If you use ZoneManager with TruePVE, you can set a zone with ZoneManager and use a ruleset in the TruePVE plugin to accomplish this. Would be easier to have a flag added to the ZoneManager plugin but I use both of those plugins to disable heli damage to buildings on my server.

After you add both plugins, create a zone around the building you want heli to deal no damage to. Then go into TruePVE.json located in oxide/config and create a new RuleSet.

    {
      "name": "nohelidamage",
      "enabled": true,
      "defaultAllowDamage": true,
      "flags": "NoHeliDamage, HumanNPCDamage",
      "rules": [
        "anything can hurt dispensers",
        "anything can hurt players",
        "players can hurt players",
        "anything can hurt traps",
        "traps can hurt players",
        "players can hurt barricades",
        "barricades can hurt players",
        "highwalls can hurt players",
        "anything can hurt heli",
        "anything can hurt npcs",
        "fire can hurt players",
        "anything can hurt resources"
      ]
    },

You can set the name (nohelidamage) to whatever you want. Change the flags and rules to whatever you prefer. "NoHeliDamage" will make it so heli deals no damage to buildings. Now get the ZoneID for that zone. Go back into your TruePVE.json file and put this in the Mappings section. (if you changed the name of the new RuleSet, put that in instead of "nohelidamage")

{
  "Config Version": "0.9.0",
  "Default RuleSet": "default",
  "Configuration Options": {
    "handleDamage": true,
    "useZones": true
  },
  "Mappings": {
    "default": "default",
    "72517616": "nohelidamage"
 },

Make sure everything else in the TruePVE.json config file is correct and it should work. (if I explained it right) Let me know if there are any issues and i'll try to help you out.

can't make it work here is my cfg file what am I doing wrong ?

{
  "Config Version": "0.9.0",
  "Default RuleSet": "default",
  "Configuration Options": {
    "handleDamage": true,
    "useZones": true
  },
  "Mappings": {
    "70131743": "nohelidamage",
    "47242872": "exclude",
    "default": "default",
    "16084370": "exclude"
  },
  "Schedule": {
    "enabled": false,
    "useRealtime": false,
    "broadcast": false,
    "entries": []
  },
  "RuleSets": [
    {
      "name": "nohelidamage",
      "enabled": true,
      "defaultAllowDamage": true,
      "flags": "NoHeliDamage, HumanNPCDamage",
      "rules": [
        "anything can hurt dispensers",
        "anything can hurt players",
        "players cannot hurt players",
        "anything can hurt traps",
        "traps can hurt players",
        "players can hurt barricades",
        "barricades can hurt players",
        "highwalls can hurt players",
        "anything can hurt heli",
        "anything can hurt npcs",
        "fire cannot hurt players",
        "anything can hurt resources"
      ]
    },
    {
      "name": "default",
      "enabled": true,
      "defaultAllowDamage": false,
      "flags": "HumanNPCDamage, LockedBoxesImmortal, LockedDoorsImmortal, AdminsHurtSleepers",
      "rules": [
        "anything can hurt dispensers",
        "anything can hurt players",
        "players cannot hurt players",
        "anything can hurt traps",
        "traps can hurt players",
        "players can hurt barricades",
        "players can hurt sam_site_turret_deployed",
        "players can hurt sam_static",
        "barricades can hurt players",
        "highwalls can hurt players",
        "anything can hurt heli",
        "anything can hurt npcs",
        "fire cannot hurt players",
        "traps can hurt npcs",
        "samrocket can hurt balloon",
        "anything can hurt resources"
      ]
    }
  ],
  "Entity Groups": [
    {
      "name": "dispensers",
      "members": "BaseCorpse, HelicopterDebris, hotair",
      "exclusions": ""
    },
    {
      "name": "players",
      "members": "BasePlayer",
      "exclusions": ""
    },
    {
      "name": "traps",
      "members": "AutoTurret, BearTrap, FlameTurret, GunTrap, spikes.floor, sam_site, ReactiveTarget",
      "exclusions": ""
    },
    {
      "name": "barricades",
      "members": "Barricade, spikes.floor",
      "exclusions": ""
    },
    {
      "name": "highwalls",
      "members": "wall.external.high.stone, wall.external.high.wood, gates.external.high.wood, gates.external.high.wood",
      "exclusions": ""
    },
    {
      "name": "heli",
      "members": "BaseHelicopter, hotair, ch47scientists.entity",
      "exclusions": ""
    },
    {
      "name": "npcs",
      "members": "ch47scientists.entity, scientist_gunner, NPCPlayerApex, NPCMurderer, murderer, NPCScientist, scientist, BradleyAPC",
      "exclusions": ""
    },
    {
      "name": "fire",
      "members": "FireBall",
      "exclusions": ""
    },
    {
      "name": "resources",
      "members": "ResourceEntity, TreeEntity, OreResourceEntity",
      "exclusions": ""
    },
    {
      "name": "samrocket",
      "members": "sam_static, sam_site_turret_deployed",
      "exclusions": ""
    },
    {
      "name": "balloon",
      "members": "hotairballoon",
      "exclusions": ""
    }
  ]
}