Ask a pilot: your flight questions answered

Illustrations: Telegramme

“Why do flights not go in a straight line to America? They seem to arc up then drop down again. Always puzzles me.”
I found this subject interesting when I did my training – however it wasn’t easy. We had the best instructor, Pete, who was so animated and entertaining! We always look to travel the shortest possible distance and, although it looks like we travel further because of the shape of the Earth, it is usually the shortest.

The Earth is a similar shape to a ball, but slightly flattened at the poles (oblate spheroid), which means it bulges at the equator. Did you know that the diameter at the equator is actually 43km larger than from pole to pole?

When a lot of world maps are drawn, they’re stretched out, especially at the poles, which makes places like Greenland look much bigger than they actually are. The most direct route from London to San Francisco would go over (a not so green) Greenland, which on a flat map looks like a big curve, known as a Great Circle route – when it’s near enough a straight line from A to B.

“How do you know where you are going when you are flying through thick cloud? Can you see anything?”
For modern commercial aircraft, we have digital displays that show our route. Our routes are built up of several waypoints, some with geographical significance, such as MAN, which is directly overhead Manchester airport. However, others are made-up names that represent a latitude and longitude. We monitor these screens throughout the flight to check we are going where we want to. Although it’s nice to be able to look out and see what is sometimes a fantastic view, it’s not necessary. 

I remember years ago driving to work through thick fog and realised that flying in zero visibility is so much easier than driving in fog!

“Why are cabin lights dimmed for take-off and landing at night? Does dimming really make a difference?”
Our eyes take 20-30 minutes to adjust to darker environments, so when flying in dusk or darkness we dim the cabin lights for the extremely rare eventuality that we may have to evacuate on take-off and landing. If we did have to evacuate an aircraft, your eyes would be in a better position to see. 

In addition to this, when the lights are off in the cabin, you can see what’s going on outside much more easily. 

“When I board a plane, it seems to take a fair time before the air-conditioning kicks in. Why is this?”
We have a small engine at the back of all British Airways aircraft, called an Auxiliary Power Unit (APU). This power unit provides electricity to the aircraft on the ground (when we’re not plugged into ground power), along with cabin air. Some airports provide ground air, which goes through a tube in the bottom of the aircraft and then gets pumped into the cabin to keep the temperature where it needs to be. 

Depending on where we are in the world, we may be restricted on the time we can start our APU before departure. As we continue striving to be a more sustainable business, we prefer using ground air where possible. However, if it is too hot or cold in the cabin, we will always do what we can to ensure the cabin temperature is comfortable for our customers.

“Without the autopilot, is it possible to manually fly a large aircraft?”
It is possible to fly an aircraft manually at any altitude we operate at. At higher altitudes, however, the aircraft is sensitive to small inputs and a small movement to the control column might be felt in the cabin. For the comfort of our customers, we ordinarily wouldn’t disconnect the autopilot at higher altitudes. The autopilot is a great piece of equipment that is designed to improve comfort and accuracy and increase the capacity of the pilots.

What is the sawing wood sound after touchdown?
Some readers asked more about the ‘sawing wood sound’ after touchdown following a previous column about the noises of the cabin  – with some asking if it was the PTU. You’re quite right, it could well be the PTU, depending on which aircraft you are on and how long after touchdown you hear the noise. 

I’ve talked about the PTU in previous columns, as I, too, remember the day I sat on an A320 and wondered, “What’s that noise?” The PTU is a power transfer unit that transfers power between the hydraulic systems on the Airbus 320 family aircraft (319/320/321). I’ve never heard of it being described as sawing wood, but I have heard of it referred to as a ‘barking dog’! 

If the pilots taxi to the stand on one engine (a sustainable, fuel-saving method to taxi an aircraft), after one engine is shut down, one of the hydraulic systems will begin to depressurise. This is a point at which you may hear the PTU transferring torque to the other system. 
 
That said, when we taxi off the runway the pilot monitoring raises the flaps and the noise you refer to could also be the flaps being retracted back into their ‘clean’ position (although I wouldn’t say that sounds like sawing wood either… and I am a country boy).

Got an aviation question you’d like to ask pilot Al? Email theclub@cedarcom.co.uk and we’ll try to answer it in a future column.