Twin engine airliners have become far more popular and practical to operate than four engine aircraft are. The current twin engine front-runners are the Boeing 737 NG series and the 777 and from the Airbus stable, the A320 series, A330 and A350.
Planning flights over oceanic or remote areas
When airline flights are planned for twin-engined aircraft, the routing that must be followed is such that at any stage of the journey the pilots will have an adequate airport to divert to should a serious in-flight abnormality or an engine failure necessitate this.
The maximum distance away from these airports that the operator may use is based on the time that aircraft is approved to fly for with only one engine operating and the resulting speed that the aircraft would achieve in that event.
The ‘One Hour Rule’
For many years this approved time was sixty minutes. Provided that a flight was not planned across an ocean or over a remote area, the “one hour rule” was easy to abide by, as no matter the route that the aircraft flew, with only one of its engines operating, it would always be within one hour of an adequate airport to divert to.
Let’s see how this rule would have been applied for a flight over the sea from Johannesburg to Mauritius. After take-off the aircraft would be planned to route overhead Maputo and then onwards over the Indian Ocean.
As it left the Eastern Coastline of the Continent, instead of routing directly out over the Indian Ocean towards Mauritius, it would turn left onto a North Easterly heading that would take it within about 250 miles of the airport at Beira.
Sixty minute circles would have been drawn around all the en-route airports that were adequate: One would be drawn around Johannesburg, the next around Maputo and then another one around Beira. In this sequence, at least two of these circles would always be intersecting and overlapping each other.
As long as the aircraft’s position was always within a circle, there would be no violation of the one hour rule.
Passing abeam of Beira the aircraft would be turned to the right onto a more Easterly heading and would then be heading directly towards Mauritius. There would then be another two circles drawn on the navigation chart, one on the Western coast of the Island of Madagascar and the final one around Mauritius.
For the four hours that it would take for the entire flight, calculated at a reduced flying speed, the aircraft would never be more than sixty minutes away from an adequate airport.
Extensions to the ‘One Hour Rule’
Based on statistical and demonstrated evidence as to the improved reliability of jet engines and also because of an ever increasing level of redundancy in aircraft technical systems, the concept of ETOPS (Extended Twin Operations) came into being.
The one hour rule and with it the circles, grew over time with extensions to 75 minutes, then 90 minutes, two hours, two and a half hours and then three hours and even more.
Twin engine airliners are now able to fly almost direct routings across the oceans from South Africa to Australia, the Orient and the Americas, where adequate airports are fewer and further apart from each other.
The requirements for airport adequacy.
Pilots need to know the adequate airports along their routes.
Within South Africa, each major city has an adequate airport in close proximity to it. Certain military airports also qualify as adequate airports. They all have runways that are long, wide and strong enough to accommodate airliners plus good taxi-ways and runway lighting systems for night time operations.
Air traffic control and firefighting services meet the necessary levels of compliance. There are let-down aids such as ILS and VOR in place and weather information is instantly available. Refuelling facilities and passenger handling services exist. In short, there is no problem flying into and out of any of these airports as airliners use them on a regular basis.
The airport that you might have to divert to because of an inflight emergency must satisfy two criteria. It has to be ADEQUATE and it has to beSUITABLE.
The requirements for airport suitability
Suitability means that the weather minima in terms of ceiling and visibility within a required validity period must be satisfied. These periods start one hour before the earliest estimated time of arrival and end one hour after the latest estimated time of arrival.
In addition, the forecast crosswind conditions and an estimated landing weight must be checked so as to ensure that a safe landing could be conducted with an engine or an aircraft system being inoperative.
Finally, NOTAMS and hours of duty must be checked so that there are no surprises on arrival!
Both adequacy and suitability are required
There you have it! There is a lot more to operating flights to remote areas, over the oceans or even late at night when most people are sleeping than just kicking the tires, lighting the fires and blasting off! The availability of adequate and suitable airports along the way is the name of the game!