Aspects of Airmanship
No 02: A PNR/PSR refresher for light aircraft aviators
Aspects of Airmanship (A of A) is a series of back-to-basic chats for low-time pilots from Mike Feeney, a retired dinosaur aviator and flight safety writer.
Please bear with me with this refresher item as I realise that point of no return, also known as point of safe return, calculations are probably not ranked high on your list of airmanship aspects about which to become wildly enthused. However, should you ever need to quickly recall how to readily do the sums, you may find yourself mentally floundering around in your memory storage bin, only to discover that you haven’t got a clue.
Do try to labour your way through this relatively brief item as I promise you that A of A no 03 is a gripping cliff-hanger tale of drama over the ocean wastes between New Zealand and Norfolk Island … be alarmed, be very alarmed!
The PNR (point of no return) aka PSR (point of safe return) determination may be quite simple or complex and becomes important if you plan to fly from A to B over oceans, vast deserts, jungles or perhaps icy wastes. It all depends on the complexity of the type of aircraft, the winds, the weather expected at your destination and departure point, the availability of suitable diversion aerodromes (and whether they have avgas, for example), you and your aircraft being IFR qualified, CAA mandated fuel reserves and your personal fuel reserve policy.
Military pilots engaged in combat exercises may well use fuel reserve figures that are much less than is usual for civil operations. In extreme situations, they may even approach the ultimate PNR case when there is virtually no fuel remaining when returning or diverting from the PNR time/position, or perhaps just enough for one approach.
But for our exercise I shall keep it as simple as is possible as the average New Zealand private pilot is most likely to want to fly to Australia via Norfolk Island, or perhaps out to the Chathams. You might also use a PNR calculation if trying to get to some distant New Zealand destination at a remote region (Haast comes to mind). Or perhaps a long trip from somewhere such as Palmerston North to Queenstown.
So for this article I shall dispense with as much fine detail as is sensible, such as taxi fuel, departure fuel, extra fuel for climb and other factors.
Let’s use a typical type that is quite capable of flights to Norfolk Island: the Cessna 206. We shall assume that it will be flown at constant power settings and fuel flows, say 140kt TAS and 60lt/hr. I hazily recall that the C206 tanks hold 88 US gal (73 imp gal/330lt). Say an endurance of 330min (5.5hr) to dry tanks.
So if the New Zealand weather was fine and stable, and there was no en-route wind (highly unlikely over the North Tasman Sea) you could deduct a sensible one-hour reserve, leaving 4.5hr for the PNR calculation which would be simply to fly outbound for 2.25hr and, if the Norfolk weather was seriously deteriorating, turn around and head for home with an hour’s fuel remaining on arrival.
Now think about the case of a tailwind component outbound of 30kt. This is where we use the calculator slide-rule thingy on your trusty navigation computer or your electronic equivalent.
Your GSo (“o” for outbound, or you may prefer to call it GS1) will be a sizzling 170kt and your GSh (“h” for homebound, or GS2) will be 110kt. How many minutes (T) will be required for the outbound leg and how many minutes for the return homebound leg? (There is another setting for distance to PNR which I shall ignore, keeping it simple.) The sum of the GSo and the GSh is to the total safe endurance time in minutes (E) as the GSh is to the time in minutes on the outbound leg (GSo).
1. First find the sum of GSo and GSh = 280kt. (If a wind change occurs during the flight, this may not be the simple method of doubling your TAS.)
2. Rotate the inner scale so the safe endurance time (E = 270min) is UNDER the sum of the two groundspeeds (280kt). 280 ******270
3. Then, UNDER 110 (GSh) on the outer scale, read the time on the outbound leg = 106min to reach your PNR.
4. Leave the setting unchanged and UNDER 170kt (GSo) on the outer scale, read the time required for the return leg to New Zealand = 164min.
By common-sense inspection, we can check that the results appear to be rational. It will take much longer to return from the PNR to New Zealand than it took to reach the PNR due to the wind components — a tailwind outbound and a headwind homebound.
If you find your brain spinning, just remember that the greater the amount of fuel you have, the closer will be your PNR to your destination. If you can carry enough, it will eventually be at your destination which, in the case of ground fog, would enable you to hold for a while before returning or diverting.
If you wish to notate the distance to PNR, simply put 60 on the inside scale against your outbound groundspeed on the outer and, against time to PNR on the inner scale, read distance on the outer scale. That is just the same as any time and distance calculation. If you know one, then you can know the other.
By adding the outbound time to your departure time and writing that on your flight plan, you have a ready quick reference should you have to make a decision about the destination weather.
Things become more complex if you are flying a helicopter to a moving destination such as a ship, and when winds differ significantly from forecast and so have to be taken into account.
I recall that some 43 years ago it took me three attempts to pass the Australian ATPL Flight Planning exam in which the big flight plan involved a Qantas Boeing 707 on a flight from Perth to Africa. The diabolical examiners introduced en-route problems such as a depressurisation, wind changes in the many zones and fuel flow reductions as weight reduced but Mach number remained constant.
There were also questions involving piston-engined airline types and engine-out critical points (equi-time points). A four-hour exam may not sound very arduous, but one really had to motor to complete it in the allotted time. On my first attempt I never even finished the portion of the paper on the B707 of which I was supposed to have been in command and I, and the examiner, were left forever in limbo somewhere over the western Indian Ocean.
And they also banned the use of the then-new and amazing electronic calculators. All the sums had to be done on one’s whizz-wheel and/or by using logarithm tables (which I still have to this day).
During the historic TEAL 150kt flying boat years, it was not uncommon on the Auckland to Sydney sector for their groundspeed home to be double that of their groundspeed outbound — 100kt westbound and 200kt eastbound. This of course put their PNR way beyond the mid-distance position. Past a certain headwind component, flights had to be suspended as it was not possible to carry enough fuel without offloading all the passengers and their baggage.
Keep it simple. The wind forecasts are pretty good these days. Just subtract a sensible reserve from your total endurance and put that E figure on the rotating inner scale against the sum of your GSo and GSh. You do not have to make another setting.
Then, as I described above, you can read off your time to PNR and write it on your flight plan in big clear numbers.
- Report by Mike Feeney
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