Still using knots, nautical miles, inches of Hg. Is that impossible to switch to SI units? Why doesn't the aviation industry use SI units?.




I believe only a few countries use meters and other SI units in air transportation, including India and China (please correct if wrong).

Using kts and NM in navy -- built from the length of the 1' arc on a meridian -- simplified nautical mental arithmetic, and likely also helped at the beginning of aviation.

Current ICAO recommendations

In Annex 5 - Units of Measurement to be Used in Air and Ground Operations, ICAO allows 3 non-SI units (knot, foot, nautical mile) to be used...


  as alternative units because of their widespread use and to avoid
  potential safety problems which could result from the lack of
  international coordination concerning the termination of their use.


... but overall states in the same document that only SI units should be used, the same units for all members. Nevertheless the termination dates for alternative units have not been specified:


  It is intended that the use of the non-SI alternative units [...]
  will eventually be discontinued in accordance with individual unit
  termination dates established by the Council. Termination dates, when
  established, will be given in Chapter 4.


Case of vertical measurements / altitude

Vertical measurement in meters may be the more complex due to the existence of flight levels in feet, with the full system of vertical separation based on FL.

This topic was discussed in Using feet vs meters for altitude?. The current question is not a duplicate of the latter.

Other non-SI units

Other non-SI measurements seem more easy to remove:


Horizontal distances in feet and nautical miles may be expressed in meters, as already done for runway lengths. It is not as important as it was to mentally translate a latitude offset in NM.
Atmospheric pressure expressed in Hg column height may be expressed in Pascal. Many pressure instruments can be already calibrated both in inHg and hPa.
Speeds expressed in knots (sometime in statute miles per hour) may be expressed in km/h.




Question

Wouldn't it be useful (and safer) in the long term if we could remove most of the imperial units from new aircraft instrumentation, procedures, ATC, documentation and maps (at least for anything other than altitudes)

Just asking what is the current view from the air transport community itself.
For me metric or imperial is not the question, from my perspective the key is standardization. Having 2 different unit systems has only got drawbacks:


Customers: many airline's customers are living in a metric system, and they do not know what's is a mile or a feet. Is a matter of making flights more comfortable.
Cost reduction: units transformation is finally a cost. Engineer at manufacturers and airlines need to spend some time using a calculator to understand what is meant by other unit system. Maybe is only 30 minutes per month, but if we are talking about 2000 people, we are reaching a cost that is not adding value of 1000h per month. Finally this cost is somehow appearing in our airplane tickets. 
Reduce human factor: we all are humans and exposed to errors. The less opportunity we have to make an error the best. Already there had been an issue with satellites with unit conversion. Reducing this difficulty is finally improving the safety of the air transport.
Increase competence in the sector: suppliers from, let's say, metric system have an slight competitive disadvantage when competing for a contract with imperial units. The more competition in airline services, airplane manufacturers suppliers, handling services the lower the price of our tickets. 


Having 2 standards is a cost that is not producing any value and is actually a sector issue, moreover, a society issue. From my point of view the industry should try to move to a single standard. Metric or Imperial, I don't mind.

Next discussion will be which side of the road we should drive...