What is PCN and ACN why is it so important for an aircraft?And working of radio altimeter.

 

PCN stands for pavement classification number and ACN for aircraft classification number this both are indicate basically the strength of the runway or taxiway wheather it can take the weight of heavy or super heavy aircrafts.

In the above mentioned diagram pavement load carrying capacity of the tel aviv airport was not sufficient for a350. 

We have to make sure that when we are landing aircraft to the any airport that pavement classification number of that airport is always be greater than the aircraft classification number. This will be look by flight dispatcher team and pilot. Otherwise our aircraft has face consequence the above situation.

Ok, as you can see in the image this are jeppsen chart for the pavement at Delhi international airport the pavement of Delhi airport is 65 which is much higher.

This will be categories as-

F- flexible pavement.

A- Sub grade strength category

W- Tyre pressure category.

Ok now we gone to look at the some category of aircrafts.

As you can see in above diagram the pavement for b767-200( aircraft classification number) is 45 and our opper limit is 65 so this aircraft can able to land safely at Delhi airport.

Pavement for a350 is nearly about 52 so this aircraft can safely land at Delhi airport.

But the pavement of worlds biggest aircraft i.e AN-225( giant mariya) this aircraft pavement is about 90 could not land at airport.

This is the major reason that bigger aircrafts like Antonov can not land at some airports.

Every airport has it's pavement classification number assign or given regulatory authority for that country.

Remember PCN is always greater than ACN.

*Working radio altimeter-

If you come for a landing the cockpit instrument guide us like how far we are away from touch down by saying 

Approaching minimums( decision height) 

50.., 40.. 30..20..10..retard.. retard..

This actually the working principle of radio altimeter so let's get started.

The voice you can hear on background is the altitude given by the radio altimeter.the concept behind is very simple we have one transmitting antenna and one receiving antenna located behind at the rear part of the aircraft like this.

Transmitting antenna transmitts the signal to the ground like this below.

And most of the signal are get back to the receiving antenna after reflected from the ground like this.

Then radio altimeter measure the time when transmitting antenna transmitts the waves and receiving by the reciver antenna as we already know that radio waves are travelling at the speed of light.

So here is the simple calculation- 

 Speed of light÷ by the measurements of time by transmitting and receiving antenna( to reach the ground)= total height above ground.

* the radio altimeter start working at 2500ft in Airbus a320 family.

And 5000 ft for b777, b747.

If the synthetic voice computer stopped working then pilot has to refer primary flight display like this.

This radio altimeter is very useful in the condition of low visibility and bad wheather conditions.

Remember radio altimeter give the Miss reading In the case of terrains like.

So we have relay on our barometric altitude or pressure altitude 

We use radio altimeter only in case of landing.