This is a straight forwards True Altitude question using the Altitude Window on the whizz-wheel.
Looking at the whizz-wheel, in the Altitude Window, you will see that the inputs are Air Temperature and Pressure Altitude.
The Pressure Altitude is what the altimeter would read in the aircraft if the pilot was to set a sub-scale pressure setting of 1013hPa. The aircraft is currently at 4500ft but on a sub-scale pressure setting of 999hPa.
To work out the Pressure Altitude, we would need to turn the sub-scale up from 999 to 1013, an increase of 14hPa.
The question states that we are to assume that 1hPa = 30ft change of altitude.
So, in increasing the sub-scale setting from 999 to 1013 (14hPa), we would increase the reading the altimeter shows by 14 x 30ft = 420ft.
In doing so, the altimeter reading would increase from 4500ft to 4500+420 = 4920ft.
Our Pressure Altitude is therefore 4920ft.
The temperature is stated as being -10oC.
Now we have both Pressure altitude and actual temperature, we can use the whizz-wheel to do the calculation to convert indicated altitude to True Altitude:
- In the Altitude window, set -10oC against 4920ft
- Against Actual Altitude (inside scale) of 4500ft, read on the outer scale the True Altitude of 4230ft.