# Aircraft Performance Questions and Answers – Standard Atmospheric Model

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This set of Aircraft Performance Multiple Choice Questions & Answers (MCQs) focuses on “Standard Atmospheric Model”.

1. International Standard Atmosphere is based on the assumption that air is a perfect gas and obeys gas law.
a) True
b) False

Explanation: International Standard Atmosphere is based on the assumption that air is a perfect gas and obeys gas law. It also neglects the effects of dust, water vapour and moisture.

2. According to ISA atmospheric model what is the value of datum atmospheric air density kg/m3?
a) 1.225
b) 12.225
c) 1.23
d) 1.000

Explanation: According to ISA atmospheric model what is the value of datum atmospheric air density is 1.225 kg/m3. It can also be written as 1225 g/m3 else 1.225 x 10-9 kg/mm3.

3. The temperature-height profile is given by ___________
a) T=Ti+Li (H-Hi)
b) T=Ti+Li (H+Hi)
c) T=Ti-Li (H-Hi)
d) T=Ti-Li (H+Hi)

Explanation: The temperature-height profile is given by T=Ti+Li (H-Hi), where
T= temperature
Ti=temperature at lower boundary layer at that height
Li=lapse rate
H=height
Hi=height at lower boundary layer at that height.

4. What is lapse rate?
a) Change of temperature with respect to altitude
b) Change of temperature with respect to pressure
c) Change of pressure with respect to altitude
d) Change of temperature with respect to density

Explanation: Lapse rate is the change of temperature with respect to altitude. The temperature lapse rate is linear at tropopause to stratosphere. The lapse rate is zero in from the altitude 11km to 20km. The temperature lapse rate becomes positive at the middle of stratosphere.

5. What is true height?
a) The vertical geometrical distance between an object and a datum level
b) The vertical geometrical distance between a certain height and a datum level
c) The horizontal geometrical distance between an object and a datum level
d) The horizontal geometrical distance between a certain height and a datum level

Explanation: True height is the vertical geometrical distance between an object and a datum level. It is denoted by –z. Unit of true height is meter.

6. What is the hydrostatic equation?
a) ΔP=-ρg∆h
b) ΔP=-ρgh
c) P=-ρgΔh
d) ΔP=ρgΔh

Explanation: The hydrostatic equation is given by ΔP=-ρg∆h. This equation is derived from equating force acting on a cylindrical flow. The unit of pressure is N/m2. It can also be represented in atm, bar, Pascal.

7. What is the pressure difference across a cylinder which consists of water and has a height difference of 20m?
a) -196.2 kN/m2
b) -1.962×10-5 bar
c) -196.2 kN/m2
d) -1.962×10-5 bar

Explanation: The answer is -196.2 kN/m2.
Given Δh=20m, g=9.81m/s2, ρ=1000kg/m3.
From ΔP=-ρg∆h,
ΔP=-1000*9.81*20
ΔP=-196200 N/m2 or ∆P=-196.2 kN/m2.

8. The difference between true height and geopotential height is relatively small.
a) True
b) False

Explanation: True height is the vertical geometrical distance between an object and a datum level whereas the geopotential height is the vertical coordinate referenced to earth’s mean sea level and the difference between true height and geopotential height is relatively small.

9. The pressure-height relationship in troposphere is given by _______
a) $$\frac{p}{p_0}$$=[1 + $$\frac{L_0}{T_0}$$H]$$\frac{-g_0}{RL_0}$$
b) $$\frac{p}{p_0}$$=[1 + $$\frac{L_0}{T_0}$$H]$$\frac{-T_0}{RL_0}$$
c) $$\frac{p}{p_0}$$=[1 + $$\frac{g_0}{T_0}$$H]$$\frac{-g_0}{RL_0}$$
d) $$\frac{p}{p_0}$$=[1 + $$\frac{L_0}{g_0}$$H]$$\frac{-g_0}{RL_0}$$

Explanation: The pressure-height relationship in troposphere is given by $$\frac{p}{p_0}$$=[1 + $$\frac{L_0}{T_0}$$H]$$\frac{-g_0}{RL_0}$$
, where
p=pressure
p0=datum pressure
L0=lapse rate
T0=datum temperature
g0=acceleration due to gravity
H=height.

10. Any atmosphere that does not conform to the ISA profile is referred to as an off-standard atmosphere.
a) True
b) False

Explanation: Any atmosphere that does not conform to the ISA profile is referred to as an off-standard atmosphere. In this atmosphere there are no assumptions taken as in that of ISA tables.

11. What is relative pressure at 101325 N/m2?
a) 1
b) 2
c) 3
d) 4

Given p=101325 N/m2,
From relative pressure (δ)=$$\frac{p}{p_0}$$
δ=$$\frac{101325}{101325}$$=1.

12. What is relative temperature at 241.45K?
a) 1
b) 0.8379
c) 0.8774
d) 0.8756

Given T=241.45K,
From relative temperature (θ)=$$\frac{T}{T_0}$$
θ=$$\frac{241.45}{288.15}$$=0.8379.

13. What is relative temperature at 1600ft? (T1600=284.98 K)
a) 0.989
b) 0.838
c) 0.877
d) 0.999

Given T1600=241.45K,
From relative temperature (θ)=$$\frac{T_{1600}}{T_0}$$
θ=$$\frac{284.98}{288.15}$$=0.989.

14. What is relative pressure at 1600ft? (P1600=95602 N/m2)
a) 0.9453
b) 0.9858
c) 0.9435
d) 0.9595

Given P1600=101325 N/m2,
From relative pressure (δ)=$$\frac{P_{1600}}{p_0}$$
δ=$$\frac{95602}{101325}$$=0.9435.

15. What will be the pressure-height ratio of air at an altitude of 9144m? (L0=-0.0065 K/m, T0=288.15K, P0=101325 N/m2)
a) 0.297
b) 0.596
c) 0.359
d) 0.458

Given L0=-0.0065 K/m, T0=288.15K, P0=101325 N/m2, H=9144m, we know g0=9.81m/s2, R=287 J/kg-K
From $$\frac{p}{p_0}$$=[1 + $$\frac{L_0}{T_0}$$H]$$\frac{-g_0}{RL_0}$$
$$\frac{P}{P_0}$$=[1 + $$\frac{-0.0065}{288.15}$$9144]$$\frac{-9.81}{287*-0.0065}$$
$$\frac{P}{P_0}$$=0.297.

Sanfoundry Global Education & Learning Series – Aircraft Performance.

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