Original Title: An Interpretation of the "Invisible Airport" on the Expressway

At the beginning of March, in a cross service joint exercise jointly organized by the US Air Force and the Marine Corps, an F-35B fighter of the US Marine Corps took off and landed on an abandoned road. This drill once again made the outside world pay attention to the value of the expressway as a combat readiness runway.

The earliest aircraft taking off and landing on the road can be traced back to World War II. At that time, Germany proposed to build aircraft runways on expressways, and many road sections were used as military airports. After the war, this practice was followed by many countries. In order to enhance the traffic emergency capacity and military deterrence, many countries incorporated the emergency airport of expressway into the overall planning of expressway construction.

In recent years, Western countries represented by NATO countries have frequently carried out takeoff and landing tests on fighter highways. So, what is highway takeoff and landing? What are the challenges in the actual use process? What is the significance of road takeoff and landing for winning the future battlefield? What is its future development trend? This article is for your interpretation.

　　 "Invisible airport" on the highway

During the exercise, the F/A-18 fighter plane of the Finnish Air Force landed on a Finnish road. Courtesy: Yang Ming

　　 Military airports have wartime "backup"

The so-called expressway runway refers to the straight and wide expressway as an emergency runway for aircraft takeoff and landing. In wartime or emergency situations, the expressway runway can quickly change from an ordinary road to a combat readiness runway, so it is also called "invisible airport".

Today, facing the modern reconnaissance system of a military power, a limited number of airports, as strategic facilities, are easy to be damaged or attacked at the beginning of the war, which makes it difficult to ensure the normal takeoff and landing of war planes. Although this dilemma will be alleviated with the improvement of runway emergency repair technology, it is obviously better to have more combat readiness airports or runways than to simply rely on the emergency repair of standing airports, considering that modern wars have the characteristics of high intensity and fast pace.

The characteristics and technical indicators of expressways are similar to those of airport runways. During construction, only the parameters such as linear length, width, vertical and horizontal gradient, subgrade speed, and pavement strength need to be modified and improved to meet the takeoff and landing requirements of aircraft. As a result, the choice of expressways as combat readiness runways has gradually become a common practice in many countries around the world.

The feasibility of changing the expressway into a combat readiness runway has long been proved by the war history.

Germany is the first country in the world to build expressways. After the outbreak of World War II, Germany was also the first to select some highway sections suitable for takeoff and landing as combat readiness runways for fighters, and to use the dense forests on both sides of the road to hide fighters, personnel and materials. Especially when attacking the Soviet Union, Germany first launched a major attack on the Soviet airport, and more than 1000 Soviet aircraft were blown up on the runway.

After the war, countries all over the world began to build expressways, and at the same time, they began to transform expressways into war readiness runways. The takeoff and landing training of expressways has gradually become the focus of war readiness training construction in many countries.

In 2018, the Russian Aerospace Army dispatched Su-30M2 and Su-34 fighter bombers to carry out emergency combat readiness takeoff and landing training on an 18 meter wide asphalt highway; On September 21, 2023, during the multinational joint exercise code named "Bana" - 23 held in Finland, two F-35A stealth fighters of the Norwegian Air Force carried out a highway takeoff and landing exercise; On September 14, 2023, the Polish Air Force completed a series of highway takeoff and landing operations involving fighter planes, fighter bombers, trainers and transport planes in the decentralized operation exercise of Route 604 held in the northern Polish town of Vilbach

A retired deputy commander of the British Air Force said that in the "era of ultra precision attack", if the air base is threatened or the airspace around the base is blocked by the enemy, the decentralized action of fighters based on highway takeoff and landing can ensure operational flexibility and prevent fighters from becoming targets of opponents.

In consideration of the future national security, some sections of highways will be specially treated during the construction of expressways. These seemingly ordinary roads can not only meet the needs of civil transportation in peacetime, but also quickly transform into combat readiness runways once wars occur, disperse the deployment of combat aircraft, and effectively improve the survivability and response speed of combat aircraft.

　　 Jointly construct the combat readiness airport system of expressway

In fact, it is not as simple as imagined to make a highway a fighter runway.

Theoretically, a fighter can take off and land only on a flat and straight road with enough length. However, the takeoff weight of modern fighter often exceeds 30 tons. During landing, the pressure on the road when its high-pressure tires contact the road greatly exceeds that of trucks with the same weight. In addition to the landing weight, the damage to the pavement caused by the great impact force generated when the aircraft lands and touches down is also unmatched by conventional road vehicles. Therefore, the roadbed of the expressway that can be used for aircraft takeoff and landing must be extremely solid. In practice, there are few expressways that can meet such conditions.

At the same time, for pilots who take off and land on the highway, the psychological challenges they need to overcome far outweigh the technical difficulties. The environment is unfamiliar, various landing conditions are not easy to grasp, and the road surface is different from the conventional airport runway. In case of emergency situations such as deviation from the runway, foreign matter inhalation, wing collision, etc. during aircraft takeoff and landing, the handling requirements are high and the room is small... These special situations require pilots to have excellent psychological quality and flexible handling ability.

In addition, if there are other complex road and railway networks around the road section for taking off and landing, it is more likely to lead to pilot deviation in identifying reference objects, which will bring greater challenges to flight. During the Cold War, during a road takeoff and landing training, an F-104 fighter plane of the West German Air Force yawed and blew a tire. A pilot at that time sighed that it was a "quite crazy experience" to control a dozen ton fighter plane to take off and land on the road.

The related guarantee and supporting facilities are another problem that cannot be ignored.

First of all, the close cooperation of various professional technical support personnel is required for the fighter to complete a takeoff and landing on the highway——

In military airports, communication, navigation, weather and other facilities to ensure aircraft takeoff are complete, while highways do not have such conditions. Fighters must use mobile workstations such as service vehicles to support takeoff and landing on roads.

The military airport has a permanent oil depot for refueling the fighters, while the fighters taking off and landing on the highway can only rely on oil tankers for refueling. If several aircraft need refueling at the same time, it is difficult for oil tankers to provide timely and effective support for several aircraft at the same time.

From the perspective of flight environment, conventional military airports will carry out some necessary road preparation before takeoff and landing, as well as highway takeoff and landing. On the one hand, there should be "clearance" around the expressway that can be used as an aircraft runway, and there should be no obstacles in the air that affect flight; On the other hand, it is necessary to ensure that there are no stones or metal particles on the pavement of the expressway, so as to avoid being involved in the aircraft engine, causing flight failure or danger.

Secondly, taking off and landing at night is another big problem that must be solved.

Different from conventional airports, the core reason for choosing expressway to take off and land aircraft is the consideration of concealment. At night, light signs and radio beacons used to provide reference for fighters are more likely to be exposed. Therefore, there is an urgent need to find a way to provide reference for pilots when flying, so that they can fly safely at night.

It can be seen that if we want to use highways to meet the requirements of combat aircraft takeoff and landing, whether it is equipped with professional service support equipment or skilled pilots, we should eventually make the highways have the function of conventional military airports. At the same time, we should normalize the takeoff and landing training of highways, and build a good airport system for highway combat readiness. Only by doing so, can the expressway runway play a "backup" role in wartime, and improve the survivability and sustained combat capability of the aviation force at a critical moment.

　　 The military application potential of expressway runway is huge

It is said that in this joint exercise jointly organized by the US Air Force and the Marine Corps in March, two pilots driving the F-35B fighter collected and summarized various flight parameters, run length, fuel consumption and other data during road takeoff and landing, and set up a professional ground team to monitor the radar response of the F-35B fighter during road takeoff and landing RF spectrum characteristics and noise intensity.

In addition, since the outbreak of the military conflict between Russia and Ukraine, European countries have begun to attach importance to the mobile deployment capability of aviation forces. Sweden, Norway, Finland, Poland and other countries have emphasized the importance of using highways as temporary airstrips.

From a worldwide perspective, with the continuous upgrading of fighter planes, the construction of expressway airport runways is becoming more and more complete. At the early stage of the development of aviation industry, the aircraft structure was relatively simple and compact, and the requirements for aircraft takeoff and landing sites were far less than those of later jet fighters. It was not difficult for aircraft to take off and land on conventional highways. Therefore, early highway airport construction generally only required the construction of an emergency runway and some simple and sporadic support facilities.

Today, the once humble support facilities are far from meeting the support needs of today's aircraft for high-density deployment and fall back.

In order to meet the needs of the future battlefield, we can see that, as a national strategic choice, in recent years, each country has its own focus and strategy in improving road takeoff and landing capacity——

More than 50% of the land in Sweden, the "Forest Kingdom", is covered by dense forests, and the highway passing through the forest is more concealed as a runway. At the same time, the expressway system in the country is developed and the quality is good, and nearly half of the road surface hardness is suitable for taking off and landing aircraft on the runway. Sweden bases a highway intensive area to form a network layout. Some roads are used for taxiing and parking of fighters, and some long straight roads are used for takeoff and landing of fighters. In addition, since Sweden began to develop its own supersonic fighter, it has taken "the fighter must be able to take off and land on the road runway" as one of its design ideas. The fighter series products launched by Sweden Saab Group, from SAAB-35 fighter to JAS-39 fighter, have excellent road takeoff and landing capabilities.

The US military chooses to conduct takeoff and landing training on some roads with poor road conditions and narrow roads, which is obviously to improve the training difficulty and exercise corresponding tactics. As a country with very developed highway network, some combat readiness runways of the United States can even take off and land large transport aircraft.

Germany has incorporated highway takeoff and landing capacity into the planning of highway construction, and constructed takeoff and landing sections and related supporting facilities according to the needs of future wars. It is reported that it has installed retractable guardrails in some highway sections. When this section of highway needs to use combat standby airport, the guardrails can be lowered at any time, without impeding aircraft takeoff and landing

The importance that different countries attach to the road takeoff and landing capability of fighter planes reflects their huge military application potential in the future battlefield: the road combat readiness runway with high concealment in wartime can not only be an important supplement to the military airport after being damaged by the enemy, ensure the emergency takeoff and landing of fighter planes, but also to a large extent eliminate the enemy's precision strike capability, It consumes the opponent's operational potential and patience. On the thousands of kilometers of highway, the opponent cannot determine which section of the runway is used for taking off and landing aircraft, so it is relatively difficult to destroy it.

In general, road takeoff and landing capability is one of the important criteria to measure a country's military strength. In order to deal with the challenges brought by the high uncertainty of the future battlefield, it is becoming a "must" to focus on improving its own road takeoff and landing capability. (Xiong Tianxia, Han Xiaodong, Du Peng)