Lets say that APUs are not really friendly to the environment on the ground, be it noise or pollution:
https://www.heathrow.com/noise/heath...s/ground-noise
Running power units on the ground
Auxiliary Power Units (APUs) are small engines and generators in the tail of an aircraft that are used to deliver electrical power and cabin conditioning while on the ground. In order to reduce noise, we specify conditions for APU usage in airport procedures to limit their use and supply alternative systems for them to use whilst on the ground including Fixed Electrical Ground Power (FEGP) and Pre Conditioned Air (PCA). This is essentially plugging aircraft into the mains electrical supply while they are parked. This directly leads to a reduction in ground noise.
https://www.sustainableaviation.co.u...e-Guidance.pdf
In many cases, the electrical and air conditioning loads normally supplied by an APU can be efficiently supplied instead by ground based systems, which use grid electricity generated at a higher efficiency and thus have a much lower carbon intensity and will in general offer a much lower cost per kWhr.
Use of aviation jet fuel in APUs is, to put it simply, expensive and inefficient. It therefore leads to a simple ground power hierarchy which if followed can save fuel, money, CO2 and NOX emissions as follows:
1. Airport Terminal, or ground based facilities such as FEGP and PCA, should always be used where provided. (described further below),
2. When they are not available, GPUs and air-conditioning units should be used as these provide a reduction in fuel, emissions and noise over APUs,
3. When FEGP, PCA or GPUs are not available, on-board APUs and associated generators and air bleeds should be used.
4. If none of the above are available, the main engine driven generators and air bleeds should be used as a last resort.7
3.3 Benefits of APU Substitution
There are clearly opportunities for reducing the time APUs are run at airports through substitution of the APU’s appropriate function (be this electrical power and or air conditioning) by airport based support systems. This has both environmental benefits for airports and operators (reduced CO2, NOx and noise) as well as in general financial savings through the saving of fuel burn by the APU.
The exact potential for savings is complex and dependent on a multitude of factors ranging from the availability of ground based systems at an airport, weather conditions and the flight schedules and local operating procedures in place9.