The main purpose of commercial aviation is getting people from varied parts of the globe closer. Qantas airplane in October last year, a Boeing 787-9 covered roughly 10,000 miles during its journey from New York to Sydney. The aircraft was in the air for a non stop 19 hrs and 16 minutes.These figures are staggering and long haul flights look more and more promising with the innovations of these aircraft’s in the future post the pandemic.The most important point to take out of this for an aviation enthusiast is how safe is the airplane when it is flying for long hours continuously in the air?
There are a number of failures that can occur during long haul flights and with the help of alternate airfields on the route, a pilot can take a decision to divert and land at one of the alternates.The big question is what if there are no alternates on the route or on one of the long oceanic legs of the flight? The answer is to that is the Critical Point calculated by pilots before their flight.
What is Critical Point
Critical Point (CP) also know known as Point Of Equal Time (PET) is the decision point between two airfields from which it would take the same time to fly to either airfield.In other words, CP or ETP is a geographical point in the flight where the aircraft would have the same flying time to continue on to a given airport or to turn back to another suitable airfield.
The knowledge of CP enables the pilot to decide which way would it be quicker for him to proceed either to the destination or return to the place of departure if they face a time critical problem such as cabin fire or an flight medical emergency.
NOTE: Before you read any further, I would highly recommend having a quick look at the Wind Triangle post in case you haven’t WIND TRIANGLE. The study of Critical Point requires concepts from the wind triangle that would help us in calculating the calculating the Ground speed so please check that out.
1.Lets consider a route from point A to point B which is 200 NM long. Our aircraft has a TAS of 100 knots and there are no winds. In this scenario, the critical point or point of equal time is the point of equal distance, that is 100 NM. What this shows that at the 100 NM mark, the time to destination or time to return back to departure airfield would be same.
2.Lets take the same example as above and consider a headwind of 20 knots.With a 20 knots headwind, the ground speed outwards (GS out) from A to B will reduce by 20 knots and become 80 knots and if you consider the opposite, that is if you return to A and direction of travel is from B to A then your ground speed will increase from 100 to 120 knots and we will call this as ground speed home (GS home).Hence as the GS home is more than GS out, the CP moves forward than what it was in still air conditions.As we saw in this example, it is safe to say that in case of headwind component, distance to CP will always be more than mid way.
3.Lets take the same example as above and consider a tailwind of 20 knots.With a 20 knots tailwind, the ground speed outwards (GS out) from A to B will increase by 20 knots and become 120 knots and if you consider the opposite, that is if you return to A and direction of travel is from B to A then your ground speed will decrease from 100 to 80 knots and we will call this as ground speed home (GS home).Hence as the GS out is more than GS home, the CP moves backward than what it was in still air conditions.As we saw in this example, it is safe to say that in case of tailwind component, distance to CP will always be less than mid way.
From the above points 2 and 3, the effect of a headwind and tailwind makes the critical point move in the direction on the wind .
4.We have addressed the effect of wind on the Critical Point but how do we know the distance to the CP (DCP) from the point we are calculating it and the time to CP. Lets consider we are traveling a distance ‘D’ from A to B and we know our TAS and winds. With the help of our TAS and winds we can calculate Ground speed.If we want to calculate our CP from position A then the ground speed from A to CP can be termed as GS OUT (O) and the ground speed from CP to A can be termed as GS HOME (H).The distance and time to CP can be found out by simple formulas as shown in the picture.
5.To get used to the calculating the DCP and time to CP we shall consider an example. Total distance from A to B is 750 NM. The aircraft TAS is 250 knots with with a tailwind of 30 knots on departure. Calculate the distance and time to CP?
6.There might be instances where the winds are directly abeam (90 degrees) to your track. In such cases, the distance to CP will always be midway and whether we proceed outbound or inbound, both ways the aircraft will face headwind.
Lets consider an example, with our route distance being 270 NM. The track of our flight is 030 degrees and the wind is coming from 120 degrees (90 degrees to the track) at 35 knots. The true air speed is 125 knots. Find the distance to CP and time to CP?
Even when we have a look at the picture on the right, the calculations show us the same thing that the critical point is mid way of the total distance.
In this weeks post we covered a very important topic of flight planning that is used by pilots.I hope you liked reading the post and gained some insight from it. Please feel free to email me or post in the comments section, any aviation related topics you would like to gain knowledge on. Until next week, stay safe and stay healthy.
Your CO PILOT