Interview Questions 3

Front 1

As weight decreases, IMN for long range cruise increases, decreases or stays the same?

Back 1

IMN = Indicated Mach number
- max range (max SAR, min SFC) cruise at 1.32 x VIMD
- VIMD = indicated min drag speed
- VIMD reduces with weight, IAS required reduces, so:
1. accelerate with design rpm = poorer drag/IAS
2. reduce rpm for correct IAS = increase SFC
3. leave design rpm + climb so IAS recuces with weight!! :)
- correct IAS for each new weight achieved by CONSTANT MACH
- also helps as below MCRIT

Front 2

Where do hurricanes/typhoons form - can they form over land?

Back 2

- non frontal low pressure systems
- develop over warm (26 deg +) waters (the tropics) due abundant supply atmospheric moisture for convective cloud growth
- do NOT form near equator
- 5 deg to 20 deg lattitude
- anticlockwise NH
- winds gale force = 34 + kts
- ST, CB, CU
- pressure below 1000hpa

Front 3

ITCZ explanation -

Back 3

-large scale convergence of trade winds
- trade wind (NE, SE) from subtropical high pressure belt to equatorial trough
- High/trough by Hadley Cell- warm air equator moving poles
- Bad widespread weather:
* deep vertical cloud (55,000ft +)
* low cloud base, +RA, TS, wind squall

Front 4

What causes wingtip vortices?

Back 4

- Lift: airflow over wing, low pressure upper, high top
- spanwise flow to wingtip
- meet at trailing edge = vortices

Front 5

What do vortex generators do?

Back 5

- stir airflow at VG = airflow turbulent boundary layer
- attached longer, higher A/A, lower AS
- reenergize airflow over control surfaces

Front 6

What is Mach Critical?

Back 6

- local mach at one point on airframe, first reaches M1.0

Front 7

Explain mach tuck -

Back 7

As a/c approach MCRIT = nose down trim caused by:
- rearward movement of CoP, due lift distribution
- Reduced download by tailplane due changing downwash

If uncorrected, a/c will accelerate more = mach trimmer

Front 8

On the B74 where does MCRIT happen first?

Back 8

- over the hump: creates divergent duct, airspeed decrease pressure + density increase

Front 9

Relationship between V1 and VMCG -

Back 9

VMCG - min control speed ground:
- min speed t/o power + config. possible to recover + maintain directional control following loss of critical engine using rudder
- no crit engine on twin jet
- nose wheel steering not considered
- varies with elevation + temp (due thrust live engine)

VMCG is below take-off decision speed V1

V1: - decision made to reject takeoff before V1 (or continue)
- V1 more than VEF (engine fail) more than VMCG + (speed gained in 2 sec reaction time)

Front 10

How would you design a high-speed wing?

Back 10

- High CL, low CD
- high span-chord ratio

CL:- camber, high A/A, high aspect ratio
CD:- supercritcal, leading edge slats/slots (reenergize boundary flow), VG (longer attachment), winglets, washout, sweep

Front 11

How are typhoons formed?

Back 11

-non-frontal low pressure system
-over water (26 deg +) abundant moisture for convective cloud
- 5-20deg of equator

Front 12

What are supercritical wings?

Back 12

- aerofoil flatten top surface - reduce top surface acceleration
- reflex camber rear undersurface - compensate for loss lift
--lift at rear +stabilises flow at TE = reduced drag
- delay onset of shockwave (fly higher MFS)
- allows increase cruise + range due reduced fuel flow

Front 13

What types of drag are there?

Back 13

Parasite:
- form: disturbance streamlined flow (separates)
- skin friction: friction between surface + air
- Interference: airflow meeting at junctions

Induced: pressure differentials, spanwise flow, wingtip vortices, downwash

Front 14

What causes induced drag?

Back 14

Lift

Front 15

What engine on a B747 would be critical on a crosswind?

Back 15

- 4 engines outboard critical due longer moment arm, different thrust lines
- xwind will be using rudder to keep a/c straight (e.g. L xwind, R rudder)
- Upwind/outboard engine crit, as yaw more L so more R rudder (but it's already being used)

Front 16

Why use the Stabilator?

Back 16

- just elevator, provides varying moments to counteract changes of CP+CG - little additional to manoeuvre a/c

Variable incidence tailplane:
- provides major trim inputs: full elevator available
- less trim drag
- BEST TO: keep stabilator at best A/A change CG to counteract pitch trim changes (e.g. by moving fuel/trim tanks)

Front 17

Is Hong Kong tropical or subtropical?

Back 17

Tropical: between Tropic of Cancer and Capricorn (23.5 deg N+S)
HK 22.20'N so Tropical

Front 18

What's more important during landing? Reverse thrust or spoilers?

Back 18

- Spoilers: dump lift so brakes more effective
- then use reverse thrust

--Clamshell (large metal plates into jet efflux)
--Cascade (airflow directed by blades/blocker doors in fan airflow)

Front 19

Why use carbon brakes?

Back 19

- high temp, thermal conductivity + specific heat
Therefore:
- longer life x2
-cost effective
- high performance (higher energy absorption)
- lighter (steel)
- lower fuel + lower CO2 emission

Front 20

Define service ceiling-

Back 20

- max alt at which a/c maintain max ROC 100fpm @ full power, gear up, max weight
- twin (one engine inop) 50 fpm, max power, prop feathered

Front 21

Define absolute ceiling -

Back 21

alt which ROC at full power is zero (no excess power available / required)

Front 22

What is 1st T/O segment?

Back 22

Used to calc a/c performance after EF before V1 - 'to go'
1st: 35ft (15ft wet) - landing gear up
- assumes V2
- twin climb: positive

Front 23

What is 2nd T/O segment?

Back 23

2nd segment: gear up - 400ft agl (or obstacle clearance)
- assumes V2
- twin climb: 2.4%

Front 24

What is 3rd T/O segment?

Back 24

3rd: 400ft (or last) - best SE Vy + flap retract
- may be flown level
- twin climb 1.2% (just in case)

Front 25

What is 4th T/O segment?

Back 25

4th: last height to 1500 ft
- twin climb 1.2%
- then a/c can land/dump fuel etc

Front 26

What would cause a No Start?

Back 26

- ie. no 'light off' indicated in 10 sec. STOP
- causes: ignition system

Front 27

What would cause a Hot Start?

Back 27

- i.e. fuel ignited before enough air around combustion chamber (req for cooling) Temp increase (EGT) rapid
- Causes: incomlete burning of fuel
- starter pressure low
- premature starter deactivation
- foreign object damage
- faulty pressure valves

Front 28

What would cause a Hung Start?

Back 28

- ie. normal 'light off' but rpm doesnt increase to normal idle rpm. rapid EGT rise
Causes- low starter pressure
- premature starter deactivation
- foreign object damage
- faulty pressure valves

Front 29

How far to start a descent from FL330 to 1500ft, which is better to descend early - heavier or lighter a/c + why?

Back 29

- 33,000ft at 3000fpm = 11min so x2 = 22min
- heavier a/c descent early: dont want to arrive with excess height or speed

Front 30

Explain the effect of constant Mach held during temp decrease -

Back 30

Mach = TAS/a
a = 39(sroot)T
- tmep decrease = SoS decrease
- to maintain constant Mach, TAS would have to decrease

Front 31

Explain TAS and pitot tube effect during changing conditions -

Back 31

- compressibilty in pitot tube above 300kts
- IAS - (positon error)= CAS - (compres: overread if not corrected) = EAS - (density: TAS must increase with height) = TAS