TrueHeading
Log inEnrol
DGCA Technical General · formula sheet

DGCA Technical General Formulas and Key Concepts

36 formulas and the ideas behind them, grouped by topic. Understand each one, then practise it on the question bank.

Practise Technical General questionsFlashcards

Jump to a topic

Aerodynamics & Performance8Airframe & Structures1Powerplant2Aircraft Systems4Pitot Statics1Radio Altimeters2Airspeeds2Mach2Gyroscope2Turn and Slip1Engine Instruments1Temperature Indicators2

Aerodynamics & Performance

See practice questions on this topic

Forces in steady level flight?

Lift balances weight; thrust balances drag.

L=12ρV2SCLL=\tfrac12\rho V^2 S C_L
D=12ρV2SCDD=\tfrac12\rho V^2 S C_D
  • Lift increases with density, speed squared, wing area and lift coefficient
  • Same form for drag with CD
LiftWeightThrustDragsteady level flight: Lift = Weight, Thrust = Drag

Parasite drag and induced drag: how do they vary?

Parasite drag grows with the square of speed. Induced drag falls with the square of speed.

CDi=CL2π e ARC_{D_i}=\frac{C_L^2}{\pi\,e\,AR}
AR=b2SAR=\frac{b^2}{S}
  • Total drag is lowest at VMD, where the two are equal
  • High aspect ratio reduces induced drag
VMDinducedparasitetotalairspeed →dragat VMD induced drag = parasite drag

What happens in a level turn?

Lift must increase, so load factor rises with bank angle.

n=1cos⁡ϕn=\frac{1}{\cos\phi}
VS,turn=VSnV_{S,\text{turn}}=V_S\sqrt n
  • 60° bank: n = 2, stall speed up by 41 %
  • Rate 1 turn: 3° per second, bank ≈ TAS/10 + 7°

Turn rate and radius formulas?

Both depend on speed and bank angle.

Rate(∘/s)≈1091tan⁡ϕV(kt)\text{Rate}(^\circ/\text{s})\approx\frac{1091\tan\phi}{V(\text{kt})}
r=V2gtan⁡ϕr=\frac{V^2}{g\tan\phi}
  • Higher speed: larger radius, lower rate
  • Steeper bank: smaller radius, higher rate

Range and endurance: where are the speeds?

Related to drag and power curves.

glide ratio=LD=distanceheight\text{glide ratio}=\frac{L}{D}=\frac{\text{distance}}{\text{height}}
  • Glide ratio = L/D = distance ÷ height, best at VMD
  • Propeller: max endurance at minimum power speed, max range at VMD
  • Jet: max endurance at VMD, max range at about 1.32 × VMD
  • Weight reduces as fuel burns, so speeds fall

What is the lift equation?

L = ½ ρ V² S CL.

L=12ρV2SCLL=\tfrac12\rho V^2 S C_L
Vs=2WρSCLmax⁡V_s=\sqrt{\dfrac{2W}{\rho S C_{L\max}}}
  • Lift increases with the square of speed
  • CL depends on angle of attack, flap and shape
  • Stall speed Vs = √(2W / ρ S CLmax)

What affects the stalling speed?

Weight (↑), load factor (↑), flap (↓ with flaps), CG forward (↑), altitude (TAS ↑, IAS same), ice (↑).

Vs,turn=VsnV_{s,turn}=V_s\sqrt{n}
  • Vs ∝ √(weight)
  • Vs in a turn = Vs × √(1/cos φ)
  • Wing contamination reduces CL max

What is the best glide ratio?

L/D at the best angle of attack: height lost × L/D = distance.

Range=h×LD\text{Range}=h\times\dfrac{L}{D}
  • Does not change with weight (speed does)
  • Headwind reduces range
  • Typical airliner L/D: 15–20

Airframe & Structures

See practice questions on this topic

Limit load and ultimate load?

Ultimate load = 1.5 × limit load. The structure must carry limit load without permanent deformation and ultimate load without failing.

Ultimate load=1.5×limit load\text{Ultimate load}=1.5\times\text{limit load}

Powerplant

See practice questions on this topic

How does a gas turbine produce thrust?

It accelerates a mass of air rearwards (Brayton cycle: intake, compression, combustion, exhaust).

F=m˙ (Ve−V0)F=\dot m\,(V_e-V_0)
BPR=m˙bypassm˙coreBPR=\frac{\dot m_{bypass}}{\dot m_{core}}
  • Turbofans accelerate a large mass by a small amount: more efficient
  • Thrust falls with altitude as density falls

What are the four strokes of a piston engine?

Induction, compression, power, exhaust.

CR=Vs+VcVcCR=\dfrac{V_s+V_c}{V_c}
  • 2 crankshaft revolutions per cycle
  • Firing order selected to balance
  • Compression ratio = (swept + clearance)/clearance

Aircraft Systems

See practice questions on this topic

Hydraulics: Pascal’s law?

Pressure applied to a confined fluid is transmitted equally in all directions.

P=FAP=\frac{F}{A}
  • Engine-driven or electric pumps, reservoir, accumulator, filters
  • Used for gear, flaps, brakes and flight controls

Describe the cabin pressurisation system.

Engine bleed air supplies the cabin; the outflow valve regulates the pressure; relief valves protect the structure.

Δp=pcabin−pambient\Delta p=p_{cabin}-p_{ambient}
  • Typical cabin altitude about 8 000 ft at cruise
  • Differential pressure max about 8–9 psi
  • Cabin altitude warning at 10 000 ft
  • Masks drop at 14 000 ft

What are the main hydraulic system components?

Reservoir, pump, accumulator, filter, relief valve, selector valve, actuator, fuses.

F=p×AF=p\times A
  • Fluid: incompressible, e.g. Skydrol (phosphate ester)
  • Pressure typically 3 000 psi
  • Force = pressure × area

Electrical system basics?

Generators (AC, 115 V/400 Hz), TRUs (AC→DC 28 V), batteries, buses, circuit breakers, RAT in emergency.

P=V×IP=V\times I
  • Power = V × I
  • Constant-speed drive keeps frequency steady
  • APU and external power on the ground

Pitot Statics

See practice questions on this topic

How does the pitot-static system work?

The pitot tube senses total pressure; static ports sense ambient pressure.

q=12ρV2=ppitot−pstaticq=\tfrac12\rho V^2=p_{pitot}-p_{static}
  • ASI uses both
  • Altimeter and VSI use static only
pitotstatic portASIALTVSIpitot (total) pressurestatic pressurebothstaticstaticASI = pitot − static · altimeter and VSI use static only

Radio Altimeters

See practice questions on this topic

How does a radio altimeter measure height?

It transmits a radio signal downward and measures the time delay or frequency difference of the reflection.

h=c Δt2h=\frac{c\,\Delta t}{2}
  • 4.2–4.4 GHz FM-CW
  • Range up to about 2500 ft above ground level
  • Used for DH, GPWS and autoland
transmitreflectedh = c × Δt ÷ 24.2–4.4 GHz FMCWheight above groundaccurate up to ~2500 ft AGL, used for autoland / GPWS

How does a radio altimeter work?

It transmits an FM (or pulse) signal to the ground and measures the time delay or beat frequency of the reflection.

h=c t2h=\dfrac{c\,t}{2}
  • 4.2–4.4 GHz
  • Range −20 to 2 500 ft AGL
  • Used for DH in CAT II/III and GPWS

Airspeeds

See practice questions on this topic

IAS, CAS, EAS and TAS?

Corrections applied in order:

TAS=EASσ,σ=ρρ0TAS=\frac{EAS}{\sqrt{\sigma}},\qquad\sigma=\frac{\rho}{\rho_0}
  • IAS → CAS: instrument and position error
  • CAS → EAS: compressibility
  • EAS → TAS: density (altitude and temperature)
  • TAS ≈ CAS + 2 % per 1000 ft
IASindicatedCAScalibratedEASequivalentTAStrueinstrument+ position errorcompressi-bilitydensity(alt, temp)IAS → CAS → EAS → TASTAS ≈ CAS + 2 % per 1000 ft (rule of thumb)in ISA at sea level all four are equal at low speed

Describe the chain IAS → CAS → EAS → TAS.

IAS + position/instrument error = CAS. CAS − compressibility = EAS. EAS ÷ √σ = TAS.

TAS=EASσTAS=\dfrac{EAS}{\sqrt{\sigma}}
  • TAS ≈ CAS + 2 % per 1 000 ft (rule of thumb)
  • σ = density ratio
  • Mach = TAS / LSS

Mach

See practice questions on this topic

Local speed of sound and Mach number?

The speed of sound depends only on temperature.

LSS=38.95T(K) kt\text{LSS}=38.95\sqrt{T(\text{K})}\ \text{kt}
M=TASLSSM=\frac{TAS}{\text{LSS}}
  • ISA sea level: 661 kt
  • At or above 36 090 ft: 573 kt (−56.5 °C)
  • Mach angle: sin μ = 1 / M
μMach cone: sin μ = 1 / Mabove Mach 1 the pressure waves pile up into a shock wave

Define Mach number and the speed of sound.

M = TAS / LSS, where LSS = 38.95 √T kt (T in kelvin).

M=TASa,a=38.95TM=\dfrac{TAS}{a},\quad a=38.95\sqrt{T}
  • LSS at sea level ISA ≈ 661 kt
  • LSS at ISA tropopause ≈ 573 kt
  • Mcrit: first local M=1
  • Mmo: operating limit

Gyroscope

See practice questions on this topic

Drift and wander of a gyro?

Earth rate gives an apparent drift; moving over the Earth adds transport wander.

Earth-rate drift=15∘/h×sin⁡ϕ\text{Earth-rate drift}=15^\circ/\text{h}\times\sin\phi
Transport wander≈VEWtan⁡ϕ60 ∘/h\text{Transport wander}\approx\frac{V_{EW}\tan\phi}{60}\ ^\circ/\text{h}
  • Zero earth-rate drift at the equator; 15°/h at the poles
  • Real drift comes from friction and imperfections

What are gyroscopic rigidity and precession?

Rigidity: the axis tends to stay fixed in space. Precession: an applied force produces a movement 90° later in the direction of rotation.

Drift=15sin⁡ϕ (∘/h)\text{Drift}=15\sin\phi\ (^\circ/\text{h})
  • Rigidity increases with mass, radius and rpm
  • Apparent drift = 15 sin(lat) °/h
  • Transport wander = (E-W GS/60) tan(lat)

Turn and Slip

See practice questions on this topic

What are the turn and slip indications?

Needle shows rate of turn. Ball shows balance: slip (inside) or skid (outside).

bank≈TAS10+7\text{bank}\approx\dfrac{TAS}{10}+7
  • Rate 1 = 3°/s; rate 2 = 6°/s
  • Bank for rate 1 ≈ TAS/10 + 7
  • Step on the ball

Engine Instruments

See practice questions on this topic

Piston and turbine engine instruments?

Piston: manifold pressure, RPM, oil pressure and temperature, CHT, EGT. Turbine: N1, N2, EPR, EGT/ITT, oil, vibration.

EPR=Pt7Pt2EPR=\frac{P_{t7}}{P_{t2}}
  • Tachometer: magnetic or tacho-generator
  • N1 is the low-pressure spool; N2 the high-pressure

Temperature Indicators

See practice questions on this topic

How is temperature measured in an engine?

Thermocouples (two dissimilar metals) produce a voltage.

TATSAT=1+0.2 K M2(temperatures in K)\frac{TAT}{SAT}=1+0.2\,K\,M^2\quad(\text{temperatures in K})
  • EGT: chromel–alumel · CHT: iron–constantan
  • Resistance thermometers for oil and air temperature
  • Total air temperature includes ram rise

What are SAT, TAT and RAT?

SAT: static (outside) air temperature. TAT: total air temperature including ram rise. RAT: the ram air temperature rise.

TAT=SAT(1+0.2M2)TAT=SAT\left(1+0.2M^2\right)
  • TAT = SAT (1 + 0.2 M²), in kelvin
  • At M 0.8, ram rise ≈ 28 °C
  • Thermocouples measure EGT

Make the Technical General formulas stick

In class every formula is built from a diagram first, then practised until it is automatic.

Enquire & enrolDGCA Technical General classes
Call WhatsApp