Smoke On Go

Nominating a take-off alternate

Airliners are operated in all sorts of adverse weather conditions. High winds, low cloud, fog, rain, thunderstorms and snow showers are all some of the very many challenges and adversities that pilots have to contend with, particularly in the departure and arrival phases of a flight.

Let us say that a crew is scheduled for a flight from Johannesburg to Cape Town in a twin-engined jet airliner that has full all-weather capability. At the time of departure, the visibility is poor, mainly because of low cloud, precipitation and mist. The visibility is down to 300 meters and the cloud-base is varying between 100 and 150 feet. Low visibility procedures are in force.

Company policies must be adhered to.

During the flight-planning process and the associated pre-flight briefing, the crew is cognisant of the need to adhere to their company’s policy in respect of the dispatch of aircraft. Given the state of the weather, the pilots would need to decide on which airports they could divert to if they were presented with a non-normal event after take-off that prevented them from landing back at Johannesburg. These airports would be known as TAKE-OFF ALTERNATES and need to be within one hour of the departure airport based on the aircraft’s single engine cruise speed. In still air, for twin-engined Category C and D airliners, this equates to about 390 nautical miles.

No return to the departure point after an engine failure.

Considering only engine failures, in particular, a return to Johannesburg would be out of the question. A company FOM (FLIGHT OPERATIONS MANUAL) would state that single engine approaches are restricted to being flown no lower than the ceiling and visibility limits that apply for Category One ILS approaches. These limits are a cloud base of 200 feet and a visibility of 550 metres. The prevailing weather for the flight is far worse than this.

What options are there for diverting to another airport?

If an engine failure occurred after take-off, Lanseria, Bloemfontein and Durban would all be adequate airports that the pilots could divert to and that would also be practical from a facilities and passenger handling point of view. However, primarily, the crew would need to see if the relationship between the forecast weather and the available approach aids would make these airports suitable for nomination as a take-off alternate or not.     

Lanseria, being right on Johannesburg’s doorstep, would be the best and most practical option to use as a take-off alternate. The weather prevailing at Lanseria is slightly better than Category One ILS limits and this would allow for an engine inoperative approach and landing. However, only one of the runways is equipped with an ILS and there is a tail-wind blowing on it that is above the maximum allowable tail-wind limitation for landings in the type of aircraft that is being flown. There is no approach aid on the reciprocal runway and weather is below VFR or circling minima. Lanseria is therefore not suitable for use as an alternate

Bloemfontein is considered next because it is closer to Johannesburg than Durban is. None of its four runways are equipped with ILS installations. Non-precision VOR approaches may be flown onto the two main runways, but the forecast cloud-base and visibility are below the published minima. Bloemfontein is therefore not suitable for nomination.

Durban, the furthest away, is the last of the three airports to be considered. It is nevertheless shorter than 390 nautical miles away, has two runways with an ILS on both of them and the weather is forecast to have broken cloud at five thousand feet with no change. Durban is therefore the airport of choice for a take-off alternate.

ATC is advised that Durban is the take-off alternate.

The pilots advise the flight-planners that Durban has been nominated as the take-off alternate. ATC is advised accordingly and the navigational data pertaining to a flight from Johannesburg to Durban is made available on the crew’s flight plan.

Formalities completed and off they go!

In spite of the poor weather and any adverse runway conditions that exist, the aircraft is within its performance envelope. It easily meets the accelerate-stop, accelerate-go and second segment climb requirements for a safe take-off. The runway in use has runway edge and centreline lights. Multiple runway visual range measuring and readouts are available. With all of these facilities, a take-off may be accomplished legally with a runway visual range of as little as 200 metres.

An engine fails at or above the V1 speed.

The aircraft commences its take off roll. As it gets airborne an engine failure occurs. The pilots  follow the published engine inoperative flight path, work through all the necessary procedures, actions and appropriate checklists, declare an emergency, brief the cabin attendants, pacify the passengers as best they can, and then arrive at the point at which they have to decide as to what they do next.

They have already read in the checklist that they should “land at the nearest suitable airport”. It is known that Durban was the only airport that met the requirements for suitability. They advise ATC as to their intentions, obtain a revised clearance, and off to Durban they go….Good airmanship and planning has resulted in the crew being pre-armed, pre-briefed and being able to keep the situation under control. Furthermore, they are able to adhere to the old aviation axiom , which says… “Never let your aircraft arrive at a point that your brain has not got to at least 10 minutes before the time”.




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