Hot and High

By Shihal Sapry

We use the term "Hot and High" to indicate an airfield that is at a rather high elevation above sea level and that has high temperatures. Some airfields are just "Hot" e.g. Dubai International Airport and Doha's Hamad International Airport, or just High e.g. Johannesburg's O.R. Tambo International Airport and Addis Ababa's Bole International Airport.

This article discusses Johannesburg's O.R. Tambo International Airport in particular. But the concept applies to all airfields that are "Hot" or/and "High". Other airfields that are hot and high are Denver (5,431ft), Salt Lake City (4,227ft), Madrid (2,000ft), Santiago (1,555ft), Mexico City (2,230ft), Nairobi (5,327ft). See our highest airfields post.

Johannesburg is at approximately 5500ft above sea level and temperatures are over 20 degrees celcius on most days. Night time temperatures are lower. If you work out the density elevation it can range from 6500 to 8200 ft. Depending on temperature and dew point.

With these conditions, aircraft performance is severely degraded. At altitude, the air is less dense, which means that the wings generate less lift. The hotter it gets, the air becomes less dense. So you have the worst of both cases at Johannesburg.

Therefore, at altitude, for the same Take-Off Weight, an aircraft needs to travel at a higher Take-Off speed  as compared to sea level Take-Off speeds. At a higher Take-Off speed, we reach tyre speed limits. This affects all aircraft. Tyres are designed for a certain speed limit. Obviously exceeding these is extremely dangerous and illegal. While the runway at Johannesburg is 14400 ft long, aircraft tend reach tyre speed limits before the runway length becomes an issue. This is the problem with the 747/777/787. On some Airbus aircraft you reach brake energy limits before tyre speed limits. In order to in order to travel at a lower take off speed, we have to make the aircraft lighter.

The next issue affects twinjets only. If there is an engine failure at rotation or just after take off, the aircraft needs to maintain a certain climb profile in order to reach a minimum safe altitude. Due to degraded aircraft performance, the remaining engine may not be able to provide the necessary thrust. Hence the aircraft may not be able to attain the required climb profile. Once again we need to lighten the aircraft in order for it to safely climb. 

We can lighten the aircraft in two ways:

• By reducing fuel load.  We carry less fuel and reduce aircraft range whilst prioritising payload (cargo and/or passengers). However, since Johannesburg is far from certain destinations, this is not common practise on long haul flights.

• By reducing payload (cargo and/or passengers).  We trade payload for range.  This is a more common practice. This makes flights less profitable but offers non stop connectivity to passengers which is vital for business travelers.