Flight Operations Support
10th Performance and Operations Conference
LOW VISIBILITY TAKE-OFF
WITH AND WITHOUT PVI / HUD - PART 2
By Captain William Wainwright
In the past, there was nothing to help the pilot take-off in poor visibility, just as there was nothing to help him land in it. It was visual or nothing. Autoland was developed to help him land, but no one has yet made an automatic take-off. However, in the same way as manual landings in poor visibility can now be done by using a HUD, both HUD and PVI have been developed to help you take-off. They differ in that you look through a HUD and you look at a PVI. Because of that, the PVI is a small unobtrusive instrument but the HUD is relatively large and cumbersome and requires extensive modification to the cockpit. Both are provided only for the left-hand seat pilot. Both are available as options; the HUD on A320/321 and A330, and the PVI on A340 and A330.
2. TAKE-OFF WITHOUT INTERNAL AID
A take-off without PVI/HUD in low visibility requires the pilot to track the centreline using his "Mark One Eyeball"; in short, he looks outside, as he always did in the past, and uses whatever he sees to keep himself on the right track. He continues looking outside to keep straight and to track the centreline until he reaches rotate speed, when he transfers his attention head down and performs the rotation on instruments. Obviously, this only works if he sees sufficiently far ahead to have some means of orientating himself.
When I was a military pilot we had to see a minimum of 2 runway lights in front of us in order to attempt a take-off, and this is the absolute minimum in order to keep straight. You are not very well placed if you have an engine failure on take-off, and you can never guarantee that this minimum forward visibility will be there for the entire take-off run. You might enter a thicker patch of fog. The Authorities impose a minimum visibility of at least 125m, depending on the runway lighting, as described in Part 1 of this presentation. Thus, some form of guidance may be useful.
3. USE OF THE HUD
The HUD is a holographic display projected onto a glass in front of the pilot. A form of PFD is displayed, but it is less dense; for example, it does not give a textural representation of the earth and sky; it is made up of lines and numbers, to give the essential information. It is also monochrome. You look through it so that you can see at the same time visual cues or the display. The display is focussed at infinity so that you do not have to re-focus your eyes when you switch from visual cues to the IHIUD information. This does not mean that you can use both simultaneously. You have to give your attention to one or the other, but you can switch between them much more quickly than you change between head up and head down.
On Airbus aircraft it is used only for take-off and landing, and it is therefore stowed for the other phases of flight. It is also rather small, because its main purpose is for monitoring the autopilot during an autoland. Therefore, the amount of information is rather limited. For take-off, a triangular aircraft symbol, a yaw bar, and an indication of speed appear as soon as the thrust levers are put into the take-off position (TOGA or Flex detent).
You then use the rudder (NWS) to keep the aircraft on the centreline by keeping the triangular symbol aligned with the vertical yaw bar. When the yaw bar is to the left of the triangular aircraft symbol, you apply left rudder to bring it back into the centre, when it will be aligned with the triangle.
4. CERTIFICATION HISTORY
Both the HUD and the PVI, when used for take-off give a reduction in the minimum visibility required to 75m. The HUD was first certified by the JAA on the A320, and this certification was later extended to the A321 and 319. Very good guidance is given on all 3 types.
The PVI has been certified for use on take-off on A340 and the HUD on A330-300. They both use the same laws for guidance, and they both give very good guidance for a normal take-off. However, we were not entirely satisfied with the guidance when associated with an engine failure; the most critical case was the acceleration/stop when there was a tendency to overcontrol.
If you try to have very good precision you have to have high gains, and this always leads to a risk of overcontrolling in dynamic situations. You have to find a compromise which gives you the precision without overcontrolling. The guidance was good, but rather active; you had to filter it and not try to follow its every movement.
We have therefore recently revised the laws to give more efficient and better quality guidance when we did the certification of the PVI on the A330-200. We are entirely satisfied with the new guidance laws. We have found the good compromise which gives you the precision without overcontrolling. In fact, we have slightly reduced the gains and significantly increased the phase advance. One of the problems with developing laws is that using a simulator, no matter how good, is not quite the same 'as using the aircraft. However, you have to use the simulator to define the laws in the first place because of the vast number of conditions that you have to cover which it is not possible to do in an aircraft. Then you have a number of flights, with a relatively few conditions imposed by the weather, to fine-tune the gains.
The law is excellent for normal take-off, for continued take-off after engine failure, and for an acceleration/stop. We have done all that ourselves and with a certification test pilot, using a screen to blank off the outside view and simulate totally blind operations. We also use the second pilot, who naturally can see outside, to bring the aircraft to a position laterally displaced from the centre line before giving it back to the pilot using the PVI who has to then bring it back to the centreline and continue the take-off.
It is now instinctive to use, and you do not have to filter the information; you just follow it. You would find it easy and comfortable to use, and you would be able to take-off with less visibility (75m) than you are authorised to do today. We will put the same PVI on the A330-300. but the decision as to whether to change the laws on the A340 PVI has not yet been taken.