I am currently waiting for my 2nd interview from Cathay Pacific that is being held next week. After browsing through the Internet, I found someone posting a few technical questions being asked during his 2nd stage interview. At this moment, I am still unable to answer those questions. Would you please give me a hand on them?
1. Why did A380 and A340 have different type of winglets? Why did Airbus use the same type of winglet originally designed for the A320 and implement it on A380 instead of using the type for A340?
2. If the static tube on the pressure head is iced up during the descent, what will the altimeter indicate? What will happen to the airspeed indicator?
3. Why modern aircraft uses the Directional Indicator instead of a compass? How does a Directional Indicator work? Does it work with a compass? Does a Directional Indicator has deviation and variation corrections as applied on a compass?
I hope to receive your response soon.
Regards,
Benny
Hi Benny,
1. First you have to understand the reasons of having winglets on planes. See answers to one of my previous FAQ.
Anyway, lets review on some issues concerning the debate on why there are no winglets on the Boeing 777 and why Airbus A330/340 has them.
Boeing chose to increase the span of the wing rather than add winglets. By increasing the wingspan, it also increases wing efficiency. Increasing the wing by 4/5 of the height of winglets will have the same effect as having a winglet. Yes, winglets reduce wing tip vortices and therefore induced drag. But they have disadvantages: If one of the winglets is damaged and removed, it would burn an extra 1 % more fuel and the take off weight may be reduced by as much as 15,000 to 30,000 kg. On the Airbus, the weight of the winglets helps to reduce bending moments on the outer wing during flight. Without it, the outer wing will be able to twist and bend to a higher degree for any given weight.
The increased wingspan on the Boeing 777 creates a higher aspect ratio (a measure of how long and slender a wing is from tip to tip - the square of the span divided by the wing area) on the wings, thus improving the take off, climb and cruise performance without adding winglets ? hence reducing the simplicity and construction cost of the wing design.
Now, to go back on your question on why the Airbus A380 did not follow the Airbus A330/340 winglet design but rather more or less imitate the old design ?wingtip fences? of the Airbus A320. Basically winglets help to reduce induced drag and improve performance (also increases aspect ratio slightly). However, the Airbus A380 has very large wing area due to the large wingspan that gives it a high aspect ratio. So, it need not have to worry about aspect ratio but needs only to tackle the induced drag problem. Therefore, it does not require the winglets, but merely ?wingtip fences? similar to those of the Airbus A320.
2. If the static tube on the pressure head is iced up during the descent, what will the altimeter indicate? What will happen to the airspeed indicator?
a. If the static tube of the pressure head is iced up, the static pressure inside the altimeter will remain constant. Therefore the altimeter will display the altitude when the static tube was iced up even though the plane may be climbing or descending. For example, the icing was encountered at 5000 feet; the height will indicate 5000 feet even if you were at 10,000 feet or 1000 feet!
b. In the case of the airspeed indicator (ASI), a static tube blockage would cause the static pressure in the ASI to remain constant. The ASI reading would be correct at constant altitude. However, during a descent, it will overread (due to trapped low static pressure of higher altitude) and underread (due to the trapped high static pressure of the low altitude) during a climb.
3. Why modern aircraft uses Directional Indicator instead of a compass? How does a Directional Indicator work? Does it work with a compass? Does a Directional Indicator has deviation and variation same as a compass?
The modern aircraft uses the Direction Indicator (DI) or directional gyro (DG) instead of a compass because it gives a more steadier (due to rigidity) heading information to the pilot than a basic compass (eg. those used by scouts or trackers on the ground).
To understand how a DI works, you must understand the principle of the gyroscope. A gyroscope is a body rotating freely in space that has the property of rigidity and precession.
The DI uses an internal gyroscope whose stability provides accurate directional information once it is set to the correct heading. Because the earth rotates at 15? per hour and because of small-accumulated errors caused by friction, the DI will drift over time. It must be reset from the compass periodically - once each fifteen minutes of flight. Failure to do will give rise to navigation errors among new pilots.
So basically, a DI is a gyroscope that displays the aircraft heading by making use of a compass rose display. The gyroscope axis is aligned to the true north.
The DI suffers from turning and acceleration errors but any deviation and variation corrections are applied to the aircraft track only. Remember, the magnetic compass works with the DI as a single instrument known as the remote indicating compass (RMI)
B777 Photo/Routes/Seats 
