That depends what aspect of the aeroplane you're measuring.
An aeroplane's length would be measured in metres, as would its height.
The mass of the aeroplane would be in kilograms.
Its speed would be in metres per second or more traditionally kilometres per hour.
A metre.
A dekameter would be the appropriate, but rarely used, unit.
di ko alam....
A litre.
None, really, because a gallon is not a metric unit. A litre is the appropriate unit to use for measuring volumes.
measuring temperature the metric system of °C is used.
The SI (metric) unit for mass is the kilogram.
monkey
joules
meter
the meter
kilometres