Things to keep in mind before diverting to the alternate aerodrome

Things to keep in mind before diverting to the alternate aerodrome

There are many things to take in account before selecting an alternate aerodrome to divert.

First of all we have to select an adequate aerodrome. But …  what is an adequate aerodrome ?

Adequate aerodrome  is the aerodrome on which and aircraft can be operated, taking account of the applicable performance requirements and runway characteristics, and of course the fuel requirements we need to reach it.

• Runway characteristics :

First of all we must check the landing distance available and landing distance required. 

• Landing distance available
• Landing distance required

Secondly: we must check  in which condition is the runway :

• Contaminated runway:

A runway is considered contaminated when more than 25% of the surface area is covered  by the following:

1. Water more than 3 mm deep, or by slush, loose snow.
2. Snow which has been compressed into a solid mass which resists further compression and holds together or break into lumps if picked up.
3. Ice, including wet ice.

Thirdly : we must check the operative restrictions . Written in the NOTAMS , status of the plane.

PLANNING CONSIDERATIONS WHEN WE CHOOSE AN ALTERNATE AERODROME

Take off alternate :

When do we have to select a take off alternate aerodrome different from which we have taken off ?

When it is not possible to return to the aerodrome which we have taken off due to weather conditions or performance ( for example we have enough runway distance for take off but not for landing).

Which aerodrome is suitable as a take off alternate ?

By law it must :

1. The weather forecast  must be appropriate during a period that starts 1 hour before and 1 h after our estimated time of arrival. Which means that must be at or above our approach minimuns taking in account the capability of our plane.
2. In case of the twin engines , the aerodrome must be within 1 hour flight time at cruise speed (wind calm)  with 1 engine inoperative ( if it is ETOPS 2 hours with an engine inoperative).

In case of three or more engines : the aerodrome must be within 2 hour flight time at cruise speed (wind calm)  with 1 engine inoperative.

 Route Alternate :

In the event of the failure of the critical engine at any point along the route, the airplane must be able to continue the flight to an aerodrome, which the landing can be performed safety.

Minimum en route altitude ( MEA) :  Segment length up to 100 NM = +/- 10 NM of track, with radio navigational aids reception and minimum of :

• 1500 ft OC elevation below 5000 ft, and between 5000 and 10000 ft. We must have 2000 ft avoidance. And above 10000 ft , we must have 10% + 1000 ft.

Minimum sector altitude (MSA): 1000 ft above above terrain and obstructions within the sector distance ( 25 NM) rounded up to 100 ft.

Minimum off-Route altitude ( MORA)

Is calculated for an area bounded by every or every second LAT/LONG square on the route facility chart, and is based on a terrain clearance as follows:

• Terrain up to 6000 ft, 1000 ft above the highest
• Terrain above 6000 ft, 2000 ft above the highest

Destination Alternate:

It must be an adequate aerodrome ( check above the definition of adequate aerodrome)

For planning reasons at least one destination alternate must be selected in case our destination does not accomplish with this requirements:

• The duration of the planned flight from take off to landing does not exceed 6 hours.
• 2 Separate runways (both usable)
• The weather 1 hour before and after from our ETA ( estimated time of arrival) must be at or above:
• The ceiling will be at least 2000ft or circling height + 500 ft ( whichever is grater)
• The visibility not must be less than 5 KM.

Summarizing :   by law if our destination does not accomplish with this requirements or you can not have weather information you must select two alternate aerodrome.

When selecting an alternate aerodrome it must be above the minima shown in the table below:

• Which means if our alternate aerodrome has CATII or CAT III approach we take the minima of CATI as it is said in the table below.

(For checking what a CATII or CAT III mean check our article : low visibility definitions ).