Hydrostatic Equilibrium Questions and Answers

Answer: C

At a cruising level of 11 kilometres the outside pressure is about 230 hectopascals against a cabin pressure of 750, giving a differential that the fuselage must contain through many thousands of cycles. Metal fatigue from that repeated cycling caused the early Comet accidents. Fuel consumption and speed limits are governed by other considerations entirely. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

Density altitude is pressure altitude corrected for temperature, and it determines aircraft performance, since lift and engine power both depend on air density. On a hot day at a high elevation airfield the density altitude can exceed the physical elevation by thousands of feet, lengthening the take-off run dramatically. Humidity contributes a smaller correction in the same direction. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Both reduced pressure and elevated temperature reduce the density, so a hot day at a mountain airfield is the worst case for aircraft performance. Adding humidity reduces the density further, since water vapour is lighter than dry air, so low humidity in fact lowers rather than raises the density altitude. Several accidents have followed from underestimating this combination. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

A pitot tube measures the total pressure of the oncoming air and a static port measures the ambient pressure, and the difference gives the dynamic pressure and hence the indicated airspeed. Because dynamic pressure depends on density, indicated airspeed differs from true airspeed at altitude. Satellite navigation gives ground speed, which differs from airspeed by the wind. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Dynamic pressure is proportional to density times the square of the speed, so at a given indicated airspeed the true speed through the air is greater where the density is lower. At 11 kilometres the difference is around 70 per cent. The aircraft responds aerodynamically to indicated airspeed, which is why that is the quantity displayed to the pilot.

Answer: D

A variometer is a sensitive rate of change of pressure instrument, indicating the rate of climb or descent of the aircraft. A total energy compensated variometer subtracts the contribution from changing airspeed, so that what remains approximates the vertical motion of the air, which is what a glider pilot actually wants to know. The distinction between the two is important in soaring flight. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Thermals are narrow columns of rising air, so the pilot circles tightly to remain within the core, climbing whenever the updraught exceeds the sink rate of the aircraft. Minimum sink speed is used while circling and a faster speed between thermals. The technique is a direct practical application of convective boundary layer structure.

Answer: B

A thermal reaching its lifting condensation level produces a cumulus cloud, so the cloud marks the top of the rising column and is the visible signature glider pilots use to locate lift. Stratus indicates a stable layer, cirrus is high ice cloud, and lenticular cloud marks wave lift rather than thermal lift. Each cloud type indicates a different kind of vertical motion. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

The lifting condensation level rises about 125 metres for each kelvin of dew point depression, so a large depression gives a high cloud base. Temperature alone does not determine it, since a warm moist parcel and a cool dry one can have very different bases. The relationship is one of the most useful rules of thumb in practical meteorology.

Answer: D

The rule follows from the difference between the dry adiabatic lapse rate of 9.8 kelvin per kilometre and the rate at which the dew point falls on ascent, about 1.8 kelvin per kilometre. The difference of 8 kelvin per kilometre gives 125 metres per kelvin. Deriving it once makes it far easier to remember than treating it as an arbitrary number.