoid 'ice bridging,' a condition in which ice can accumulate over and conform to the shape of a boot that has been left inflated for too long, making that ice difficult to shed even once the boot is subsequently cycled; correctly timed cycling, generally allowing some ice to accumulate before cycling, helps the boot shed ice effectively rather than simply flexing beneath an already-bridged ice layer.

TP 690 -- Airframes, Engines and Systems: Ice and Rain Protection

A fuel collector tank (or collector cell), positioned around a tank's boost pump inlet, is designed to

• serve as the aircraft's sole fuel tank, replacing the need for any main wing fuel tank, which overlooks the structural, thermal, or operational considerations that actually govern this aspect of the design, a characterization that does not match how this system is actually designed, certified, or maintained in service.
• heat incoming fuel before it reaches the collector, performing the same function as a fuel-oil heat exchanger, which overlooks the structural, thermal, or operational considerations that actually govern this aspect of the design, a characterization that does not match how this system is actually designed, certified, or maintained in service.
• maintain a small, reliable reservoir of fuel immediately available to the boost pump regardless of the aircraft's attitude or manoeuvring, helping ensure an uninterrupted fuel supply even when the main tank's fuel level is low or momentarily shifted away from the pump inlet.
• measure total fuel quantity across the entire aircraft, replacing the need for a separate fuel quantity indicating system, which overlooks the structural, thermal, or operational considerations that actually govern this aspect of the design, a description not supported by the applicable design standards or normal maintenance practice.

Explanation: A collector tank or cell surrounds the boost pump inlet with a smaller, localized reservoir of fuel that tends to remain filled even when the aircraft manoeuvres or when the main tank's overall fuel level runs low, helping ensure the boost pump continues to draw a reliable, uninterrupted fuel supply rather than momentarily running dry if fuel in the broader tank shifts away from the pump inlet during an unusual attitude, a low-fuel condition, or aggressive manoeuvring.

TP 690 -- Airframes, Power Plants, Propellers and Aircraft Systems: Fuel Systems