DALLAS — Few sights are more enthralling than flying over the snow-capped Himalayas, with their jagged peaks stretching as far as the eye can see.
A glance at a navigation chart reveals that few routes allow aircraft to fly over the Himalayan mountain range rather than simply around it. To the south of the mountains, a vast network of routes crisscrosses in an east-west direction, while to the north of the mountain range in China, the skies see very few overflying aircraft.
Pakistan and China collaborated in 1997 to develop a new route connecting the southwest of China to the northeast corner of Pakistan. This region of the world is never far from areas of airspace entangled in geopolitical tension and conflict, and this new route was intended to circumnavigate such hotspots.
Airways are the highways in the sky that aircraft fly along, and the portion of this new airway in Pakistani airspace has been named ‘Golf 325’ and is shown on aeronautical charts as G325.
The boundary between Pakistani and Chinese airspace is denoted by a waypoint called PURPA, and it is at this point that airway G325 connects to the Chinese airway ‘Whisky 112’, or W112.
The unforgiving terrain beneath this airway is what sets this route apart. The minimum safe altitude that can be flown in an emergency to keep an aircraft clear of terrain between the Pakistan town of Gilgit, which is nestled high in the mountains, and the border with China is 27,600 feet. No other airway in the world, to the best of my knowledge, has such high terrain directly beneath it.
On a clear day, the views are breathtaking, and I remember my first PURPA crossing more than a decade ago with fondness. It is common for some cabin crew to visit the flight deck for a better view while flying past the route’s highest peaks, rather than squinting through a cabin window.
K2 is visible on a clear day to the east, and it’s always fun to try to match the various peaks to what’s depicted in my atlas.
Despite the high terrain, flying along this route is not difficult, but one must take the time to prepare in advance. What may surprise you is that an engine failure on this route is not the most dangerous malfunction that could occur. Instead, the worst-case scenario is a rapid loss of cabin pressure because there is a greater urgency to descend as soon as possible to denser air that facilitates unaided breathing.
However, due to the extremely high terrain, you are unable to perform this rapid descent. Following an engine failure, however, a gradual descent to an altitude that will allow flight on the remaining engine(s) is required. While a powerplant failure is still a serious situation, it is not as critical as a rapid decompression in this situation, and the initial actions can be carried out more slowly.
Let’s take a look at what would happen if there was a rapid decompression. There are two essential tasks that the flight crew needs to perform. Firstly, to don their oxygen masks, and secondly, to descend as quickly as possible to a lower altitude where the air is dense enough to breathe without an oxygen mask.
When cabin pressurization functions normally and whilst cruising at thirty-plus thousand feet, the air in the cabin has a density that corresponds to an altitude of around 4000-6000 feet, so just the same if you were on the ground in Denver, Colorado, or in Nairobi, Kenya.
A rapid loss of pressure in the cabin now causes the cabin altitude to be the same as the altitude of the aircraft, so if you lose pressure, you will be exposed to significantly less dense air outside of the fuselage. Following this, pilots typically descend to 10,000 feet above sea level, which is often the highest altitude at which they can breathe unaided.
However, passenger rules differ from country to country, and in some cases, passengers are permitted to breathe unaided at altitudes slightly higher than 10,000 feet.
One interesting observation is that the oxygen masks used on the flight deck are larger than the ubiquitous yellow masks that drop down from the overhead panels in the cabin. This is for good reason, as the oxygen masks provided to pilots are designed to be more robust in more hostile environments, such as a smoke-filled flight deck.
But what if the terrain prevents you from descending to 10,000 feet, as is the case with the route’s very high mountains?
This is when the flight crew will use ‘escape routes,’ which provide the crew with lateral and vertical paths to steer clear of the higher terrain and then gradually descend from one altitude to the next as soon as possible. In the airline industry, these escape routes are frequently defined by the airline.
The oxygen supply available to passengers via the yellow drop-down masks is limited and is only intended to keep you breathing until the flight crew can descend to denser air.
Rather than using large gaseous cylinders, these masks are typically supplied with a restricted flow of oxygen that is produced chemically. Passenger oxygen masks provide oxygen for short periods, such as around 20 minutes, and in our scenario, this is where things can get interesting.
Rapid depressurization is usually associated with some kind of structural failure, and if something is loose on your car, the faster you drive, the more damage you could cause. The same logic can be applied here. If the fuselage has structural damage, flying faster and exposing the structure to faster airflow may aggravate the problem further.
“No problem,” you say, “we just need to fly slower.” The problem with that logic is that you may need to fly at least 200 nautical miles before reaching a low enough altitude with dense enough air to breathe unaided, and covering that distance in 22 minutes will require a ground speed of 546 knots!
Even in the eyes of the eternal optimist, this would be a massive challenge, as a noticeable tailwind would be required to achieve that speed over the ground on any airliner!
As a result, there is a delicate balancing act between flying as fast as possible to exit the high terrain under your route and flying slow enough to safeguard against any structural damage that may have caused the rapid depressurization.
Because terrain and airport proximity change all the time, it’s a good idea to divide the route into segments, each with its own escape path away from terrain and a selection of possible airports to divert to. Staying a few steps ahead of where you are now allows you to plan and know what route and airport you would divert to before reaching the relevant portion of the route that you are interested in.
Interestingly, the weather reporting systems used by some smaller Chinese airports do not integrate with wider global networks, which often means you have no weather for some possible diversion airports until you are within the radio range of Air Traffic Control (ATC) in China.
If a diversion is required, both pilots must agree on where to divert, because deciding where to divert while communicating while wearing an oxygen mask is difficult at best! All critical tasks that must be completed whenever a new segment approaches include reviewing the current weather and the likely runway in use at potential diversion airports, as well as preparing backup routes for use in the flight management computer.
As a side note, while traveling through this region, you may encounter a half-dozen escape routes that cover a portion of the route that easily stretches over 1,000 nautical miles. Although the Himalayas are confined to southwest China, there is still very high terrain that extends deep into central China.
Communications with ATC on this route have greatly improved over time, but because Very High Frequency (VHF) radios rely on line-of-sight communications, the extremely high peaks in this area pose a logistical challenge.
The ATC agency responsible for the portion of G325 that lies in Pakistan is based in Lahore, and it is common to lose radio contact with the Lahore-based controller for the final few minutes or the first few minutes if entering Pakistan airspace after leaving China.
On the other hand, the Chinese ATC agency in charge of the southwest portion of the country’s airspace is based in the city of Urumqi, in northwest China. The Chinese have invested heavily in radio infrastructure, as one can normally clearly hear the Urumqi controller before leaving Pakistan-controlled airspace.
In the last few decades, Controller Pilot Data Link Communications (CPDLC) has made life on the flight deck much easier. This is a system that allows ATCs and pilots to communicate using short text messages, and it has transformed how aircraft communicate with ATCs over long distances. The Chinese do not use this technology, and while it was reportedly installed at the ATC center in Urumqi for a brief period, it was rarely used.
One plausible explanation is that CPDLC can only transmit alphanumeric characters from the English alphabet, but Chinese pilots who only fly domestically are not required to speak English, so ATC communications are always performed in Chinese. This, I believe, explains why a CPDLC system that uses English characters would be unusable for many Chinese pilots.
On the whole, Chinese air traffic controllers speak reasonable English, with basic aviation terminology easily understood. Many controllers in larger metropolitan areas, such as Beijing and especially Shanghai, clearly have a much stronger command of the language.
However, when flying over provincial areas, there is a distinct possibility of communication difficulties when using non-aviation standard terminology on the radio, such as when you have a technical failure and need to use plain language to describe what has gone wrong.
While cabin crew are not expected to communicate with ATC in the course of their duties, in such exceptional circumstances, a colleague in the cabin who can converse with a controller in their native language can often be a very valuable asset.
This route never fails to excite the minds of even the most experienced pilots. It is arguably one of the most beautiful parts of the world to fly over, and it is an awe-inspiring series of moments to look down on such harsh terrain beneath while sipping your coffee from the flight deck!
The PURPA crossing is something I’ll never get tired of, and the plethora of ‘what-if’ scenarios that must be constantly evaluated make it a never-ending learning experience.
Featured image: Crossing the Himalayas. Photo: Chris Smith/Airways